No electromechanical system (which includes robots) is perfect and there will always be losses in efficiency. These losses can arise in many different areas, the most common of which are below:
Battery pack
Most rechargeable battery packs are never fully recharged; they oxidize or lose capacity over time. A perfect battery might be able to retain over 99% of its charge over a large number of cycles, though a less efficient pack might lose 0.5% or more per recharge. Also, some batteries succumb to “memory effect”; if you do not fully discharge the batteries before recharging them, they will lose capacity even faster.
Motors
Finding a motor that is 100% efficient is not possible. This means that 100% of the electricity supplied to the motor is converted into useful mechanical energy. Highly efficient motors are able to operate at 80-90% efficiency, but this is only under nominal conditions. An inefficient motor can even operate at 40% efficiency or less. Efficiency losses in motors can be attributed to heat, friction (bearings at both ends of the shaft are ideal), wind resistance (any motors have integrated cooling fans), coiling of the wires and control method. Some manufacturers are kind enough to include the graphs of efficiency versus voltage and rpm though others prefer not to make that information public.
Gear Down
Gear downs are solely mechanical systems and as such are subject to wear and tear, friction, manufacturing defects and more. The most efficient type of commonly available gear down is planetary, followed by spur gears and then worm gears. A planetary gear down can be upwards of 80-90% efficient.
Heat
Losses due to heat can occur almost everywhere in a robot. The area on a robot where the most heat is lost tends to be the motor controller, especially when the motor is subjected to a heavy load. Many motor controllers are sold with a heat sink to quickly dissipate the heat, and heat sinks are highly suggested if the controller does not come with one. However, this does not stop the total energy loss but rather lowers the temperature of microchip and H-bridge faster. If your robot is going to encounter situations where a lot of energy will be expended (such as an inclined surface, lifting weights or others), then it is best to consider a maximum of 80% efficiency. However, if your robot is moving on a flat surface, and the motors and motor controller you selected are properly matched to the robot's weight and specifications, then you might consider 90 to 100% efficiency.
Friction
Friction dissipates energy wherever moving parts are concerned. This includes the motor, gear down and wheels. This value is very difficult to calculate or measure. Friction losses are usually coupled with heat losses, though friction can also include wear and tear and losses in material due to grinding action.
Calculating
Instead of adding, efficiencies are multiplied. If you deemed the main sources of losses in efficiency to be the motors, gear down and motor controller, with respectively 80%, 90% and 95% efficiencies, the overall efficiency of the system will be:
0.8 * 0.9 * 0.95 = 0.684 = 64.8% overall efficiency.
When you design your robot, make sure to include losses in efficiency, otherwise your robot will not perform as you expected.
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