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In the world of modern automation, robotics, and electric motion systems, the ability to detect and control position, speed, and direction with accuracy is essential. Among the many devices designed for this task, one often-overlooked yet highly reliable technology stands out: the resolver.
Resolvers have been around for decades, long before digital encoders became widespread. Despite their age, they remain a critical component in industries where durability, reliability, and precision are non-negotiable. This article explores what a resolver is, how it works, its advantages, applications, and why it continues to be relevant in today's increasingly digital world.
A resolver is an electromagnetic transducer that measures the angular position of a rotating shaft with high reliability. In simple terms, it tells us the exact angle at which a shaft is positioned, and from this angle, engineers can also derive valuable information such as rotation speed and direction. Because of its ability to convert mechanical motion into electrical signals, a resolver plays an essential role in modern control systems.
Resolvers are often referred to as analog rotary transformers. Unlike digital encoders, which generate binary outputs, resolvers produce continuous analog signals that vary smoothly with the shaft's position. These signals can then be interpreted by electronic controllers or drive systems, making them indispensable in applications where precision and durability are required, such as aerospace, robotics, electric vehicles, and industrial automation.
In its most basic construction, a resolver consists of the following main components:
Stator Windings – The stationary part of the device, which is supplied with an excitation voltage (usually sinusoidal AC). This excitation creates a magnetic reference field.
Rotor Windings – The rotating element, mechanically attached to the shaft whose angle needs to be measured. As the rotor turns, its winding interacts with the magnetic field of the stator.
Electromagnetic Coupling – The interaction between the magnetic fields of the rotor and stator windings produces output signals. These signals vary continuously according to the angular position of the shaft.
Resolvers operate on the principle of electromagnetic induction, much like transformers. Here's a step-by-step explanation:
Excitation Signal
A reference AC voltage (usually sinusoidal) is applied to the stator's primary winding.
Induced Rotor Signals
As the rotor turns, its position alters the coupling between stator and rotor windings. This generates secondary voltages in the rotor proportional to the shaft angle.
Sine and Cosine Outputs
The resolver typically provides two key signals: a sine output and a cosine output. These are functions of the shaft angle relative to the reference signal.
Angle Calculation
By processing these sine and cosine signals, the control electronics can compute the exact angle of the shaft.
This system ensures that a resolver can deliver continuous, absolute position feedback without requiring a reference point reset, which is often necessary with incremental encoders.
Resolvers are not one-size-fits-all devices. They come in several types, each suited to specific applications:
Brushless Resolvers
The most common type, using electromagnetic induction without mechanical brushes.
Offers longer service life and lower maintenance requirements.
Brush-Type Resolvers
Utilize mechanical brushes for electrical connection.
Less common today due to wear and tear, but still used in certain legacy systems.
Single-Speed Resolvers
Provide one electrical cycle per mechanical revolution, ideal for basic position sensing.
Multi-Speed Resolvers
Deliver multiple cycles per revolution, enhancing accuracy and resolution.
Why do many industries still use resolvers, even with the availability of high-tech encoders? The answer lies in their unique strengths:
High Reliability
Resolvers are rugged, withstanding heat, shock, vibration, and harsh environments where delicate electronics may fail.
Absolute Position Feedback
They always provide an exact position, even after power loss. No need for recalibration or “homing” like some digital devices.
Wide Temperature Range
Operates effectively in extreme temperatures, often from –55°C to +150°C.
Electromagnetic Immunity
Resistant to electrical noise and interference, making them suitable for industrial and military environments.
Longevity
With fewer fragile electronic components, brushless resolvers can last decades with minimal maintenance.
High Accuracy
Especially multi-speed resolvers, which deliver fine resolution required for precision control.
Resolvers are widely used across industries that demand robust and precise position sensing. Some key applications include:
Aircraft control systems rely heavily on resolvers to measure the angle of control surfaces (rudders, ailerons, etc.) and for navigation equipment. Their reliability under extreme conditions makes them invaluable in this sector.
In robotic arms and industrial automation, resolvers help ensure accurate joint positioning and smooth motion control, essential for tasks like welding, assembly, and material handling.
Resolvers are often paired with servomotors and synchronous machines to provide position and speed feedback for closed-loop control systems.
They play a role in electric power steering systems, traction control, and hybrid/electric vehicle drive systems where durability and accuracy are crucial.
Locomotives and railway signaling equipment use resolvers for monitoring rotational components, ensuring safety and efficiency.
In wind turbines, resolvers measure the yaw and blade pitch angle to optimize energy generation while protecting equipment from high wind stresses.
At first glance, both resolvers and encoders serve similar functions: they provide position feedback. However, there are key differences:
Signal Type
Resolvers provide analog sine and cosine signals.
Encoders provide digital pulses or codes.
Durability
Resolvers excel in harsh environments.
Encoders, especially optical types, may fail under dust, vibration, or temperature extremes.
Accuracy
High-resolution encoders can outperform resolvers in ultra-precise applications.
Multi-speed resolvers, however, offer excellent accuracy for most industrial tasks.
Maintenance
Brushless resolvers are low-maintenance.
Encoders may require more frequent attention depending on the type.
In short, resolvers and encoders complement each other. Resolvers are favored where reliability and ruggedness matter most, while encoders dominate applications needing very high resolution in controlled environments.
Even in a world increasingly driven by digital technology, resolvers continue to play a pivotal role. Their robustness, long lifespan, and immunity to harsh conditions ensure they will not become obsolete anytime soon.
Advancements in signal processing and integration with modern drive electronics have further expanded their capabilities. Hybrid systems combining resolvers with digital electronics now offer the best of both worlds—rugged sensing with precise digital interpretation.
As industries move toward electrification, automation, and sustainable energy, the need for reliable position feedback devices like resolvers will only grow.
So, what is a resolver? It is a time-tested, highly reliable electromagnetic sensor that converts shaft rotation into precise angular data. Its ability to operate in extreme environments while delivering accurate and absolute feedback makes it a cornerstone of industries ranging from aerospace to robotics.
While encoders may dominate in some high-resolution applications, resolvers remain indispensable where ruggedness, dependability, and long-term accuracy are essential.
If your business is looking for high-quality resolvers or related motion control solutions, it is worth exploring suppliers with expertise in advanced motor technologies. One such company is NINGBO START MOTOR TECHNOLOGY Limited, which specializes in providing reliable and innovative motor-related products. Reaching out to them could help you find the right resolver solutions tailored to your specific needs.
