By Richard Anslow and Michael Jackson
It has been estimated that electric motors represent approximately 70% of industrial electricity consumption*, making them an obvious target for sustainability improvements and encoders will have a pivotal role in delivering this. This blog series will answer some of the most common questions about motor encoders, covering encoder technologies (including relative advantages and disadvantages), performance metrics and specifications for specific applications, signal-chain electronics, and motor control design challenges as well as examining some emerging trends. This first blog looks at how the changing nature of motion control applications is driving the need for more high-performance motor encoders and compares the benefits of different types of encoder technologies.
Open Loop versus Closed Loop Motor Control
There has been a steady but significant transformation of industrial rotating equipment in recent decades, moving away from grid-connected motors towards inverter-driven motors. This has resulted in considerable process cost and energy savings as motor-driven end equipment became more efficient. Open loop motor speed control can be implemented by applying a variable frequency voltage to the motor (through pulse width modulation at the inverter). This approach works reasonably well in a steady state or in slowly varying dynamic conditions, and many motor drives in lower-performance applications use open loop speed control. However, this approach also has several disadvantages:
Motor performance and efficiency can be significantly improved by combining a power inverter with high-performance motor speed/position sensing and current/voltage closed-loop feedback for the power stage (Figure 1). Implementing this approach requires a motor encoder but in return, it delivers higher-quality motor control performance and synchronization in even the most demanding applications.
Figure 1 Closed-loop inverter-powered motor control feedback system
What does an Encoder do?
An encoder tracks the speed and position of a rotating shaft - information that can be provided in a closed feedback loop. In general-purpose servo drives, an encoder measures shaft position, which can also infer speed. An encoder gives the precise and repeatable shaft position information required by robotics and discrete control systems.
What are the main Encoder Technologies?
Encoders with either optical or magnetic transducers are among the most widely used, but capacitive encoders are also available.
An optical encoder (Figure 2) typically consists of an opaque glass (or plastic) disk with fine transparent lithography slots and is attached to a rotating shaft. When the disk rotates, light from an LED is either blocked or allowed to pass through a slot as it passes by. A photodiode detects this light and converts it into an electrical signal which is amplified and digitized, and then sent to a motor controller over wired cabling. Optical encoders offer high resolution and accuracy but are susceptible to dust and dirt, which can obstruct the light source. In addition, their glass or plastic construction makes them vulnerable to strong vibrations and high temperatures.
Figure 2 Motor encoder using an optical transducer
A magnetic encoder (Figure 3) consists of a spinning metallic disk with alternating magnetic poles and a Hall-effect or magneto-resistive sensor. It operates by detecting changes in magnetic flux fields as the disk rotates. The magnetic field sensor provides sine and cosine analog output signals, which are amplified and digitized before being sent to a controller. Compared to an optical encoder, a magnetic encoder offers rugged performance in the presence of contaminants (like oil and dirt) and is less susceptible to shock and vibration. However, magnetic encoders are sensitive to magnetic interference caused by electric motors and have a limited operating temperature range. While magnetic encoder performance has recently improved, they also typically deliver lower resolution and accuracy than their optical counterparts.
Figure 3 Motor encoder using a magnetic transducer
The following blog in this series will look at encoder performance metrics and specifications and how to decide which are the most critical in a specific application.
* World Energy Outlook 2019. International Energy Agency, 2019.
