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35 Engineering a More Sustainable Future | ADI The Importance of Motor Encoder Accuracy and Repeatability Pick and place machines/robots are commonly utilized automation machines in the food packaging and semiconductor manufacturing industries. A machine or robot with high accuracy and repeatability is required for process efficiency. The accuracy, repeatability, and efficiency are achieved using high performance motor encoders. Figure 4 illustrates an encoder use case in robotics. Motors drive each joint in a robot arm via precision speed reduction gearboxes. The robot joint angles are measured via a precision motor-mounted shaft angle encoder (θm) and often additional arm-mounted encoder (θj). For robots, the main performance specification listed on data sheets is repeatability and typically the order of magnitude is at a submillimeter level. By knowing the repeatability specification and the robot reach, you can extrapolate back to the rotary encoder specifications. The angular repeatability (θ), required at the joint encoder, can be derived from trigonometry: tan inverse of robot repeatability divided by reach. Figure 4: Angular repeatability at the motor encoder (θ m ) and the joint encoder (θ j ), with the robot reach (L). 4 Table 2: Encoder Repeatability and Robot Repeatability Specification Robot System Robot 1 Robot 2 Assumed Gear Ratio, G 100 Repeatability Spec ±0.05 mm ±0.01 mm Reach, L 1.30 m 1.10 m Encoder Repeatability Spec θ 0.0022° 0.0005° θj/10¹ 0.00022° (~20- bit) 0.00005° (~22-bit) θ m = θ j × G 0.02° (~14-bit) 0.005° (~16-bit) 1 Individual encoder must be higher accuracy to achieve overall system accuracy as the robot reach consists of multiple joints. Multiple joints combine to achieve the overall robot reach. The sensor should have a higher performance than the target angular accuracy. The repeatability specification per joint must be improved, and a factor 10 improvement is assumed here. For the motor encoder, the repeatability is defined by the gearbox ratio (G). For example, robot systems shown in Table 2, 20-bit to 22-bit repeatability specifications are required for the joint encoder while 14-bit to 16-bit resolutions are necessary for the motor encoder. Adobe Stock / WilliamJu – stock.adobe.com