Choosing the right DC Motor (or DC gear motor) for a specific application can be a daunting task and many manufacturers only provide basic motor specifications. These basic specifications might not be sufficient for your needs. Listed below are ideal motor specifications and whenever possible, ways to approximate values.  

BASIC SPECIFICATIONS

Below is a list of the most common specifications a DC motor manufacturer might list. For most hobbyists the basic information is enough to make an informed decision about which motor to purchase.

Nominal Voltage:

The voltage that corresponds to the highest motor efficiency. Try to choose a main battery pack which most closely matches the nominal voltage of your drive motors. For example, if the motor’s nominal voltage is 6V, use a 5x 1.2V NiMh pack to get 6V. If your motor operates at 3.5V nominal, you can use either a 3xAA or 3xAAA NiMh pack or a 3.7V LiPo or LiIon pack. If you operate a motor outside of its nominal voltage, the efficiency of the motor goes down, often requiring additional current, generating more heat and decreasing the lifespan of the motor. Aside from a “nominal voltage” DC motors also have an operating voltage range outside of which the manufacturer does not suggest operating the motor. For example a 6V DC Gear motor may have an operating range of 3-9V; it will not operate as efficiently as compared to 6V, but it will still run well.

No Load RPM:

This is how fast (angular velocity) the final output shaft will rotate assuming nothing is connected to it. If the motor has a gear down and the motor’s speed is not indicated separately, the no load rpm value is the shaft speed after the gear down. The motor’s RPM is proportional to the voltage input. “No Load” means the motor encounters no resistance whatsoever (no hub or wheel mounted to the end). Usually the No Load RPM provided is associated with the nominal voltage.

Power rating:

If a motor’s power is not listed, it can be approximated. Power is related to current (I) and voltage (V) by the equation P = I*V. Use the no load current and nominal voltage to approximate the motor’s power output. The motor’s maximum power (which should only be used for a short time) can be approximated using the stall current and nominal voltage (rather than maximum voltage).

Stall Torque:

This is the maximum torque* a motor can provide with the shaft no longer rotating. It is important to note that most motors will sustain irreparable damage if subjected to stall conditions for more than a few seconds. When choosing a motor, you should consider subjecting it to no more than ~1/4 to 1/3 the stall torque.

Stall Current:

This is the current the motor will draw under maximum torque* conditions. This value can be very high and should you not have a motor controller capable of providing this current, there is a good chance your electronics will fry as well. If neither the stall nor the nominal current are provided, try to use the motor’s power rating (in Watts) and the nominal voltage to estimate the current: Power [Watts] = Voltage [Volts] x Current [Amps]

General Specifications:

A DC motor’s general specifications usually include weight, shaft length and shaft diameter as well as motor length and diameter. Other useful dimensions include the location of mounting holes and thread type. If only the length or diameter are provided, refer to an image, photo or scale drawing to get a sense of the other dimensions based on the one known value.

Torque

*”Torque” is calculated by multiplying a force (acting at a distance away from a pivot) by the distance. A motor rated at a stall torque of 10Nm can hold 10N at the end of 1m. Similarly, it could also hold 20Kg at the end of 0.5m (20 x 0.50 = 10) and so on. Note: 1 Kg * force of gravity (9.81m/s2) = 1N

 

IDEAL SPECIFICATIONS

Many motor manufacturers are now listing additional information that can be very useful when selecting the right motor. Below is some additional information you might come across when searching for DC motors:

Voltage vs. RPM:

Ideally, the manufacturer would list the graph of a motor’s voltage vs. rpm. For a quick approximate, consider using the no-load rpm and nominal voltage: (nominal voltage, rpm) and the point (0, 0). See “gear down” below for motors with a gear down.

Torque vs. Current:

Current is a value that cannot be easily controlled. DC motors use only as much current as they need. Ideal specifications include this curve, and approximations are not easily reproduced. The stall torque is related to the stall current. A motor that is prevented from turning will consume maximum (“stall”) current and produce the maximum torque possible. The current required to provide a given torque is based on many factors including the thickness, type and configuration of the wires used to make the motor, the magnets and other mechanical factors.

Technical specifications or 3D CAD drawing:

Many robot builders like to draw their robot on the computer before purchasing the necessary parts. Although all motor manufacturers have a CAD drawing with the dimensions, they rarely make it available to the public. Ideal motor dimensions include the basics listed above, as well as mounting hole locations and thread type. Ideally the materials used to make the motor, gears and winding as well as separate dimensions for the motor and the gear down would also be given.

Gear down:

DC motor manufacturers that also produce the corresponding gear down for a motor must list the gear down ratio. The gear down acts to increase torque and reduce rpm. The No Load RPM value given is always that of the last output shaft after the gear down. To find the angular velocity of the motor shaft before the gear down, multiply the value by the gear ratio. To obtain the motor’s stall torque before the gear down, divide the stall torque by the gear down. The material used to make the internal gears is usually plastic or metal and are chosen to be able to withstand the maximum torque rating. Calculate the gear down below given the values before and after gear down:

Accessories: An optical encoder is the most common accessory for a gear motor. Finding the right size of optical encoder for your motor can be very difficult if it is not made from the same company. An optical encoder allows you to track both the direction of rotation and number of revolutions of the motor. With the right code, an optical encoder can also give you the angle of the shaft.

Hubs and Shaft Couplers:

Secondary items such as hubs (used to connect the output shaft to other items) are slowly becoming available for varying sized output shafts. Only a few manufacturers provide generic shaft couplers. If you cannot find the appropriate coupler, consider using spur gears to offset the shaft to that of a different size. The image below shows three different types of couplers. The hole in the hub is for a threaded screw (“set screw”) which presses tightly against the shaft.

It is becoming increasingly important for schools to improve and broaden their scope and selection of technological subjects/topics in order to compete in a global market. More and more robotics manufacturers are noticing this potentially huge market and are making their products more “user friendly” so that teachers can incorporate them into their courses more easily. The big winners seem to be those that offer the most “bang for the buck”; easy to use and inexpensive products like Arduino are becoming more widespread and children are being introduced to robotics at younger and younger ages.

Robotics offers the perfect outlet for students not only because it is at the forefront of technology, and appeals to (almost) everyone, but also because it:

• offers a hands-on learning approach which students love
• incorporates multiple disciplines and opens career possibilities
• promotes technological literacy
• students must demonstrate resource and time management
• encourages teamwork and problem solving
• curriculum material is becoming increasingly available
• increasing support from both the public and private sectors

If you have a story you would like to share about how robotics played an important role in your school, or how you would have liked to have had the opportunity to use robotics in school, we invite you to share your experiences and comments in the comments section below.

For a robot to react to its environment, it must first obtain information about its surroundings. Robots are not limited to just sight, sound, touch, smell and taste. Robots can not only sense but give accurate values for a variety of environmental factors that humans are otherwise unaware of or incapable of doing.

 Measure Distance

• Contact (when an object is touched) and Proximity (object is close)
• Ultrasonic Range Finders (variable distances)
• Infrared and laser (used for a specific distance or range)
• Accelerometers (distance based on initial position, integrated twice),
• IMU‘s (distance based on initial position and includes rotation)
• Encoders and Disks (location based on number of evolutions of the wheel)
• Linear Potentiometer (location along slider)
• Resistive band (location along a band)
• Stretch and Bend Sensors (distance stretched or related)

 

Measure Rotation

• Potentiometer (usually 0 to 300 degrees)
• Gyroscope (angle based on initial value)
• IMU‘s (angle based on initial value and includes possible linear acceleration)
• Encoders and Disks (angle based on number of fractional turns)
• Stretch and Bend Sensors (moving closer or farther – use geometry)

 

Measure Environmental Conditions

• Light Sensor (specific color, intensity or range)
• Sound sensor (specific decibel reading or range)
• Thermal Sensors (specific temperature or temperature range)
• Infrared (used to measure temperature)
• Temperature and Humidity (range of values)
• Cameras and Vision Sensors (measure depth, color, proximity and object type)
• Barometric Sensor (measure pressure)

 

Measure Orientation and Position

• Compass (orientation with respect to earth’s magnetic field)
• Accelerometers (distance based on initial position, integrated twice),
• IMU‘s (distance based on initial position and includes rotation)
• Gyroscope (angle based on initial value)
• Stretch and Bend Sensors (orientation of specific parts)
• Localization and GPS (fixed sensors locate an object in 3D space)

 

Miscellaneous

• Current and Voltage Sensors (range of values)
• Magnetic Sensors (intensity of magnetic field)
• Vibration (specific value or range of values))
• Stretch and Bend Sensors (object warping or changing shape/position)

On the weekend of March 2nd to March 4th, a Global Cloud Robotics Hackathon will be held in several cities around the world. This initiative is born in Montreal, Canada where the main event will be held but other cities are challenged to participate and to show off their robot hacking skills.

What is a “hackathon”?

The word “hackathon” is a combination of “hack” and “marathon” and designates an event where hackers, makers and tinkerers from teams and get together for a short time (a weekend for instance) to create and hack technology. The most hard-core hackers code and build during the entire event without sleeping and only taking minimal breaks.

What is this “cloud” thing?

“Cloud robotics” refers to the use of web services (that run on the internet, a.k.a. “the cloud”) in order to make robots smarter. This hackathon is focused in catalyzing the creation of cloud robotics applications by using the MyRobots.com API, Android, and ROS.

Why participate?

Many reasons: It is a fun opportunity to hack  and learn about cloud robotics, it brings roboticists to work with web developers and android programmers, and each team gets a FREE DFRobotShop Rover. If you want to be a hero in the robotics community and get started in the very exciting and emerging field of cloud robotics, register your team now! Find out more about the Cloud Robotics hackathon on roboticshackathon.com

RobotShop, June 2008

This tutorial is intended to complement RobotShop’s Drive Motor Sizing Tool by providing you with a step by step explanation as to the calculations behind the dynamic tool. In the image below, half a mobile robt is shown. Although in this scenario only two out of the four wheels are driven, the equations below can be used for any number of passive and driven wheels, as well as for tank tracks. The equations are presented without units (units are presented with the drive motor selection tool).

To calculate the required torque, power, current and battery pack required by a wheeled mobile robot, there are several principles that must be understood: concept of vectors; 2D Force balance; Power; Current and Voltage. If you do not understand these concepts, you are encouraged to research them prior to reading this tutorial. In order to roll on a horizontal surface, a wheeled robot’s motors must produce enough torque to overcome any imperfections in the surface or wheels, as well as friction in the motor itself. Therefore theoretically, a robot (small or large) does not require much torque to move purely horizontally. Obviously there will be more friction and resistance in a large robot than in a small robot, though it is still exponentially less than when a robot encounters an incline. In order for a robot to roll up an incline at a constant velocity (no acceleration or deceleration) it must produce enough torque to “counteract” the effect of gravity, which would otherwise cause it to roll down the incline. On an inclined surface (at an angle theta) however, only one component of its weight (mgx parallel to the surface) causes the robot to move downwards. The other component, mgy is balanced by the normal force the surface exerts on the wheels.

In order for the robot not to slide down the incline, there must be friction between the wheel and the surface. The motor in a heavy truck may be able to produce 250 horsepower and significant torque, but we have all seen (in person or in video) large trucks simply spinning their wheels as they fall backwards on an icy street. It is friction (f) that “produces” the torque.

The torque (T) required is:

To select the proper motor, we must consider the “worst case scenario”, where the robot is not only on an incline, but accelerating up it. Note now that all forces (F) are along the x and y axes. We balance the forces in the x-direction:

 Inserting the equation for torque above, and the equation for mgx, we obtain:

Rearrange the equation to isolate T:

This torque value represents the total torque required to accelerate the robot up an incline. However, this value must be divided by the total number (N) of drive wheels to obtain the torque needed for each drive motor. Note that we do not consider the total number of passive wheels as they have no effect on the torque required to move the object aside from adding weight.

  The final point to consider is the efficiency (e) in the motor, gearing and wheel (slip).

  This increases the torque required and compensates for inefficiencies. Total power (P) per motor can be calculated using the following relation:

  T is known from above and the angular velocity (w) is specified by the builder. It is best to select the maximum angular velocity to be able to find the corresponding maximum power. Knowing the maximum power and the supply voltage (V) which the builder chooses, we can find an idea of the maximum current (I) requirements:

The two equations above are used to produce the following relation:

  Finally, the capacity (c) of battery pack required can be estimated using the equation:

You may wonder why such a large value is needed. This is because when choosing a battery pack, the rated amp hours are not an accurate indicate of the maximum current the pack can produce for extended periods of time. Also, the total charge is rarely retained over time. This way you will ensure the battery pack you select will be capable of producing the current your motors require, for the time you require and with the inefficiencies inherent in recharging battery packs. Note: This is the battery required PER MOTOR. To obtain a total battery pack required for the robot, multiply this value by the number of drive motors.