Bench Talk

Optical, Magnetic, Resolver, or Capacitive for Your Motor Encoder?

(Source: Vera Aksionava/stockabobe.com)

In many motion control designs, it’s essential to know the key parameters of the motor’s rotor, such as its angular position, speed, and direction. One component that is vital to understanding these parameters is an encoder, which is an add-on sensor that determines and reports rotor angular position to the system controller (all analog, hybrid analog/digital, or entirely digital). The encoder takes repeated position “snapshots” at defined time intervals. Using this series of position information readings, the controller can determine rotational velocity and direction. (Note that the term “encoder” also refers to many other electronic circuits and components, so there's potential for confusion.)

This blog reviews how the main encoder technologies function and the considerations users must take into account when choosing one for motor control.

Encoders at a Glance

Not all motor applications need this sensor-provided feedback and the associated rotor parameters. Low-end products such as toys do not need—and cannot afford—an encoder. At the other extreme, advanced motor control algorithms that use field-oriented control (FOC)—also called vector control—don’t require it. However, even the FOC arrangement may want to include the encoder to monitor and verify the presumed position and motion of the rotor in critical applications.

Four types of encoder technologies are widely used to determine rotor position: optical, magnetic, electromagnetic induction (resolver), and capacitive. Each has relative advantages and disadvantages, so the “right” choice is determined by the application’s requirements, priorities, and cost.

Some encoder implementations can provide absolute rotor-position information, which is needed in some cases on power-up or restart, while others can provide only relative position information; however, this may be all that is required for the end application.

The desired encoder is selected by the motor customer assembling it into a final product and is usually added to the motor during production at the customer’s site. Many motor vendors also offer encoders of their own or from third parties and will incorporate them during the motor production stage at their factory as well. With diverse choices available, let’s look closer at the four widely used encoders.

Optical Encoders

Optical encoders use an LED as a light source, a code wheel connected to the shaft, and a photo sensor, supported by a power circuit to drive the LED and an output circuit to produce clean pulses from the sensor.

Optical encoders come in two forms. In the transmissive design, the LED and sensor are on opposite sides of the code disk; in the reflective design, they are on the same side. The reflective design is thinner but has more critical alignment and potential for reduced sign-to-noise ratio (SNR) compared to the transmissive design.

The code wheel is the critical element here. It is a thin glass or plastic disk that is patterned with opaque lines where light cannot pass and clear areas where it does pass. For reflective design, the opaque areas are instead reflective. As the code wheel turns, the light sensor produces a serial train of pulses corresponding to the wheel’s motion. The resolution of the code wheel can be as high as 1,024 pulses per revolution (PPR), or even 4,096 PPR.

The basic optical encoder is an incremental sensor that only shows relative motion. The encoder system can determine direction using the relative phase difference between track outputs by adding a second optical track offset by 90° (quadrature) from the first track, along with another sensor.

Two approaches are used to obtain absolute position, which is needed in many applications. In one approach, a third code-wheel track is added with a single index marker, which registers once per revolution to set a starting point. The other approach is used in more stringent cases where absolute position is needed on power-up, so the encoder disk has several tracks with different patterns to produce a unique code output for each encoded position.

Optical encoders are very popular due to their simple design, ease of interfacing, and high resolution. Some trade-offs to consider is that they are somewhat fragile due to the disk, can be affected by ambient dust and dirt, and require a modest amount of operating power.

Magnetic Encoders

There are several types of magnetic-based encoders. In one widely used version, a permanent magnet is attached to the tip of the rotor shaft, and magnetic sensors are mounted nearby so they can sense the field of that magnet. When the permanent magnet attached to the motor shaft rotates, the direction of the magnetic field, as detected by the magnetic sensors, also changes.

Sensing the relative strength of the field allows the encoder to detect the rotational position and speed of the motor shaft. It does this by using two Hall effect sensors, which are oriented orthogonal to each other, using one to sense the X-axis component (Bx) magnitude and the other for detecting the strength of the Y-axis component (By).

Then, using standard trigonometric identities, the two outputs can be demodulated to determine the shaft angle. This can be done using an analog circuit or computed numerically with digitized values of both its X- and Y-components.

These magnetic encoders can provide resolution as high as 4,000 PPR and are rugged enough for use in dust, oil, and even moisture applications. However, it is affected by strong nearby magnetic fields, such as those from the motor, monitoring, or nearby wires. 

Another magnetic encoder, which is not as widely used, employs a code wheel with alternating north and south pole magnets on the outer edge of the code wheel. The magnetic sensor detects changes in magnetic polarity when the poles pass by. This design has a lower resolution than the previous approach but may be adequate for some situations. Since there’s no need to power the light emitter and receiver, the magnetic encoder uses less power than an optical encoder.

Resolvers

A very different type of encoder based on magnetic principles is the resolver. This rotary transformer determines the angle and displacement speed of its rotor using an arrangement similar to a small synchronous motor.

An AC signal, typically around 10kHz, is applied to the rotor and functions as the primary-side winding. The stator has two secondary windings placed at 90° to each other, referred to as sine and cosine windings. As the rotor turns, it induces relative changes and differences in the signals between the sine and cosine secondary-side windings. These can be decoded via demodulation to provide absolute position, again using trigonometric identities.

It should be noted that despite their size, weight, and interface-circuitry requirements, resolvers were once widely used in extreme-stress applications, such as missile guidance-system inertial platforms, due to their accuracy and ruggedness. At the time, they were the only angular-position encoder option despite being larger, more costly, and consuming more power than most newer options.

In general, resolvers produce sinusoidal analog signals while encoders provide digital on/off outputs, although there is some overlap. As a result, each requires a different decoding scheme to produce the desired motor information. The encoder’s output is more compatible with modern electronics and processors and is easier to use “as is” in a motor control system.

Capacitive Encoders

The capacitive encoder technique implements a newer rotary-motion sensing technology, although it has been used for years in digital high-accuracy, low-cost calipers. In this arrangement, there is a rotor imprinted with a conductive sinusoidal pattern around its perimeter, a stationary transmitter, and a stationary receiver. As it rotates, the high-frequency reference signal of the transmitter is modulated in a predictable way. The encoder detects the changes in capacitance-reactance on the receiver board and translates these changes into increments of rotary motion, again using a demodulation algorithm.

Since there is no LED, capacitive encoders have a longer lifetime, smaller footprint, and lower current consumption (typically just 6mA to 18mA) than an optical encoder. Additionally, these encoders are also fairly immune to magnetic interference and electrical noise. Still, airborne contaminants can affect the apparent capacitive coupling and thus impact performance consistency.

Choosing the “Best” Encoder

High-performance encoders are available to meet the diverse needs of varied applications. Given the many encoder options, determining which is the presumed right or best one for a given application is difficult. The decision includes factors such as resolution, interfaces, power requirements, physical ruggedness, EMI considerations, size, and cost.

Vendors of specific encoder types naturally maintain that theirs is the “best,” but the reality is more nuanced. For every general statement about encoders (“this type is good for this parameter” or “this type is perhaps not-so-good on this other parameter"), there are many valid exceptions to any general guidelines.

Although a comparative table of the relative attributes of each encoder type would seem to make sense, it would also need many explanatory footnotes to clarify exceptions. The reality is that making an encoder decision requires careful examination of what’s available, balanced against the design's objectives, the project’s priorities, and the trade-offs that must be made when selecting an encoder type.