Compact, light, cost effective; how resonant inductive sensing compares to resolvers.

CambridgeIC’s team has led the development of resonant inductive sensing solutions for more than 20 years, producing solutions for a wide range of industrial applications.

Compared to a resolver, this approach allows for a more compact, light and cost effective solution, ideal for integrating into designs. This short white paper summarises the similarities and differences between the two approaches.

Shared features of resonant inductive and resolvers

• Both determine position by measuring inductive coupling factors between a moving target/rotor and fixed sensor/stator.
• Both have coupling factors vary sinusoidally with angle and at least 2 phases of sensor coils and include an atan2(cos_coupling, sin_coupling) function for determining position.
• Both are robust and suited to a harsh engine environment

Differences

Fig1. Resolver “motor-like” wound construction (left) and resonant inductive coils build on a PCB (right)

• Resonant inductive sensors are usually built from a PCB while resolvers are wound components constructed like a motor
• Resonant inductive sensors use a resonant target with relatively high Q factor, so that the target can be excited by current in the sensor separately to receiving signals back from it (“pulse echo”).  This means there is no chance of the “breakthrough” that resolvers suffer from, which is an important cause of linearity errors, especially when misaligned.  See Fig 2 and also https://www.cambridgeic.com/technology/interrogation. 

Fig 2: Sequence of events in a pulse echo cycle for Resonant Inductive System

• Resonant inductive sensors operate at a higher frequency, while the steel in a traditional resolver prevents operation at high frequencies.
• Resonant inductive sensors have a much lower impedance than resolvers due to having only a few coil turns.

Advantages of resonant inductive sensors

• Fundamentally lower cost sensor and target
• Sensor and target can use standard PCB fabrication/assembly from any convenient manufacturer, and are not tied to a specialist supplier
• Lower linearity error due to greater scope for design optimisation, including when sensor and target are misaligned form the rotation axis.
• Lower linearity error due to “pulse echo” sensor interrogation, so that it is easier to suppress once per revolution position errors.
• Fewer analog components external to processor.  This is because passive (RC) filters can be used in place of active (op-amp) filters, because the sensor coils have a low impedance.
• Sensor can include “fine” and “coarse” sensor coil pairs for an output that is full absolute across 360° of rotation (from “coarse” coils with one pole pair per revolution) while still achieving high accuracy and resolution (from multi-pole “fine” sensor coils).  In some applications absolute 360° is a requirement.  However even if it isn’t, it can help indirectly, for example by allowing the customer to infer how error changes across a full 360° and hence correct for that error and deliver greater drive smoothness.
• Fundamentally lower group delay between actual angle and angle reported to motor controller, due to higher operating frequency, so that velocity and position control loops can have higher bandwidth and/or margins and perform better.
• Resonant inductive sensors and targets have more design flexibility, allowing a tighter and more optimised integration with the customer’s machine.

Disadvantages of resonant inductive sensors

• A faster ADC rate is required to digitise the incoming sensor signals
• It takes more digital signal processing to process sensor signals because the detected frequency is a variable (equal to the target’s resonator frequency) and not fixed (the same as the excitation frequency)
• They use a higher excitation frequency, so the waveform edges used to excite the sensor need to be adequately slowed down to suppress electrostatic emissions
• They’re a recent technology (~ 10 years) and don’t have the deep history of resolvers (~70 years)

Find out more

This white paper is intended to provide background and guidance; for more detailed insights and assistance with questions on resonant inductive sensing and whether it is suitable for your application, get in touch with CambridgeIC’s engineers using our live chat or email links.