Lessons Menu:

• Lesson 1 – Getting Started
• Lesson 2 - Choosing a Robotic Platform
• Lesson 3 - Making Sense of Actuators
• Lesson 4 - Understanding Microcontrollers
• Lesson 5 - Choosing a Motor Controller
• Lesson 6 – Controlling your Robot
• Lesson 7 - Using Sensors
• Lesson 8 - Getting the Right Tools
• Lesson 9 - Assembling a Robot
• Lesson 10 - Programming a Robot

Now that you have chosen all the basic building blocks used to make a robot, the next step is to design and build a structure or frame which keeps them all together and gives your robot a distinct look and shape.

Making the Frame

There is no “ideal” way to create a frame since there is almost always a trade-off to be made; You may want a lightweight frame but it may need to use expensive materials or end up too fragile. You may want a robust or large chassis but realize it will be expensive, heavy or hard to produce. Your “ideal” frame may be complex and take too much time to design and create when a simple frame may have been just as good. There is also rarely ever an “ideal” shape, but some designs can certainly look more elegant in their simplicity, while others can attract attention because of their complexity.

Materials

There are many materials you can use to create a frame. As you use more and more materials to build not only robots but other devices, you will get a better feeling as to which is most appropriate for a given project. The list of suggested building materials below include only the more common ones, and once you have tried a few, feel free to experiment with ones not on the list, or merge some together.

Use existing commercial products

You have likely seen school projects which were based on existing mass produced products such as bottles, cardboard boxes, Tupperware, etc.  This is essentially “re-purposing” a product and has the potential to either save you a lot of time and money, or create added hassle and headache. The amazing RoboBrrdto the left is a very good example of how to repurpose materials and make a very capable robot out of them.

Basic construction material

Some of the most basic construction materials can be used to make excellent frames. One of cheapest and most readily available materials is cardboard, which you can often find for free and can be easily cut, bent, glued and layered. Example: You can create a reinforced cardboard box which looks a lot nicer and is more proportional in size to your robot. You can then spread epoxy or glue to make it more durable and then paint it.

Flat structural material

One of the most common ways to make a frame is to use a standard material such as a sheet of wood, plastic or metal, and add holes for connecting all the actuators and electronics. A durable piece of wood tends to be fairly thick and heavy, whereas a thin sheet of metal may be too flexible. Example:   A flat ⅛” piece of dense wood can be easily cut with a saw, drilled (without fear of shattering), painted, sanded, and more. You can connect devices to both sides (for example connect the motors and caster wheels to the bottom, and the electronics and battery to the top) and the wood will still remain intact and solid.

Laser cut / bent plastic or metal

If you are at the stage where you are prepared to have a frame outsourced, the best options are still to have the part precision cut using a laser or water jet. Having a company produce a custom part is ideal only if you are confident in all your dimensions, since mistakes can be quite costly. Companies which offer computer controlled cutting services many also offer a variety of other services including bending and painting.

3D printing

3D printing a frame is rarely ever the most structurally sound solution (because it is built up in layers), but this process can produce very intricate and complex shapes which would not be possible (or very difficult) by other means. A single 3D printed part can contain all the necessary mounting points for all electrical and mechanical components while saving considerable weight. As 3D printing becomes more popular, the price of producing parts will also go down.  A very prominent advantage of 3D printing is not only that your design is easy to reproduce, it is also, easy to share. For instance, you can click on the turtle shell example on the left and get all the design instruction and CAD files.

Polymoph

Polymorph is really in a class on its own; at room temperature, Polymorph is a hard plastic, but when heated (in hot water for instance), it becomes malleable and can be shaped into intricate parts, which then cools and solidifies into durable plastic parts. Normally, plastic parts require high temperatures and molds, making them off-limits to most hobbyists. Example: You can combine different shapes (cylinders, flat sheets etc) to form complex plastic structures which look production. You can also experiment with basic molding, the Polymorph robotic arm is a good example of what you can achieve with this material.

Putting the Robot Together

Given the selection of materials and methods, how do you get started? Follow the steps below to create an aesthetic, simple and structurally sound smaller sized robot frame.

1. Settle on a construction material choice.
2. Get all the parts that you robot will require (electrical and mechanical) and measure them. If you don’t have all your parts on hand, you can refer to the dimensions provided by manufacturer’s
3. Brainstorm and sketch a few different designs for the frame. Don’t go into too much detail.
4. Once you settle on a design, make sure the structure is sound and that the components would be well supported.
5. Draw each part of your robot in paper or cardboard at 1:1 scale (real size). You can also draw them using CAD software and print them out.
6. Test your design in CAD and in real life with your paper prototype by test fitting each part and connections.
7. Measure everything again! and once you are absolutely sure your design is correct, start cutting the frame into the actual material. Remember, measure twice and cut once!
8. Test fit each component before assembling the frame in case modifications are require.
9. Go crazy and assemble your frame using hot glue, screws, nails, Duck tape or whatever other binding technique you choose for your robot.
10. Fit all the components onto the frame and voila: you have just created a robot from scratch!

Assembling the Robot Components

Step 10 from the list above deserves to be elaborated upon. In previous lessons, you had chosen the electrical components and actuators. Now, your need to get them all working together. For the following section we will use generic cable colors and terminal names that only cover the common case. As always, the datasheet and manuals are you bests friends when understanding how robotic equipment works.

Connecting Motors to Motor Controllers

A DC (gear) motor, or DC linear actuator will likely have two wires: red and black. Connect the red wire to the M+ terminal on the DC motor controller, and the black to M-. Reversing the wires will only cause the motor to spin in the opposite direction. A servo motor, there are are three wires: one black (GND), red (4.8 to 6V) and, yellow (position signal). A servo motor controller has pins matching these wires so the servo can be plugged directly to it.

Connecting Batteries to a Motor Controller or a Microcontroller

Most  motor controllers have two screw terminals for the battery leads labelled B+ and B-. If your battery came with a connector and your controller uses screw terminals, you may be able to find a mating connector with pigtails (wires) which you can connect to the screw terminal. If not, you may need to find another way to connect the battery to the motor controller while still being able to unplug the battery and connect it to a charger. It is possible that not all the electromechanical products you chose for your robot can operate at the same voltage and thus may require several batteries or voltage regulation circuits. See bellow the usual voltage levels involved in common hobby robotics components:

• DC gear motors – 3V to 24V
• Standard Servo motors – 4.8V to 6V
• Specialty Servo motors – 7.4V to 12V
• Stepper motors – 6V to 12V
• Microcontrollers usually include voltage regulators – 3V to 12V
• Sensors – 3.3V, 5V and 12V
• DC motor controllers – 3V to 48V
• Standard batteries are 3.7V, 4.8V, 6V, 7.4V, 9V, 11.1V and 12V

If you are making a robot with DC gear motors, a microcontroller and maybe a servo or two, it is easy to see how one battery may not be able to power everything directly. We recommend nevertheless, choosing a battery which can directly power as many devices as possible. The battery with the greatest capacity should be associated with the drive motors. For example, if the motors you chose are rated a nominal 12V, your main battery should also be 12V, then you can use a regulator o power a 5V microcontroller. Without going into details, NiMH and LiPo are the top two choices for small to medium-sized robots. Choose NiMh for a cheaper price and LiPo for a lighter weight. Warning: Batteries are powerful devices and can easily burn your circuits if they are connected incorrectly. Always triple check that the polarity is right and that you device can handle the energy provided by the battery. If you are not sure, don’t “guess”. Electricity is much faster than you, by the time you realize something is wrong, the magic blue smoke already escaped your device.

Connecting Motor controllers to Microcontroller

A microcontroller can communicate with motor controllers in a variety of ways:

• Serial: The controller has two pins labelled Rx (receive) and Tx (transmit). Connect the Rx pin of the motor controller to the microcontroller’s Tx pin and vice versa.
• I2C: The motor controller will have four pins: SDA, SCL, V, GND. Your microcontroller will have the same four pins but not necessarily labelled, simply connect them one to one.
• PWM: The motor controller will have both a PWM input and a digital input for each motor. Connect the PWM input pin of the motor controller to a PWM output pin on the microcontroller, and connect each digital input pin of the motor controller to a digital output pin on the microcontroller.
• R/C: To connect a microcontroller to an R/C motor controller, you need to connect the signal pin to a digital pin on the microcontroller.

Regardless of the communication method, the motor controller’s logic and the microcontroller need to share the same ground reference (this is achieved by connecting the GND pins together) and the same logic high level (this can be achieved by using the same V+ pin to power both devices). A logic level shifter is required if the devices don’t share the same logic levels (3.3V and 5V for instance)

Connecting Sensors to a Microcontroller

Sensors can be interfaced with microcontrollers in a similar way than motor controllers. Sensors can use the following types of communication:

• Digital: The sensor has a digital signal pin that connects directly to a digital microcontroller pin. A simple switch can be regarded as a digital sensor.
• Analogue: Analogue sensors produce an analogue voltage signal that needs to be read by an analogue pin. If your microcontroller does not have analog pins, you will need a separate analog to digital circuit (ADC). Also, some sensors some with the required power supply circuit and usually have three pins: V+, GND and Signal. If a sensor is a simple variable resistor for instance, it will require you to create a voltage divider in order to read the resulting variable voltage.
• Serial or I2C: the same communication principles explained for motor controllers apply here.

Communication device to microcontroller

Most communication devices (e.g. XBee, Bluetooth) use serial communication, so the same RX,TX, GND and V+ connections are required. It is important to note that although several serial connections can be shared on the same RX and TX pins, proper bus arbitration is required in order to prevent cross-talk, errors and madness in general. If you have very few serial devices, it is often simple to use a single serial port for each one of them.

Wheels to motors

Ideally, you would have chosen wheels or sprockets which are designed to fit the shaft of the motor you chose. If not, hopefully there is a hub which fits between the two. If you find that the wheel and motor you have chosen are not compatible with one another and cannot find a suitable hub, you may need to find another hub which connects to the wheel but has a smaller bore, you would then drill out the hub’s bore to the same diameter as the shaft.

Electrical components to frame

You can mount electronics to a frame using a variety of methods. Be sure that whatever means you use do not conduct electricity. Common methods include: hex spacers, screws, nuts, double-sided tape, Velcro,  glue, cable ties, etc.

Practical Example

1. Settle on a construction material choice.
2. We are getting the following parts in order to measure and test fit them:
• 2x Gear motors with 90 degree shafts
• 1x Arduino Uno
• 1x Johnny Robot Standard GM Track Kit
• 2x Arduino Motor controller Shield
• 2x Mini Breadboard
3. We will try to stay close to a 6 sided box, but may have had to make modifications in order to accommodate for all parts
4. Some modifications need to be done to the design in order to accommodate for all parts such as:
• Add more mounting holes for the battery pack
• Add more mounting points for servos or other accessories
• Refined the hole placement.
5. The cardboard frame will be made by printing the design onto white cardboard (or gluing a printed paper sheet onto cardboard), cutting it, bending it and using (hot) glue in order to reinforce the bends, edges and surfaces.
6. We completely assembled the robot using the cardboard frame in order to make sure everything fits properly.
7. We measure everything again and once we were absolutely sure about the design, we had it professionally manufactured.
8. Test fit each component in case modifications are require.
9. The frame is made in one piece so no assembly is required
10. Assembled the robot incorporating lots of accessories.

For further information on learning how to make a robot, please visit the RobotShop Learning Center. Visit the RobotShop Community Forum in order to seek assistance in building robots, showcase your projects or simply hang-out with other fellow roboticists.

Lessons Menu:

• Lesson 1 – Getting Started
• Lesson 2 - Choosing a Robotic Platform
• Lesson 3 - Making Sense of Actuators
• Lesson 4 - Understanding Microcontrollers
• Lesson 5 - Choosing a Motor Controller
• Lesson 6 – Controlling your Robot
• Lesson 7 - Using Sensors
• Lesson 8 - Getting the Right Tools
• Lesson 9 - Assembling a Robot
• Lesson 10 - Programming a Robot

The definition we have chosen for a “robot” requires the device to obtain data about its environment, make a decision, and then take action accordingly. This does not exclude the option of a robot being semi-autonomous (having aspects which are controlled by a human and others which it does on its own). A good example of this is a sophisticated underwater robot; a human controls the basic movements of the robot while an on-board processor measures and reacts to underwater currents in order to keep the robot in the same position without drifting. A camera onboard the robot sends video back to the human while onboard sensors may track the water temperature, pressure and more. If the robot loses communication with the surface, an autonomous program may kick-in causing it to surface. If you want to be able to send and/or receive commands from your robot, you will need to determine its level of autonomy and if you want it to be tethered, wireless or fully autonomous.

Tethered

Direct Wired Control

The easiest way to control a vehicle is with a handheld controller physically connected to the vehicle using  a cable (i.e. a tether). Toggle switches, knobs, levers, joysticks and buttons on this controller allow the user to control the vehicle without the need to incorporate complex electronics. In this situation, the motors and a power source can be connected directly with a switch in order to control its forward/backwards rotation. Such vehicles usually have no intelligence and are considered to be more “remote controlled machines” than “robots”.

Advantages

• The robot is not limited to an operating time since it can be connected directly to the mains
• There is no worry about loss of signal
• Minimal electronics and minimal complexity
• The robot itself can be light weight or have added payload capacity
• The robot can be physically retrieved if something goes wrong (very important for underwater robots)

Disadvantages

• The tether can get caught or snagged (and potentially cut)
• Distance is limited by the length of the tether
• Dragging a long tether adds friction and can slow or even stop the robot from moving

Wired Computer Control

The next step is to incorporate a microcontroller into the vehicle but continue to use a tether. Connecting the microcontroller to one of your computer’s I/O ports (e.g. a USB port) allows you to control its actions using a keyboard (or keypad), joystick or other peripheral device. Adding a microcontroller to a project also may require you to program how the robot reacts to the input. Instead of using a laptop or desktop computer, netbooks are often a desirable choice because of their low price, small size and low weight.

Advantages

• Same advantages as with direct wired control
• More complex behaviours can be programmed or mapped to single buttons or commands.
• Larger controller choice (mouse, keyboard, joystick, etc.)
• Added onboard intelligence means it can interface with sensors and make certain decisions on its own

Disadvantages

• Cost is higher than a purely tethered robot because of the added electronics
• Same disadvantages as with direct wired control

Ethernet

A variation on computer control would be to use an Ethernet interface. A robot that is physically connected to a router (so it could be controlled via the Internet) is also possible (though not very practical) for mobile robots. Setting-up a robot that can communicate using the internet can be fairly complex, and more often than not, a WiFi (wireless internet) connection is preferable. A wired and wireless combination is also an option, where there is a transceiver (transmit and receive) connected physically to the internet and data received via the internet is then sent wirelessly to the robot.

Advantages

• Robot can be controlled trough the Internet from anywhere in the world
• The robot is not limited to an operating time since it could use Power over Ethernet (PoE).
• Using Internet Protocol (IP) can simplify and improve the communication scheme.
• Same advantages as with direct wired computer control

Disadvantages

• Programming involved is more complex
• The tether can get caught or snagged (and potentially cut)
• Distance is limited by the length of the tether
• Dragging a long tether adds friction and can slow or even stop the robot from moving

Wireless

Infrared

Infrared transmitters and receivers cut the cables connecting the robot to the operator. This is usually a milestone for beginners. Infrared control requires “line of sight” in order to function; the receiver must be able to “see” the transmitter at all times in order to receive data. Infrared remote controls (such as universal remote controls for televisions) are used to send commands to an infrared receiver connected to a microcontroller which then interprets these signals and controls the robot’s actions.

Advantages

• Low cost
• Simple TV remote controls can be used as controllers

Disadvantages

• Needs to be line of sight
• Distance is limited

Radio Frequency (RF)

Commercially available Remote Control (R/C) units use small microcontrollers in the transmitter and receiver to send, receive and interpret data sent via radio frequency (RF). The receiver box has a PCB (printed circuit board) which comprises the receiving unit and a small servo motor controller. RF communication requires either a transmitter matched/paired with a receiver, or a transceiver (which can both send and receive data). RF does not require line of sight and can also offer significant range (transmission distance). Standard radio frequency devices can allow for data transfer between devices as far away as several kilometres and there is seemingly no limit to the range for more professional RF units.

XBee and Zigbee modules use RF for communication, but allow the user to vary many of the communication parameters involved. These modules have a specific footprint (layout) and are only produced by certain companies. Their main advantage is that they provide a very robust easy to set up link and take care of all of the communication protocol details.

Many robot builders choose to make semi-autonomous robots with RF capability since it allows the robot to be as autonomous as possible, provide feedback to a user and still give the user some control over some of its functions should the need arise.

Advantages

• Considerable distances possible
• Setup can be straightforward
• Omni directional (impeded but not entirely blocked by walls and obstructions)

Disadvantages

• Very low data rate (simple commands only)
• Pay attention to the transmission frequencies – they can be shared

Bluetooth

Bluetooth is a form of RF and follows specific protocols for sending and receiving data. Normal Bluetooth range is often limited to about 10m though it does have the advantage of allowing users to control their robot via Bluetooth-enabled devices such as cell-phones, PDAs and laptops (though custom programming may be required to create an interface). Just like RF, Bluetooth offers two-way communication.

Advantages

• Controllable from any Bluetooth enabled device (usually additional programming is necessary) such as a Smartphone, laptop, desktop etc.
• Higher data rates possible
• Omnidirectional (does not need line of sight and can travel a little through walls)

Disadvantages

• Devices need to be “paired”
• Distance is usually about 10m (without obstructions)

WiFi

WiFi is now an option for robots; being able to control a robot wirelessly via the internet presents some significant advantages (and some drawbacks) to wireless control. In order to set up a WiFi robot, you need a wireless router connected to the internet and a WiFi unit on the robot itself. For the robot, you can also use a device that is TCP/IP enabled with a wireless router.

Advantages

• Controllable from anywhere in the world so long as it is within range of a wireless router
• High data rates possible

Disadvantages

• Added programming required
• Maximum range is usually determined by the choice of wireless router

GPRS / Cellular

Another wireless technology that was originally developed for human to human communication, the cell phone, is now being used to control robots. Since cellular frequencies are regulated, incorporating a cellular module on a robot usually requires added patience for programming as well as an understanding of the cellular network system and the regulations.

Advantages

• Robot can be controlled anywhere it has a cellular signal
• Direct satellite connection is possible

Disadvantages

• Setup and configuration can be complex – NOT for beginners
• Each network has its own requirements / restrictions
• Cellular service is not free; usually the more data you transmit/receive the more money you will need to pay.
• System is not (yet) well setup for robotics use

Autonomous

The next step is to use the microcontroller in your robot to its full potential and program it to react to input from its sensors. Autonomous control can come in various forms: pre-programmed with no feedback from the environment, limited sensor feedback and finally complex sensor feedback. True “autonomous control” involves a variety of sensors and code to allow the robot to determine by itself the best action to be taken in any given situation.

The most complex methods of control currently implemented on autonomous robots are visual and auditory commands. For visual control, a robot looks to a human or an object in order to get its commands. Getting a robot to turn to the left by showing a piece of paper with arrow pointing left is a lot harder to accomplish than one might initially suspect. An auditory command such as “turn left” also requires quite a bit of programming. Programming a variety of complex commands like “get me a drink from the fridge” or “get my shoes, they’re near the front door” is no longer fantasy but requires a very high level of programming, and a lot of time.

Advantages

• This is “real” robotics
• Tasks can be as simple as blinking a light based on one sensor readings to landing a spacecraft on a distant planet.

Disadvantages

• It’s only as good as the programmer; if it’s doing something you don’t want it to do, the only option you have is to check your code, modify it and upload the changes to the robot.

Practical Example

For our project, the goal is to create an autonomous rover capable of making a decision based on external input from sensors. Should the robot “misbehave” it will be physically close and shutting it off will not be an issue. However having the option of semi-autonomous (wireless) control to allow us the option of making a remote-controlled vehicle is also attractive. We will not have the need for tethered control.

The microcontroller chosen in the previous lesson uses what are called “shields” which are essentially ad-on boards specific to the Arduino’s pin layout. There are many shields, including ones that allow for Ethernet, Xbee, or Bluetooth communication. There is even a shield that allows for GPRS (i.e. cellular) communications. The basic robot will therefore have no additional modules, though it is important to note that it does have wireless communication capability.

For further information on learning how to make a robot, please visit the RobotShop Learning Center. Visit the RobotShop Community Forum in order to seek assistance in building robots, showcase your projects or simply hang-out with other fellow roboticists.

In an effort to make robotics accessible to everyone, we will be posting robotics project ideas and guides. This will provide you some inspiration to start using your newly acquired skills from the How to Make a Robot Tutorial Series.

Stay tuned for new projects to come. In the meantime, you can check out the RobotShop Learning Center for some cool Project Ideas.

Lessons Menu:

• Lesson 1 – Getting Started
• Lesson 2 - Choosing a Robotic Platform
• Lesson 3 - Making Sense of Actuators
• Lesson 4 - Understanding Microcontrollers
• Lesson 5 - Choosing a Motor Controller
• Lesson 6 – Controlling your Robot
• Lesson 7 - Using Sensors
• Lesson 8 - Getting the Right Tools
• Lesson 9 - Assembling a Robot
• Lesson 10 - Programming a Robot

Making Sense of Actuators

Now that we learned about robotics in general in Lesson 1 and decided on the robot to make in Lesson 2, we will now choose the actuators that will make the robot move.

What is an actuator?

An “actuator” can be defined as a device that converts energy (in robotics, that energy tends to be electrical) into physical motion. The vast majority of actuators produce either rotational or linear motion. For instance, a “DC motor” is therefore a type of actuator.

Choosing the right actuators for your robot requires an understanding of what actuators are available, some imagination, and a bit of math and physics.

Rotational Actuators

As the name indicates, this type of actuators transform electrical energy into a rotating motion. There are two main mechanical parameters distinguishing them from one another: (1) torque, the force they can produce at a given distance (usually expressed in N•m or Oz•in), and (2) the rotational speed (usually measured in revolutions per minutes, or rpm).

AC Motor

AC (alternating current) is rarely used in mobile robots since most of them are powered with direct current (DC) coming from batteries. Also, since electronic components use DC, it is more convenient to have the same type of power supply for the actuators as well. AC motors are mainly used in industrial environments where very high torque is required, or where the motors are connected to the mains / wall outlet.

DC Motors

DC motors come in a variety of shapes and sized although most are cylindrical. They feature an output shaft which rotates at high speeds usually in the 5 000 to 10 000 rpm range. Although DC motors rotate very quickly in general, most are not strong (low torque). In order to reduce the speed and increase the torque, a gear can be added. To incorporate a motor into a robot, you need to fix the body of the motor to the frame of the robot. For this reason motors  often feature mounting holes which are generally located  on the face of the motor so they can be mounted perpendicularly to a surface. DC motors can operate in clockwise (CW) and counter clockwise (CCW) rotation. The angular motion of the turning shaft can be measured using encoders or potentiometers.

Geared DC Motors

A DC gear motor is a DC motor combined with a gearbox that works to decrease the motor’s speed and increase the torque. For example, if a DC motor rotates at 10 000 rpm and produces 0.001 N•m of torque, adding a 256:1 (“two hundred and fifty six to one”) gear down would reduce the speed by a factor of 256 (resulting in 10 000rpm / 256 = 39 rpm), and increase the torque by a factor of 256 (0.001 x 256 = 0.256 N•m). The most common types of gearing are “spur” (the most common), “planetary” (more complex but allows for higher gear-downs in a more confined space, as well as higher efficiency) and “worm” (which allows for very high gear ratio with just a single stage, and also prevents the output shaft from moving if the motor s not powered). Just like a DC motor, a DC gear motor can also rotate CW and CCW. If you need to know the number of rotations of the motor, an “encoder” can be added to the shaft.

R/C Servo Motors

Industrial Servo Motors

An industrial servo motor is controlled differently than a hobby servo motor and is more commonly found on very large machines. An industrial servo motor is usually made up of a large AC (sometimes three-phase) motor, a gear down and an encoder which provides feedback about angular position and speed. These motors are rarely used in mobile robots because of their weight, size, cost and complexity. You might find an industrial servo in a more powerful industrial robotic arm or very large robotic vehicles.

Stepper Motors

A stepper motor does exactly as its name implies; it rotates in specified “steps” (actually, specific degrees). The number of degrees the shaft rotates with each step (step size) varies based on several factors. Most stepper motors do not include gearing, so just like a DC motor, the torque is often low. Configured properly, a stepper can rotate CW and CCW and can be moved to a desired angular position. There are unipolar and bipolar stepper motor types. One notable downside to stepper motors is that if the motor is not powered, it’s difficult to be certain of the motor’s starting angle. Adding gears to a stepper motor has the same effect as a adding gears to a DC motors: it increases the torque and decreases the output angular speed. Since the speed is reduced by the gear ratio, the step size is also reduced by that same factor. If the non geared down stepper motor had a step size of 1.2 degrees, and you add a gear down of 55:1, the new step size would be 1.2 / 55 = 0.0218 degrees. Linear Actuators A linear actuator produces linear motion (motion along one straight line) and have three main distinguishing mechanical characteristics: the minimum and maximum distance the rod can move “a.k.a. the “stroke”, in mm or inches),  their force (in Kg or lbs), and their speed (in m/s or inch/s).  

DC Linear Actuator

A DC linear actuator is often made up of a DC motor connected to a lead screw. As the motor turns, so does the lead screw. A traveller on the lead screw is forced either towards or away from the motor, essentially converting the rotating motion to a linear motion. Some DC linear actuators incorporate a linear potentiometer which provides linear position feedback. In order to stop the actuator from destroying itself, many manufacturers include limit switches at either end which cuts power to the actuator when pressed.  DC linear actuators come in a wide variety of sizes, strokes and forces.  

Solenoids

Solenoids are composed of a coil wound around a mobile core. When the coil is energized, the core is pushed away from the magnetic field and produces a motion in a single direction. Multiple coils or some mechanical arrangements would be required in order to provide a motion in two directions. A solenoid’s stroke is usually very small but their speed is very fast. The strength depends mainly on the coil size and the current going trough it. This type of actuator is commonly used in valves or latching systems and there is usually no position feedback (it’s either fully retracted or fully extended).

Muscle wire

Muscle wire is a special type of wire that will contract when an electric current traverses it. Once the current is gone (and the wire cools down) it returns to its original length. This type of actuator is not very strong, fast or provides a long stroke. Nevertheless, it is very convenient when working with very small parts or in a very confined space.

Pneumatic and Hydraulic

Pneumatic and hydraulic actuators use air or a liquid (e.g. water or oil)  respectively in order to produce a linear motion. These types of actuators can have very long strokes, high force and high speed. In order to be operated they require the use of a fluid compressor which makes them more difficult to operate than regular electrical actuators. Because of they high force speed and generally large size, they are mainly used in industrial environments.     Choosing an Actuator To help you with the selection of an actuator for a specific task, we have developed the following questions to guide you in the right direction. It is important to note that there are always new and innovative technologies being brought to market and nothing is set in stone. Also note that an single actuator may perform very different task in different contexts. For instance, with additional mechanics, an actuator that produces linear motion may be used to rotate an object and vice versa (like on a car’s windshield wiper). (1) Is the actuator being used to move a wheeled robot? Drive motors must move the weight of the entire robot and will most likely require a gear down. Most robots use “skid steering” while cars or trucks tend to use rack-and-pinion steering. If you choose skid steering, DC gear motors are the ideal choice for robots with wheels or tracks as they provide continuous rotation, and can have optional position feedback using optical encoders and are very easy to program and use. If you want to use rack-and-pinion, you will need one drive motor (DC gear is also suggested) and one motor to steer the front wheels). For stirring, since the rotation required is restricted to a specific angle, an R/C servo would be the logical choice.  

(2) Is the motor being used to lift or turn a heavy weight?

  Lifting a weight requires significantly more power than moving a weight on a flat surface. Speed must be sacrificed in order to gain torque and it is best to use a gearbox with a high gear ratio and powerful DC motor or a DC linear actuator. Consider using system (either with worm gears, or clamps) that prevents the mass from falling in case of a power loss.

(3) Is the range of motion limited to 180 degrees?

If the range is limited to 180 degrees and the torque required is not significant, an R/C servo motor is ideal. Servo motors are offered in a variety of different torques and sizes and provide angular position feedback (most use a potentiometer, and some specialized ones use optical encoders). R/C servos are used more and more to create small walking robots.

(4) Does the angle need to be very precise?

Stepper motors and geared stepper motors (coupled with a stepper motor controller) can offer very precise angular motion. They are sometimes preferred to servo motors because they offer continuous rotation. However, some high-end digital servo motors use optical encoders and can offer very high precision.

(5) Is the motion in a straight line?

Linear actuators are best for moving objects and positioning them along a straight line. They come in a variety of sizes and configurations. Muscle wire should be considered only if your motion requires very little force. For very fast motion, consider pneumatics or solenoids, and for very high forces, consider DC linear actuators (up to about 500 pounds) and then hydraulics. Tools In order to compute the strength (or torque), and speed required for your application, many (rather complex) computations are required involving the physics of the machine to be created. In order to simplify the design process, we have put together a few tools that can help you out.

• DC Drive Motor Selector (useful for robots with wheels or tracks). Also consult the Drive Motor Sizing Tutorial for further details.
• Robot Leg Torque Tutorial
• Robot Arm Torque Calculator

Practical Example

In lesson 1 we determined the objective of our project would be to get a better understanding of mobile robots, while keeping the budget to about $200 to a maximum of $300. In lesson 2 we decided we wanted a small tank (on tracks) that could operate on top of a desk.

First, let us determine the type of actuators that would be required by answering the five  aforementioned questions:

1. Is the actuator being used to move a wheeled robot? Yes. A DC gear motor is the suggested type of actuator and skid steering is appropriate for a tank, which means that each track will need it;s own motor.
2. Is the motor being used to lift or turn a heavy weight? No, a desktop rover should not be heavy.
3. Is the range of motion limited to 180 degrees? No, the wheels need to urn continuously.
4. Does the angle need to be precise? No, our robot does not require positional feedback.
5. Is the motion in a straight line? No, since we want the robot to turn and move in all directions.

Since rotating a wheel needs rotational motion, we could quickly eliminate all linear actuators and choose a DC gear motor. The next logical question was “which one?”A search online shows that there are not too many track systems intended for small robots, which in itself would restrict which motors we could consider.

The Currently Available Track Systems

 

At 2″ and 3″ wide, the Lynxmotion tracks are more intended for medium sized robots, so we’ll omit them. The price does fall within the budget though.

The Vex Tank Tread Kit is definitely a good option, but it would restrict us to one specific motor.

The Tamiya Track and Wheel Set is definitely a good option, and would limit our choices to Tamiya motors  and gearboxes. This would also be within the budget.

There are several Johnny Robot Track Kits, one for a Hitec continuous rotation servo (which is essentially a gear motor in a servo’s body) another for a Futaba continuous rotation servo, one for Tamiya motors and another for Pololu or Solarbotics motors. This is definitely a good option and also within our budget. Mainly because of aesthetic and motor compatibility reasons, we are going to stick with this choice.

There is always the option of hacking a toy such as an R/C tank and convert it into a robot.  This option would also give us compatible motors, however, the objective is to design our own robot and not hack another product.

Computing the motor requirements

The next step is to fill out the DC Drive Motor Selector Tool, using approximate values.

Data Details

• Total mass of robot:200 g  should include everything:  motors, frame, batteries and all.
• Number of drive motors:Two motors are required for skid steering.
• Radius of drive wheel: from 0.5” to about 1” should be an appropriate size for a desktop robot.
• Velocity of robot:0.2 m/s would be nice for a desktop robot.
• Maximum incline: Climbing some books would be cool, let us choose 30 degrees.
• Supply Voltage:Uncertain at the moment, so we choose the default 12 V
• Desired Acceleration:Not sure, so choose default 0.2 m/s²
• Desired operating time: 30 minutes is reasonable between charges.
• Total efficiency:Not sure, so we choose default 65%

Using 0.5 as the wheel radius we obtain 150 rpm @ 1.4 oz-in. When using 1”, the calculator provides 75rpm @ 2.8 oz-in.

Selecting the Motor

Thus, the motors we are looking for must turn at approximately 150 rpm and provide roughly 1.4123 oz-in of torque. We can use the DC motor Comparison Table in order to find the appropriate motor. There are many motors available that fit the Johnny Robot Track Kit : The Solarbotics GM8 and GM9 feature 70 rpm @ 43 oz-in and 66 rpm at 43 oz-in respectively. Both sell for $5.46 each. All Tamiya gearbox ad motor combinations sell for approximately $11 and up and provide a wide range of torques and speeds. Hitec continuous rotation servo and Futaba continuous rotation servos sell for  $17  and $14 respectively. In the end, we opted to use a pair of Solarbotics GM9 in order to use skid-drive, mainly because of their low cost. It is important to note that although the calculator specified we needed about 150rpm, we chose the motor anyway, knowing it would move at about half the original (desired) velocity. The torque produced by this motor  is significantly greater than what we needed, which means it can carry additional weight, or climb stepper angles.

For further information on learning how to make a robot, please visit the RobotShop Learning Center. Visit the RobotShop Community Forum in order to seek assistance in building robots, showcase your projects or simply hang-out with other fellow roboticists.

Lessons Menu:

• Lesson 1 – Getting Started
• Lesson 2 - Choosing a Robotic Platform
• Lesson 3 - Making Sense of Actuators
• Lesson 4 - Understanding Microcontrollers
• Lesson 5 - Choosing a Motor Controller
• Lesson 6 – Controlling your Robot
• Lesson 7 - Using Sensors
• Lesson 8 - Getting the Right Tools
• Lesson 9 - Assembling a Robot
• Lesson 10 - Programming a Robot

Choosing a Robotic Platform Following the first lesson, you now have a basic understanding of what a robot is and what current robots normally do. Now, it is time to decide on the type if robot you are going to build. A custom robot design often starts with a “vision” of what the robot will look like and what it will do. The types of robots possible are unlimited, though the more popular are:

• Land wheeled, tracked, and legged robots
• Aerial planes, helicopters, and blimp
• Aquatic boats, submarines, and swimming robots
• Misc. and mixed robots
• Stationary robot arms, and  manipulators

This lesson is intended to help you decide what type of robot to build to best suite your mission. Since you have brainstormed on what tasks or functions you want it to accomplish (after lesson 1),  you can now choose the type of robot that will best suite your needs. Below, you will find a description of all the major robot types. Land Land-based robots, especially the wheeled ones,  are the most popular mobile robots among beginners as they usually require the least investment while providing significant exposure to robotics. On the other hand, the most complex type of robots is the humanoid (akin to a human), as it requires many degrees of freedom and synchronizing the motion of many motors, and uses many sensors.

Wheeled Robots

Wheels are by far the most popular method of providing mobility to a robot and are used to propel many different sized robots and robotic platforms. Wheels can be just about any size, from a few centimetres  up to 30 cm and more . Tabletop robots tend to have the smallest wheels, usually less than 5 cm in diameter. Robots can have just about any number of wheels, although 3 and 4 are the most common. Normally a three-wheeled robot uses two wheels and a caster at one end. More complex two wheeled robots may use gyroscopic stabilization. It is rare that a wheeled robot use anything but skid steering (like that of a tank). Rack and pinion steering such as that found on a car requires too many parts and its complexity and cost outweigh most of its advantages. Four and six wheeled robots have the advantage of using multiple drive motors (one connected to each wheel) which reduces slip. Also, omni-directional wheels or mecanum wheels, used properly, can give the robot significant mobility advantages. A common misconception about building a wheeled robot is that large, low-cost DC motors can propel a medium sized robot. As we will see later in this series, there is a lot more involved than just a motor.

Advantages

• Usually low-cost compared to other methods
• Simple design and construction
• Abundance of choice
• Six wheels or more rival a track system
• Excellent choice for beginners

Disadvantages

• May lose traction (slip)
• Small contact area (only a small rectangle or line underneath each wheel is in contact with the ground)

Tracked Robots

Tracks (or treads) are what tanks use. Although tracks do not provide added “force” (torque), they do reduce slip and more evenly distribute the weight of the robot, making them useful for loose surfaces such as sand and gravel. Also, a track system with some flexibility can better conform to a bumpy surface. Finally, most people tend to agree that tank tracks add an “aggressive” look to the robot as well.

Advantages

• Constant contact with the ground prevents slipping that might occur with wheels
• Evenly distributed weight helps your robot tackle a variety of surfaces
• Can be used to significantly increase a robot’s ground clearance without incorporating a larger drive wheel

Disadvantages

• When turning, there is a sideways force that acts on the ground; this can cause damage to the surface the robot is being used on, and cause the tracks to wear
• Not many different tracks are available (robot is usually constructed around the tracks)
• Drive sprocket might significantly limit the number of motors that can be used.
• Increased mechanical complexity (idler placement and number, # of links) and connections

Legs

An increasing number of robots use legs for mobility. Legs are often preferred for robots that must navigate on very uneven terrain. Most amateur robots are designed with six legs, which allow the robot to be statically balanced (balanced at all times on 3 legs); robots with fewer legs are harder to balance. The latter require “dynamic stability”, meaning that if the robot stops moving mid-stride, it might fall over. Researchers have experimented with monopod (one legged “hopping”) designs, though bipeds (two legs), quadrupeds (four legs), andhexapods (six legs) are the  most popular.

Advantages

• Closer to organic or natural motion
• Can potentially overcome large obstacles and navigate very rough terrain

Disadvantages

• Increased mechanical, electronic and coding complexity (not the easiest way to get into robotics).
• Lower battery size despite increased power demands
• Higher cost to build

Air

A AUAV (Autonomous Unmanned Aerial Vehicle) is very appealing and is entirely within the capability of many robot enthusiasts. However, the advantages of building an autonomous unmanned aerial vehicles, especially if you are a beginner, have yet to outweigh the risks.  When considering an aerial vehicle, most hobbyists still use existing commercial remote controlled aircraft. On the professional side, aircraft such as the US military Predator were initially semi-autonomous though in recent years Predator aircraft have flown missions autonomously.

Advantages

• Remote controlled aircraft have been in existence for decades (so there is a large community, at least for the mechanics)
• Excellent for surveillance

Disadvantages

• The entire investment can be lost in one crash.
• Limited robotic community to provide help for autonomous control

Water

An increasing number of hobbyists, institutions and companies are developing unmanned underwater vehicles. There are many obstacles yet to overcome to make underwater robots attractive to the wider robotic community though in recent years, several companies have commercialized pool cleaning “robots”. Underwater vehicles can use ballast (compressed air and flooded compartments), thrusters, tail and fins or even wings to submerge. Other aquatic robots such as pool cleaners are useful commercial products.

Advantages

• Most of our planet is water, so there is a lot to explore and discover
• Design is almost guaranteed to be unique
• Can be used and/or tested in a pool

Disadvantages

• Robot can be lost many ways (sinking, leaking, entangled…)
• Most electronic parts do not like water (also consider water falling on electronics when accessing the robot after a dive)
• Surpassing depths of 10m or more can require significant research and investment
• Very limited robotic community to provide help
• Limited wireless communication options

Miscellaneous and hybrid combinations

Your idea for a robot may not fall nicely into any of the above categories or may be comprised of several different functional sections. Note again that this guide is intended for mobile robots as opposed to stationary or permanently fixed designs (other than robotic arms and grippers). It is wise to consider when building a hybrid design, to use a modular design (each functional part can be taken off and tested separately). Miscellaneous designs can include hovercraft, snake-like designs, turrets and more.

Advantages

• Designed and built to meet specific needs
• Multi-tasking and can be comprised of modules
• Can lead to increased functionality and versatility

Disadvantages

• Possible Increased complexity and cost
• Often times, parts must be custom designed and built

Arms & Grippers

Although these do not fall under the category of mobile robotics, the field of robotics essentially started with arms and end-effectors (devices that attach to the end of an arm such as grippers, electromagnets etc). Arms and grippers are the best way for a robot to interact with the environment it is exploring. Simple robot arms can have just one motion, while more complex arms can have a dozen or more unique degrees of freedom.

Advantages

• Very simple to very complex design possibilities
• Easy to make a 3 or 4 degree of freedom robot arm (two joints and turning base)

Disadvantages

• Stationary unless mounted on a mobile platform
• Cost to build is proportional to lifting capability

Practical Example In our case, we have opted for building a robot that will provide the maximum exposure to robotics. A programmable tracked platform that can accommodate a variety of sensors and gripper sees ideal in this case, specially since we consider tank tracks  are far cooler than wheels. In order to keep the costs down, we opted to build a small desktop robot that will be able to roam indoors and on tabletops. We also have taken into consideration the fact that there are not many tracks available, and to keep things simple, we’ll only consider a single drive sprocket and single idler sprocket system, this should not be a problem since the robot will be very light weight. The preliminary CAD below summarized the features describes so far.

Next, we will be choosing the right actuators (e.g. motors) for your robot. For further information on learning how to make a robot, please visit the RobotShop Learning Center. Visit the RobotShop Community Forum in order to seek assistance in building robots, showcase your projects or simply hang-out with other fellow roboticists.