Inductive encoders for motors

Cost effective motor commutation and positioning
Reliable, robust to dirt, dust, condensation and magnetic fields
Flexible integration with motor
Short axial length
Through shaft without compromising on accuracy
Robust to mounting tolerances - no bearings, completely wear free
Wide temperature range, -40°C to 150°C
Scales to match motor poles and resolution requirements
Functional Safety, automotive ASIL D, similar to SIL3
Redundant configurations or mix technologies to meet demanding safety and reliability requirements

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Motor encoders and technology choice

Electric motors are fitted with position sensors, or encoders whenever the end application requires a high level of position control, such as a servo motor, or the motor type itself demands it. One of the most efficient and power dense motor types, permanent magnet synchronous motors (PMSM) requires the rotor position to be known. A motor drive can often find rotor position in a “sensorless” manner, but sensorless schemes struggle at low speed. An encoder is needed to get full torque at standstill and low speeds or just predictable, controlled motor movements at start-up. At any speed, an encoder enables a more responsive control loop.

Just as selecting the right motor type for your application can make a big difference, so it is for the encoder. Two of the key choices are the encoder’s measurement technology and whether it should have bearings or not. The most commonplace technologies today are optical, magnetic and resolvers. With inductive set to become much more widely available.

Inductive Encoders

Inductive position sensing technology is well established in automotive applications and a few other areas. However, patent coverage, the need for dedicated integrated circuits (IC) and sensor design expertise has created barriers to its wider availability, particularly for those looking to balance performance and cost - this is no longer the case.

Sensor methods make inductive sensing technology accessible to OEMs with development services that include specification support, IC selection, sensor coil design, sample preparation and end of line programming and testing support.

Development sevices

Inductive Encoders

Inductive encoder mounted inside the rear cover of a motor

Target for inductive encoder mounted on the shaft of motor

Inductive motor encoders consist of a PCB with transmitting and receiving coils driven at MHz frequency by a dedicated chip on the same PCB. This measures the position of a shaped metal target mounted on the rotor.

Sine and cosine position signals are produced as the metal target moves over the coils; this gives absolute position within a sensor’s electrical period. This is typically matched to the motor’s electrical period, making it the ideal encoder partner for PMSM and other motors where absolute position within a pole pair is needed for commutation.

Inductive sensors are intrinsically robust to oil, dust, condensation and magnetic fields, environments which are problematic for optical and magnetic technologies, respectively. Temperature has no significant impact on the inductive sensing principle, or the materials used, meaning the specification is limited only by the driving IC, -40°C to 150°C. The MHz operating frequency of inductive sensors means they are fast. The maximum delay from rotor movement to sensor output is less than 6 microseconds, which is considerably faster than a resolver, which has similar environmental robustness.

The metal target element and the sensing coils are readily customised to adapt to the application needs for target separation, mounting tolerances and accuracy/resolution, all without significant cost impact. The position measurement is effectively distributed over the whole sensor and target area. This means that much of the influence from mounting and dynamic tolerances is averaged out of the measurement. This allows a lower cost, wear free encoder in place of technologies requiring costly and performance limiting encoder bearings and couplings. To an extent optical and magnetic technologies can cope with misalignment too, but this is often more limited and fixed as it “baked in” to the high tooling cost parts: the optics or Hall array IC.

Inductive sensors have a short axial length, with the form factor, of a pancake. The sensors are typically 25-50mm in diameter and can accommodate almost any size through hole. A multipitch Inductive sensor need not occupy a full 360 degrees and can be realised with an arc or segment PCB. With segment sensors, careful consideration of geometric tolerances is required, the same as for any position measurement technology used in a “read head” configuration.

An inductive sensor with a metal target is very robust and has few failure points. Nevertheless, redundant configurations can be created readily and compactly. With a technology heritage in safety critical applications like traction motors, functional safety is built into the sensor’s IC. The IC is suitable for integration into ASIL D systems in automotive (ASIL D has some equivalence with SIL3, a more general less industry specific standard).

Sensor methods

Sensor methods provide all the application engineering required to create an inductive encoder using readily available application specific standard products (ASSPs) from IC manufactures. This includes specification support, IC selection, sensor coil design, tolerance analysis, sample preparation, end of line programming and testing.

Once a design has been created and qualified, the standard construction of inductive sensors, a PCB assembly and shaped metal target means that all or part of the encoder could be within your existing manufacturing capabilities or those of your preferred partners for contract electronics manufacturer. While Sensor methods can supply encoders via manufacturing partners, manufacturing your own encoder gives you more supply chain control, added value in your manufacturing and a reduction in your bill of materials. Sensor methods have experience in supporting manufacturing sites and engineers worldwide.

Sensor methods has design and simulation tools and extensive expertise in creating sensor modules and encoders integrated into products. Our expertise can be integrated flexibly too, Sensor methods can offer development services from specification to qualification, or simply provide inductive designs and expertise into your established development team as required.

Sensor methods

Optical encoders

Optical encoders consist of a patterned disk mounted to the shaft and optical read head positioned at one point towards the edge of the disc. This generates an incremental signal as the rotating patterned disc alternatively shades or exposes the receiver to the light. Further patterned tracks, along with accompanying receiver complexity can expand the measurement capability to provide commutation or even absolute position. The finely patterned disc means high resolution comes naturally to optical encoders, and when it is mounted without any eccentricity, accuracy is particularly good too. The optics can be sized to be tolerant of some axial and radial alignments.

Optical technology has some drawbacks: Any, dirt, grease etc will interfere with or even prevent the measurement being made, as does condensation. The disc materials which enable the highest resolutions have temperature limitations too.

Some of these limitations can be overcome by a high ingress protection (IP) rated package, but this does not just add cost, but also introduces a speed limitation and wear issues due to the encoder bearings.

Optical encoders can be found at almost all price points and if your environment, tolerances and requirements for absolute position at power-on allow it, it is often a good choice.

Simplified optical encoder, with code disc and optical read head

Magnetic encoders

Magnetic encoders typically place a magnet on the rotating shaft and measure the change in magnetic field direction as the shaft rotates. The magnetic field can be sensed in a chip with Hall or MR technology, and at the field sensing level both technologies are fast and accurate. Like inductive, magnetic encoders are not affected by contaminates like, dust, grease, dirt and condensation, but external magnetic fields can be a problem and only rarely do they have equivalent temperature range. High temperatures can be accommodated, but only with some trade-offs creating some variation in temperature and other specifications points for magnetic encoders.

A small diameter encoder can be created with an on-axis arrangement with the sensor and magnet at the end of the shaft. This produces sine and cosine signals with periods of 360 mechanical degrees creating an absolute encoder which is relatively tolerant to some, but not all, mounting tolerances. As with any sine-cosine signals these are readily digitised to 12-14 bits.

End of shaft or axially mounted magnetic encoder, Hall or MR chip directly beneath a magnet

Off axis magnetic encoder, with Hall IC radially displaced from a multipole magnet on the shaft

If more resolution is required a multipole magnetic wheel can be used: The Hall IC moves to an ‘off-axis’ or 'read-head' configuration. The diameter increases, axial length decreases, and a through shaft encoder is now possible, however accuracy is significantly impacted. In this configuration sine and cosine signals naturally have different amplitudes and many of the mounting and dynamic positioning tolerances which did not affect the measurement in the end of shaft configuration now have a significant impact on the measurement accuracy.

Sensing a ‘static’ magnetic field direction leaves the measurement vulnerable to external fields, or those created by nearby high currents in the motor and drive. Magnetic shielding can be of some help but introduces design constraints and costs. Another way to provide some magnetic field immunity is to sense the field at several points, a few mm apart in the chip to get a measure of the ‘curve’ of the magnetic field. Measuring local differences in the field is harder and not without trade-offs elsewhere. One of which is to leave the encoder vulnerable to a specific combination of tilt and axial misalignment which occurs whenever an axial or rotor is loaded at one end.

An end of shaft magnetic encoder is a particularly good, low cost, small and robust solution if the application’s resolution, mounting tolerances and external magnetic field requirements are all met.

Resolvers

Resolvers are essentially transformers, typically with no electronics included. Although based on inductive principles they are rarely if ever referred to as “inductive sensors”, still many of the environmental advantages are shared, resolvers are robust to oil, dust, dirt, condensation and can even be made radiation hard as no they contain no electronics. The resolver driving electronics in the motor controller uses a relatively low frequency, meaning the response time to position and speed changes is slow.

As the measurement takes place in a distributed manner, over the whole circumference and over the axial length too, a resolver can maintain its accuracy over a variety of mounting tolerances. However, the gap between rotor and stator needs to be minimised to maintain a sufficient signal level, so this imposes quite tight constraints on the mounting tolerances.

Resolvers can be made with multiple electrical periods to match the motor pole pairs, enhancing accuracy and resolution. However, construction costs scale up with pole pairs and physical space constraints are soon felt. In contrast, modern inductive sensors can have more poles in the same space; their coils have much fewer turns, there is no magnetic material, and precision coil placement is standard in PCB manufacture.

Resolvers span a wide range of price-performance points and are often chosen whenever environmental robustness is the key criteria. Low cost resolvers were seen in many of the early hybrid and electric vehicle models, these are now being replaced by modern inductive encoders in today's vehicles.