Motion System Selection Considerations for Wheeled Robots

As robots become an integral part of our lives in both industry and in our homes, wheeled robots will be at the forefront of the progression. Their motion systems usually consist of brushed or brushless DC motors coupled with compact planetary gearboxes. When selecting a motor and gearbox, you will need to determine both the product’s operating and maximum conditions and confirm the required speed and torque experienced at the wheel output. Here are some tips to consider to help with your selections:

Torque. First, determine the output torque required and then work backwards to find your motor and gearing. The torque on the wheel should be based on robot acceleration, wheel diameter and carrying capacity. Be sure the carrying capacity accounts for pulling the entire robot in case some actuators fail or wheels slip, as well as the minimum slope and potential obstacles to overcome.

Speed. In order to calculate the speed at which the wheel needs to rotate, you must define your desired speed — or final output — for the wheel. Based on that information, you can choose the motors and gearing. The robot manufacturer generally finalizes the robot’s driving speed, and the wheel diameter determines the required speed at the wheel output.

Packaging space. Make sure the motor stack, which consists of the encoder, brake, motor and gearing, fits into the robot and can be packaged cleanly. The encoder allows you to measure how much the motor shaft turns, and the brake system helps to hold the torque and provide dynamic stopping.

Voltage. The operating voltage is used to power the motor. Typically, the higher the voltage, the higher the speed capability of the motor. The motor’s datasheet will provide the voltage constant for calculating how fast the motor will rotate per volt.

Operating temperature. Although temperature often is not an issue, it is still important to make sure your motor stack does not overheat — particularly if it is enclosed. The gearbox is another concern since temperature can affect its lubrication life and performance over time.

Weight. You will need to estimate the mass, or know the actual mass, to determine motor torque. If you base your design on a mass estimate, we recommend applying a 25 percent safety margin. The motor datasheet indicates the torque constant so you can determine how much torque output you will achieve per amp.

Cost. While building a gearbox from scratch may be less expensive, the time and effort required to design, assemble and test the new gear can make a standard gearbox more economical.

Precision, accuracy and efficiency. How much lag can you afford in your gearing? Some wheel motor robot applications allow a little less precision and accuracy depending on terrain and torque profile. And, since not all applications demand low noise or vibration, lower precision can be tolerated. However, robotic arms or instruments often need low backlash systems that are more precise and accurate.

Reliability and noise. Most applications require higher reliability, so the motor stack must survive the required working points. Some critical applications like surveillance robots demand both low noise and higher reliability, and the motor and gearbox have to satisfy both criteria.

Portescap offers a broad range of standard and custom miniature motors with high torque density, reliability and efficiency along with engineering support to ensure your wheeled robotic systems can go where they’re needed and get the job done.

To learn more, read the complete white paper or contact an engineer.