Position Sensing is a measurement system of capturing the traction motor mechanical motion data to ensure that the system functions in a synchronized way. To capture such data, various types of Rotary Position Sensors are utilized in traction motors intended for Electric Vehicle (EV) applications. Once the angular position data is captured, it is converted into electric signals for activating the feedback loop via a Motor Controller or Motor Control Unit (MCU). The selection of these sensors involves a trade-off between operating environment, accuracy, resolution, and cost.

WHY POSITION SENSING IS IMPORTANT?

Traction Motor and Motor Controller forms the core of an EV powertrain that particularly requires motor position data for motor control algorithms to work efficiently. The control loop in the Motor Controller takes feedback from rotary position sensors in the motor and directly controls the amount of current flowing into each of the motor phases. This current control is established by firing the MOSFETs/IGBTs in each of the phases, being controlled independently, to drive the required current into motor phases and the resulting torque to propel the vehicle.

Also, when the driver presses or releases the acceleration pedal, the motion data of the motor is captured and shared with the MCU which in turn controls the speed of the motor. Thus, position data is sufficient for implementing the speed control loop, thereby eliminating the need for a separate speed sensor.

HOW POSITION SENSING WORKS?

Rotary Position Sensors have been utilized to determine the position of the motor shaft since the inception of motor control mechanisms. The operating principles for determining such positions differ significantly that results in variation in corresponding accuracy, resolution, and data signal quality. Consequently, the application and cost act as the final criteria for selecting a particular sensor. Some of the commonly deployed Position Sensors in the EV Traction Motor includes:

a) Hall Effect Sensor:

Hall Effect Sensors provide three overlapping signals phase shifted either by 60° or 120°, thereby making the speed information available by capturing the time interval between two phases. Further, the algorithms in the Controller are utilized to numerically ascertain the position data required for sharp firing of MOSFETs.

Application: Due to their low cost and optimal accuracy in position sensing, these sensors are used in BLDC/PMSM motors with applications spanning across Low and High-Speed Electric 2-Wheelers (e-2W) along with Electric 3-Wheelers (e-3W).

b) Encoders:

Encoders involve optically coded disks with single track or quadrature resolution. When the motor shaft rotates, an infrared source emits a light which is converted to digital signals by an optical sensor. To determine the direction of rotation, two streams of pulses are generated, A&B, at 90° timing angles. Encoders are further classified into following sub-types viz;

i) AB Encoders:

Two streams of pulses, A&B, phase shifted by 90° provides specific number of equally spaced pulses per revolution for each stream with starting position of rotor acting as a reference. Direction of motion is detected by the phase relationship of one channel leading or trailing the other channel. These sensors are commonly utilized in Induction Motors (IM).

ii) ABZ Encoders:

Here also, two streams of pulses, A&B, phase shifted by 90° are generated while a marker pulse I/Z provides a position reference once per revolution. Additionally, a fourth signal in Pulse Width Modulation (PWM) gives the actual rotor position in degrees based on duty cycle. Direction of motion is detected by the phase relationship of A & B channels leading or trailing the other channel. ABZ Encoders are used in Permanent Magnet Synchronous Motors (PMSM).

iii) Differential ABZ Encoders:

As opposed to ABZ Encoders, six signals are generated when using Differential ABZ Encoder. A+, A-, B+, B-, Z+ and Z- are such signals with A+ and B+ phase shifted by 90° while A- & B- acting as complimentary signals. Z+ & Z- are another set of complimentary signals which indicate actual rotor position. Similar to ABZ Encoder, these sensors are also used in Permanent Magnet Synchronous Motors (PMSM).                                           

Application: Due to higher accuracy & resolution relative to Hall Sensor, Encoders are utilized in both Induction and PMS Motors with applications cutting across Electric High-Speed 2-Wheelers (e-2W), Electric 3-Wheelers (e-3W) and Light Commercial Vehicles (e-LCV)

c) Resolvers:

A Resolver is an analog sensor with an operating principle similar to a transformer. The sensor has one primary and two secondary windings. Primary Winding is linked to the rotor while secondary windings oriented at 90° are linked to the stator. An AC Voltage is induced in the primary winding to excite coils. Back EMF generated from primary winding is further induced in secondary windings with a Sine and Cosine output, respectively. Such outputs are further processed in AD Converters (ADC) for generating digital signals. Resolvers are commonly interfaced in PMSM motors.

Application: Resolver has the highest accuracy and resolution for position data while high tolerance for rugged environments. Such features make them suitable for PMS motors deployed in Heavy Duty applications including Electric Passenger and Commercial Vehicles.

COMPARISON