Can You Resolve an Encoder from a Resolver?

By Richard Anslow and Michael Jackson

The previous blog in this series showed how the resolution of an optical encoder can be increased by modifying the components in the signal chain instead of changing the disk. This final blog addresses a common cause of confusion for many engineers - the difference between an encoder and a resolver (Figure 1) and their use in motor control applications. It shows how components from Analog Devices can be used to quickly implement the signal chain for a resolver for use in industrial applications.

Figure 1 Internal components in a resolver

What is a Resolver?

Resolvers are electromechanical sensors operating as variable coupling transformers that convert a rotating shaft's precise angular position and/or velocity to electrical signals. The signals are proportional to the sine and/or cosine of the shaft angle. The amount of magnetic coupling between the primary winding and two secondary windings varies according to the rotating element's position (rotor), which is typically mounted on the motor shaft. They are used in industrial motor controls, servos, robotics, power-train units in hybrid- and full-electric vehicles, and many other applications that require precise shaft rotation. Standard resolvers have a primary winding on the rotor and two secondary windings on the stator. Variable reluctance resolvers, on the other hand, have no windings on the rotor. Their primary and secondary windings are all on the stator, but the rotor's saliency (exposed poles) couples the sinusoidal variation in the secondary with the angular position. Figure 2 shows classical and variable reluctance resolvers. 

Figure 2 Classical and variable reluctance resolvers

The sine and cosine of the shaft angle modulate the two output signals from a resolver. A graphical representation of the excitation signal and the sine and cosine output signals is shown in Figure 3. The sine signal has a maximum amplitude at 90° and 270° and the cosine signal has a maximum amplitude at 0° and 180°.

Figure 3 Resolver output signals are analog

 How do Encoders and Resolvers Differ?

The differences can be explained under the following headings:

• Accuracy: An encoder's accuracy depends on the disk used, whereas the accuracy of a resolver largely depends on the number of its poles (on sine/cosine secondary windings). The greater the number of poles re-located on its stator, the higher the accuracy. In general, resolvers are not as accurate as encoders.

• Reliability: Encoders (especially optical) are susceptible to interference from dirt and moisture, which can negatively impact their accuracy. Magnetic encoders are more robust, but generally, encoders cannot be used in applications where temperatures exceed 120 °. Mechanical shocks and vibration also affect their performance. In contrast, resolvers are less sensitive to environmental factors, making them more reliable even in extremely harsh operating conditions.

• Cost: Encoders are more challenging to manufacture, which makes them more expensive than resolvers, which have a more straightforward design, allowing them to be produced more cheaply and making them easier to repair and maintain.

• System Integration: Encoders generate a quadrature digital signal, which can be connected directly to a microcontroller. Resolvers generate analog signals which require a resolver-to-digital converter to connect to a controller.

• Range of Offerings: There are many types of encoder transducer technologies (optical, magnetic, capacitive) that can provide different types of information about the position of a motor shaft (absolute or relative). There are fewer types of resolvers, and they are categorized by the number of poles (two-pole or multiple types) and their operating principle - receiver and differential.

What are the Critical Design Parameters for a Resolver?

A resolver sensor has a unique set of parameters that should be considered during the design phase. The most critical electrical parameters and the respective typical specifications are summarized in Table 1.

Table 1 Resolver design parameters

Practical Resolver Signal-Chain

Figure 4 shows the block diagram of a resolver signal chain. The AD2S1200 resolver to digital converter takes the signal from the resolver and converts it to an angle/angular rate represented in a digital format. Two amplifiers are used to create a third-order Butterworth low-pass filter to pass the resolver signals to the AD2S1200. The LTC4332 SPI extender is recommended to save space and reduce design complexity. The LTC4332 enables system partitioning, providing the option of placing the microcontroller at the servo, rather than at the encoder. If a microcontroller is required at the encoder, the MAX32672 SPI interface directly links to the AD2S1200 and the ADM3065E RS-485 transceiver can be used instead of the LTC4332. If the LTC4332 is used, the AD2S1200 SPI output is converted to a robust differential fieldbus interface. The LTC4332 includes three slave select lines, so additional sensors, such as MEMS and temperature sensors, can be wired on the same bus as the AD2S1200.

Click here to learn how a high-accuracy resolver simulator can evaluate the resolver performance without setting up a complicated motor control system.

Figure 4 Resolver signal chain and recommended components from Analog Devices

Summary

This blog series addressed the role of encoders in motor control applications, exploring them under various headings, including transducer types, applications, and emerging trends. Whatever your motor control requirements, Analog Devices will likely have a solution that meets your needs. Click here to learn more.