How to improve the performance of inverter and motor drive systems?

Moving from fixed speed motors to variable speed motors with position and current feedback offers a path to significant process and energy savings. This article provides an overview of motor encoders (position and speed), including types and technologies, as well as application use cases. It also answers key questions, such as what encoder performance metrics are most important for my system. Major future trends in electronics used in encoder applications will be discussed, including machine health monitoring, smart and robust long-life sensing. Finally, we will explain why a complete signal chain design is critical to designing the next generation of motor encoders.

Motor Encoder Performance Specifications, Trends and Electronics

After reading this article, you should be able to answer the following key questions:

What is an encoder and how can it improve the performance of my inverter and motor drive system?

What encoder performance metrics are most important to my system? After reading this article, you will understand how to match encoder resolution, accuracy, and repeatability specifications to motor and robotic systems.

What are the common electronic components used in encoders and what are the future trends? After reading this article, you will understand how machine health monitoring, edge intelligence, robust sensing and high-speed connectivity will support future encoder designs.

Closed-loop motor control feedback system

Over the past few decades, there has been a steady progression from traditional grid-tied motors to inverter-driven motors. This has been and continues to be a major shift in industrial rotating equipment, enabling huge process and energy savings through more efficient use of motors and end equipment. Higher performance motor control for variable speed drives and servo drive systems provides improved quality and synchronization for the most demanding applications. As shown in Figure 1, motor performance and efficiency are improved through the use of power inverters, high performance position sensing, and closed-loop feedback of current/voltage at the power stage.

Open-loop speed control of the motor is possible by applying a variable frequency voltage to the motor using pulse width modulation in the inverter. This approach works quite well under steady-state or slowly changing dynamic conditions, and many motor drives in low-performance applications use open-loop speed control without the need for an encoder. However, this approach has several disadvantages:

Speed ​​accuracy is limited due to lack of feedback

The motor efficiency is low due to the non-optimal current control

The transient response must be tightly limited so that the motor does not lose synchronization

Figure 1. Closed-loop motor control feedback system.

What is a position encoder?

Encoders provide closed-loop feedback signals by tracking the speed and position of a rotating shaft. Optical and magnetic encoders are the most widely used technologies, as shown in Figure 2. In general-purpose servo drives, encoders are used to measure the position of the shaft and, from this, the speed of the drive. In robotics and discrete control systems, accurate and repeatable shaft position is required. Optical encoders consist of a glass disk with fine photoetched grooves. A photodiode sensor detects changes in light as it passes through or is reflected by the disk. The analog output of the photodiode is amplified and digitized, then sent to the inverter controller via a wired cable. Magnetic encoders consist of a magnet mounted on the motor shaft, and a magnetic field sensor provides sine and cosine analog outputs that are amplified and digitized. The optical and magnetic sensor signal chains are similar, as shown in Figure 2.

Motor encoder types, technologies and performance specifications

After power is applied, an absolute single-turn encoder returns the absolute position within the mechanical or electrical 360°. The position of the motor shaft can be read immediately. An absolute multi-turn encoder includes absolute functionality and counts the number of 360 revolutions. In contrast, an incremental encoder provides the position relative to the starting point of the rotation. An incremental encoder provides an index pulse indicating 0 and a single pulse that counts the number of revolutions or a double pulse that gives direction information.

Figure 2. (a) Optical encoder and (b) magnetic encoder.

The resolution of an encoder is the number of positions that can be distinguished per 360° rotation of the motor shaft. Generally speaking, the highest resolution encoders use optical technology, while medium resolution/high resolution encoders use magnetic or optical sensors. Resolvers (rotary transformers) or Hall sensors are used for low and medium resolution encoders. Optical or magnetic encoders use high resolution signal conditioning. Most optical encoders are incremental. Encoder repeatability is a key performance indicator and a measure of how consistently the encoder returns to the same commanded position. This is critical for repetitive tasks, such as robots or pick-and-place machines used for semiconductor placement during PCB manufacturing.

Figure 3. Encoder types.

Table 1. Key performance indicators of encoders

The Importance of Motor Encoder Accuracy and Repeatability

Pick and place machines/robots are popular automated machines used in food packaging and semiconductor manufacturing industries. To improve processing efficiency, machines or robots with high accuracy and repeatability are required. Using high-performance motor encoders can achieve accuracy, repeatability, and efficiency.

Figure 4 shows an encoder use case in robotics. A motor drives each joint of the robotic arm through a precision reduction gearbox. The robot joint angles are measured via precision shaft encoders (θm) mounted on the motor and usually an additional arm-mounted encoder (θj).

For robots, the main performance specification listed in the data sheet is repeatability, which is usually on the order of sub-millimeter. By knowing the repeatability specification and the reach of the robot, you can extrapolate back to the rotary encoder specifications.

Figure 4. Angular repeatability at the motor encoder (θm) and joint encoder (θj), and the reach (L) of the robot.

The required angular repeatability (θ) of the joint encoders can be derived from trigonometry: the inverse tan of the robot’s repeatability divided by its reach.

Multiple joints are combined to achieve the full range of the robot. The sensor should have better performance than the target angle accuracy. The repeatability specification of each joint must be improved, assuming a 10-fold improvement here. For the motor encoder, the repeatability is defined by the gearbox ratio (G).

For example, in the robotic system shown in Table 2, the joint encoders require a 20-bit to 22-bit repeatability specification, while the motor encoders require a 14-bit to 16-bit resolution.

Table 2. Encoder repeatability and robot repeatability specifications

Future Trends in Motor Encoder Technology

Figure 5 describes future encoder trends and the technologies that support them.

Figure 5. Encoder trends and the technologies that support them.

A study by Rockwell1 on servo drives, encoders, and encoder communication ports showed that transceivers used for feedback communication are growing at 20% per year. Single-pair Ethernet (SPE) transceivers (IEEE 802.3dg standard 100BASE-T1L) [1] that support two-wire 100 Mbps communication are currently under research, and future encoder-drive interfaces will benefit from low latency, with a target of ≤1.5 s. This low latency will enable faster feedback data acquisition and faster control loop response times.

Condition-based monitoring (CbM) of robots and rotating machines such as turbines, fans, pumps, and motors records real-time data related to machine health and performance to enable targeted predictive maintenance and optimized control. Targeted predictive maintenance early in the machine life cycle can reduce the risk of production downtime, thereby improving reliability, significant cost savings, and increasing factory productivity. Use MEMS accelerometers placed in encoders to provide vibration feedback to machines where quality control is critical. It is very convenient to add MEMS accelerometers to encoders because the encoders already have wiring, communications, and power to provide vibration feedback to the controller. In some applications, such as CNC machine tools, the MEMS vibration data sent from the encoder to the servo system can be used to optimize system performance in real time.

Extending the life of industrial assets using CbM can be complemented by durable, long-life position sensors. Magnetic sensors produce an analog output that indicates the angular position of the surrounding magnetic field and can be used in place of optical encoders. Magnetic encoders can be used in areas with high humidity and high levels of dust. These harsh environments compromise the performance and life of optical solutions.

For robotics and other applications, the position of a mechanical system must always be known, even in the event of a power outage. One of the major costs and inefficiencies associated with standard robots, cobots, and other automated assembly equipment is the downtime required to reconnect and initialize power after a sudden power outage during operation. The magnetic multi-turn memory developed by Analog Devices [2] records the number of turns of an external magnetic field without the need for an external power source. This results in a reduction in system size and cost.

For robots and collaborative robots, motor encoders and joint encoders typically require 16-bit to 18-bit ADC performance, and in some cases 22-bit ADCs are required. Some optical absolute position encoders also require high-performance ADCs with up to 24-bit resolution.