Mastering The Metrics Makes Specifying Encoders Simpler

by Richard Anslow and Michael Jackson

The first blog in this series described the benefits encoders bring to closed-loop control motor applications and looked at the relative advantages and disadvantages of optical and magnetic transducers. This blog presents key encoder metrics and shows how to specify the performance level for encoders in a robotic pick-and-place application.

Figure 1 Robotic arm in a pick-and-place application

Information derived from Encoder Outputs

Specifying an encoder for an application depends on the type of information it is intended to provide:

• In general-purpose servo drives and velocity control systems, output signals from encoders are used to derive speed information. Speed is calculated by measuring the change in the position of a shaft during a sample period of the control loop.
• In servos, robotics, and discrete control systems, encoder signals provide information about shaft position in a feedback loop.
• Encoders can also assist with motor commutation, ensuring that the currents in the motor windings have the correct phase relationship to the rotating magnetic field from the rotor (either a rotating magnetic field from magnets or induced fields in an induction motor).

Key Encoder Performance Metrics

Having a grasp of the key performance metrics is essential for correct encoder specification. These include:

• Resolution: The number of uniquely identifiable positions (codes) in a single 360-degree rotation of a motor shaft. Generally, the highest-resolution encoders use optical transducers, while medium-resolution/high-resolution encoders use magnetic transducers. Resolvers (a type of rotary transformer) or Hall sensors are suitable for applications requiring low to medium resolution. 16-24 bit encoders are typically chosen for high-resolution applications, 13-18 bit encoders deliver medium resolution, while 12 bits (or lower) provide low resolution.
• Repeatability: Measures how consistently an encoder returns to the same commanded position and is a critical performance metric in applications that require precise, repetitive movements, like robotics or pick-and-place machines.
• Absolute accuracy: The difference between the actual and reported positions for a single shaft rotation. It is analogous to a data converter's integral non-linearity (INL). Absolute accuracy is critical in position control applications.
• Differential accuracy: Quantifies the difference in the reported distance between two adjacent codes as a shaft rotates and is an essential metric in speed control applications.

How to specify an Encoder for a Pick-and-place Robot

Pick-and-place robots are a regular feature of the food packaging and semiconductor manufacturing industries. In this application, high accuracy and repeatability are vital to ensure process efficiency, and using high-performance motor encoders can help deliver this. In Figure 2, motors drive each joint in a robotic arm via precision speed-reducing gearboxes. The joint angles are measured using a precision motor-mounted shaft angle encoder (m) and sometimes also using an additional arm-mounted encoder (j). 

Figure 1 Angular repeatability at the motor encoder (m) and joint encoder (j)  with the robot reach (L)

Here, repeatability is the most critical datasheet performance metric, typically specified at the sub-millimeter level. Knowing repeatability and robot reach enables the calculation of rotary encoder specifications using the following formula:

Since multiple joints contribute to the overall reach in a robotic system, the chosen encoder must exhibit performance that exceeds the target angular accuracy. The repeatability specification per joint should scale by a factor of 10 (typically), while the gearbox ratio (G) determines the repeatability of the motor encoder. For the example robotic system described in Table 1, 20 to 22-bit repeatability specifications are required for the joint encoder, while 14-to-16-bit resolutions are necessary for the motor encoder.

Table 1 Specifying encoders for a pick-and-place robot

The following blog in this series will discuss the differences between absolute and incremental encoders. In the meantime, visit here to learn more about Analog Devices’ industrial robotics solutions.