What is Resolver in Electric Motor & How it Works?

AI-Generated Motor Resolver to determine the position of the rotor.

                             

The resolver is an electromechanical transducer or a rotary transformer that determines a motor's rotor position. The resolver is sometimes called as a Control Transmitter or Analog Trigonometric Function Generator.

The resolvers were initially developed for military applications to withstand harsh environments. Wherever we need to find the angular position of a rotary shaft we can use resolvers to measure it accurately and reliably.

Encoder:

Don't confuse a Resolver with an Encoder, Both are used to measure the speed and angular position of the rotating shaft, the main difference is the resolver is a mechanical device & the encoder is an electrically operated device and the output is analog in nature for resolver and digital in the encoder.

Let's dive deeper into how a resolver works.

Core Principle: Electromagnetic Induction

At its heart, a resolver functions based on the principle of electromagnetic induction, just like a transformer. This means that a changing magnetic field can induce an electrical current in a nearby conductor.

Think of it like a very precise dial:

• Imagine a dial that can tell you exactly how far it has turned. A resolver does something similar, but instead of a physical dial, it uses electrical signals.
• It has two main parts: a rotor (the part that turns) and a stator (the stationary part).
• These parts have wire coils that create magnetic fields. As the rotor turns, these magnetic fields change.
• These changes in the magnetic fields produce electrical signals that can be used to precisely determine the angle of the rotor.

Components:

1. Rotor: The rotating part of the resolver, typically with a single winding (coil of wire) called the excitation winding or primary winding.
2. Stator: The stationary part, usually with two windings placed at a 90-degree angle to each other. These are called the sine and cosine windings or secondary windings.

Working Mechanism:

R - Rotor (Excitation Winding)

S1 - Stator Winding

S2- Stator Winding

Note: The resolver's output is two sine and cosine waves that have a 90 electrical degree phase difference with respect to the reference voltage given in the Primary Excitation Winding.

Excitation: An AC (alternating current) voltage is applied to the rotor's excitation winding. This creates a magnetic field within the resolver.

From where the voltage is supplied from:

• Resolver-to-Digital Converter (RDC): An RDC is a specialized integrated circuit (IC) designed specifically to work with resolvers. It does several key things:

• Generates the AC excitation signal: The RDC produces the precise AC voltage at the correct frequency and amplitude needed for the resolver's reference winding.
• Processes the resolver's output signals: The RDC takes the sine and cosine signals from the resolver and converts them into a digital representation of the angle.

A simple electronic oscillator circuit can generate an AC signal. However, this is less precise and flexible than an RDC.

RDCs are designed to provide a very stable and accurate AC signal, which is crucial for precise angle measurement.

Magnetic Field Interaction: As the rotor turns, its magnetic field interacts with the stator windings. The strength of this interaction changes depending on the rotor's angular position.

Induced Voltages: This interaction induces AC voltages in the stator windings. The amplitudes (strengths) of these voltages vary sinusoidally with the rotor's angle.

• The voltage induced in one stator winding is proportional to the sine of the rotor angle.
• The voltage induced in the other stator winding is proportional to the cosine of the rotor angle.

Mathematical Representation:

If the excitation voltage applied to the rotor is:

• V_excitation = V_peak * sin(ωt) (where ω is the frequency of the AC signal)

Then the voltages induced in the stator windings are:

• V_sine = K * V_peak * sin(ωt) * sin(θ)
• V_cosine = K * V_peak * sin(ωt) * cos(θ)

Where:

• K is a constant related to the resolver's construction.
• θ is the angle of the rotor.

Determining the Angle:

By measuring the amplitudes of the sine and cosine voltages from the stator, the angle θ can be precisely calculated using trigonometric relationships.

Still confused about resolver, here is an analogy:

Imagine a flashlight inside a tube.

• The tube: This is like the body of the resolver.
• The flashlight: This is like the "reference winding" in the resolver. It sends out light (or in the resolver's case, an electrical signal).
• Two light sensors on the tube's wall: These are like the "sine and cosine windings." They measure how much light hits them.

Now, imagine you can rotate the flashlight inside the tube.

• When the flashlight points directly at one sensor, that sensor gets the most light. The other sensor gets very little.
• As you rotate the flashlight, the amount of light hitting each sensor changes. Sometimes one gets more, sometimes the other.

This is similar to how a resolver works:

• The "flashlight" (reference winding) sends out an electrical signal.
• As the resolver's shaft rotates, it changes how much of that signal reaches the two "sensors" (sine and cosine windings).
• The amount of signal each sensor receives follows a sine wave and a cosine wave pattern as the shaft turns.

Hope you get an idea of how a resolver works inside an edrive or motor.

Why are resolvers used?

Resolvers are popular in applications where accuracy and reliability are crucial, such as:

• Robotics: To precisely control the movement of robotic arms.
• Aerospace: In aircraft control systems and navigation.
• Industrial automation: In machines that require precise positioning.
• Electric vehicles: To accurately control the motor and ensure smooth operation.

Advantages of resolvers:

• High accuracy: They can provide very precise measurements of rotation.
• Robustness: They can withstand harsh environments, including high temperatures, vibration, and shock.
• Reliability: They have a long lifespan and require minimal maintenance.

In essence, a resolver is a robust and accurate sensor that provides precise information about the rotational position, making it essential in various demanding applications.