Motor sensors
Electric motors with permanent magnets are manufactured with various rotor position sensor types. The controller supports only some of them and paying attention to its selection is necessary.
Supported rotor position sensors are:
- Sensorless
- 3 Hall
- Sin-Cos
- Resolver
No additional HW is necessary, the controller simply "computes" rotor position from the measured signals and motor parameters. The sensorless operation does not depend on the Motor sensor assembly variant.
BLDC driver algorithm does not require full motor identification for the sensorless operation.
| Pros | Cons |
|---|---|
| Simple and robust - no additional HW required | Relies on motor parameters which can change with the temperature |
| Very precise at higher speeds | Can have problems with operation at zero RPM under load |
| The rotor position calculation loads the main processing unit |
The signal is usually produced by three Hall sensors placed inside the motor in a 120° (rarely by 60°) span along with one electrical revolution. It can be also emulated by some advanced sensors, such as RLS AM4096. UVW commutation signal is composed of three digital signals. Each signal has two switchpoints per electrical revolution (the first switchpoint is from logical HIGH to log. LOW, second is from log. LOW to log. HIGH). Signals are shifted by 120° from each other (variants with signals shifted by 60° also exist). Examples of the signals are shown in the picture below.
When the UVW commutation signal is processed, it gives six discrete levels of rotor position for one electrical revolution. In the six switchpoints between the levels, the motor position is known with the least ambiguity. This information is enough when the BLDC motor driver algorithm is used. If the VECTOR control algorithm is used, these six switchpoints are not enough and positions between them have to be extrapolated. UVW commutation signal may not be the ideal choice (especially in applications where a high precision/motion control is required at low RPM) for the VECTOR driver algorithm since position estimation is needed.
Rotor position measurement using the UVW commutation signal is shown in the picture below.
Three hall sensors are supported by "a" or "h" Motor sensor assembly variant.
| Pros | Cons |
|---|---|
| Sense electrical (not mechanical) revolutions. No angle multiplication error occurs – it is suitable for motors with higher number of pole pairs. | Interpolation is needed if used with the VECTOR control algorithm. |
| Low-frequency digital signal – good immunity against electrical interference. | 13% ripple of generated torque during steady operation |
| Ideal for BLDC motors | About 13% to 50% torque ripple during stall or very low-speed operation |
| Cheap |
Sin-Cos signal is composed of two analogue signals of sinusoidal shape. Signals are phase-shifted by a quarter of the period and one period of sine (or cosine) signal corresponds to one mechanical turn of the motor (see pic. below). This type of signal is usually produced by a sensor consisting of a cylindrical permanent magnet glued to the rotor and a sensor chip located on the stator at a defined distance from the cylindrical magnet.
Sin-Cos motor sensor is supported by "a" Motor sensor assembly variant.
BLDC driver algorithm does not support Sin-Cos sensor
| Pros | Cons |
|---|---|
| Absolute and continuous position sensing | Typically sense mechanical revolutions. Angle multiplication error could occur when using a motor with many pole pairs. |
| Suitable for VECTOR driver algorithm | Analog interface – could be sensitive to electrical interference. |
| Suitable for position servo drives | Needs output offset calibration (can be done automatically by the controller) |
Resolver is a motor angle sensor with one excitation winding and two sense (sine and cosine) windings, electrically perpendicular to each other. The resolver is fed by AC voltage of known amplitudes and frequency to the excitation winding. Voltage is measured on both sense windings (sine and cosine winding). The voltage across these sense windings has the same waveform as the voltage across the excitation winding. The amplitude of the sensed voltages is modulated by rotor position, as shown in the figure below. Resolver is a common rotor angle sensor in the industry, it is used in high-end drives and servos. Resolvers are usually constructed to sense mechanical revolutions (one period of the modulated sine or cosine voltage corresponds to one mechanical turn). Some resolvers have more pole pairs and can sense electrical revolutions.
Resolver motor sensor is supported by "r" Motor sensor assembly variant.
BLDC driver algorithm does not support Resolver sensor
| Pros | Cons |
|---|---|
| Absolute and continuous position sensing | Typically sense mechanical revolutions. Angle multiplication error could occur when using a motor with many pole pairs. |
| Suitable for VECTOR driver algorithm | Analog interface – could be sensitive to electrical interference. |
| Suitable for position servo drives | Higher weight, larger dimensions |
| No offset in sense winding voltage – easy to set up the drive | Higher price |
| Robust solution with no semiconductors or active electronics |
SC, SL, AX and TX controller usually needs to attach a driver for the excitation coil to get a quality signal reading from the resolver.