What Is a Resolver?

Precision and reliability remain top priorities in industrial manufacturing, and servo motors can help factory managers achieve these goals. Servo motors offer precise control capabilities over motion, speed, torque and position in various processes, ensuring seamless operations. However, when these motors encounter issues, they can disrupt production schedules and ultimately impact a factory’s bottom line.

That’s why understanding servo motor technology and components is crucial, and one of these components is the resolver. What is a servo motor resolver? Learn the inner workings of these components and how they can impact production processes below. 

What Is a Motor Resolver?

A motor resolver is an analog rotary electromagnetic transducer. The resolver generates voltage signals to help measure the angular position of a rotating machine shaft. In other words, it’s a robust measuring instrument.

A motor resolver’s design has three major components mounted on the motor shaft: 

1. Rotor: The rotating part of the resolver that houses the primary winding. 
2. Stator: The stationary part of the resolver that houses the secondary windings. 
3. Windings: The coils of wire found on the rotor and stator. 

A motor resolver can be single- or multi-speed. A single-speed resolver’s windings create one complete Sine and Cosine curve in one mechanical revolution. However, a multi-speed resolver creates multiple Sine and Cosine curves in one mechanical revolution. A single-speed resolver can provide absolute feedback, whereas a multi-speed resolver cannot. That said, a multi-speed resolver can provide higher accuracy and smoother motion.

How Does a Motor Resolver Work?

The motor resolver’s inner workings involve applying an AC voltage to its primary winding in the rotor. The primary winding induces voltage to two secondary windings — Sine and Cosine. These windings are orientated orthogonally (90 degrees apart), which allows the resolver to resolve the angular position into Sine and Cosine components. This process is analogous to a vector’s X and Y components in mathematics.

The resolver’s output voltages vary sinusoidally with the shaft’s rotational angle. At the Electrical Zero (EZ) position, the Sine winding outputs its maximum voltage while the Cosine winding outputs its minimum voltage — and vice versa. The rotor’s angle can be determined using the arctangent function of the ratio of Sine voltage to Cosine voltage. This method inherently reduces noise and provides temperature composition due to its ratio metric format.

These parameters can affect resolver performance:

• Accuracy
• Maximum input current 
• Input excitation voltage and frequency
• Transformation ratio from input to output voltage 
• Phase shift between input and output voltages 
• Null voltage characteristics 

Resolvers vs. Encoders

Although resolvers and encoders are used in servo motors, they have key differences. Here’s a brief comparison: 

• Principle of operation: Encoders use light or magnetic sensors and generate digital signals (binary or Gray code). Resolvers operate as transformers, inducing analog signals (Sine and Cosine). 
• Parts: Encoders use electronic parts, and resolvers use no electronic parts. 
• Input: An encoder requires a DC input, whereas a resolver requires an AC input. 
• Output: Encoders output phases A, B, and Z impulses. Resolvers output Sine and Cosine signals. 
• Cost: Encoders are generally more cost-effective, making them ideal for machines and lifts. Resolvers may be more expensive, but their reliability makes them ideal for jet engine fuel systems and even radiated environments. 
• Environment suitability: An encoder can be sensitive to dust, moisture and vibrations. In comparison, a resolver can withstand harsh conditions such as temperature extremes and electromagnetic interference (EMI). 

Resolver Applications

The resolver’s simple yet robust design makes it a reliable option for extreme applications in harsh environments, such as:

• Servo motor feedback 
• Speed and position feedback for steel and paper mills
• Oil and gas production 
• Jet engine fuel systems 
• Communication position systems 
• Control systems for military vehicles
• Aircraft flight surface actuators

Advantages and Challenges of Resolvers

Resolvers can come with the following benefits: 

• Robustness: Resolvers are highly durable. They can operate in adverse environments exposed to high temperatures, vibrations and EMI. 
• Accuracy: They can provide accurate angular position feedback, made possible by their inherent analog sinusoidal output. 
• Noise immunity: The ratio metric output inherently cancels out common-mode noise. Thus, it may ensure a stable performance even in electrically noisy spaces. 
• Temperature stability: Resolvers offer stable performance levels across a wide temperature range. As such, they’re suitable for industrial automation and aerospace applications. 

However, they do face some challenges, including: 

• Signal conversion: Their analog output may require additional components — such as resolver-to-digital converters (RDC) — to integrate into digital control systems. This kind of integration adds a layer of complexity. 
• Cost: At lower resolutions, resolvers can be more expensive than digital encoders. Additionally, if an RDC is required, it can add to the price tag.
• Maintenance: While robust, resolvers may require periodic calibration and maintenance for optimal performance and feedback accuracy. This may be the case in demanding industrial environments.

How to Troubleshoot a Resolver

Troubleshooting a resolver requires a structured approach to identify and address issues:

1. Visual Inspection

Start by visually inspecting the resolver. Check for physical damage, loose connections, signs of wear and tear or cable deterioration. Ensure all its connections are secure and properly seated.

2. Input Excitation and Verification

Measure the AC voltage input to the resolver. Ensure the results match the specified excitation voltage and frequency provided by the servo drive or controller. 

3. Output Signal Check 

Use an oscilloscope or a resolver signal analyzer to check the output signals. Verify that they’re present within the expected voltage ranges. Also, confirm that they exhibit the correct phase relationship relative to the excitation signal.

4. Zero Angle Adjustment 

If possible, adjust the resolver’s zero angle. Adjusting it can ensure the resolver accurately represents the zero position of the motor shaft. 

5. RDC Check

If applicable, check the RDC for proper operation. It must receive and process the resolver’s analog signals correctly. 

6. Grounding and Shielding

Make sure there’s proper cable grounding and shielding to prevent noise interference. Additionally, ensure cables are routed away from EMI sources.

If your resolver issues persist, you’ll want to contact a professional repair service that can diagnose the issue and apply the correct fixes.

Learn About Our Servo Motor Repair Services Today

At Global Electronic Services, we specialize in repairing servo motors to keep your production lines running. Our expert technicians are equipped to handle all makes and models, offering quick turnaround times and cost-effective solutions.

Your company can rely on us for expert support and guaranteed quality, helping you minimize downtime and get your equipment back in working order. Contact us to request a quote, or call us at 877-249-1701 and learn how we can support your servo motor repair needs.