The internal-combustion car historically had only one high-current electric motor: the starter. This 12-V/100-A (typical) unit does not need sophisticated control, as it is just an on/off, “give it the full 12 V DC and let it rip” motor. The few other motors were small and easily controlled, such as for the windshield wipers.
Jump to the present: Today's cars have dozens of small and medium-size motors for power windows, seat adjustment, outside mirrors, and vent controls, for example. The number of wiring loops in a car for all these functions became so high that car motors are now networked, as individual wire runs were taking up too much space, adding considerable weight, hard to trace and debug, and exploding the BOM cost.
Further, motor demands have increased dramatically with electric vehicle (EV) and hybrid electric vehicle (HEV) automotive designs. The vehicle electric systems now must route power at many hundreds of amps and several hundred volts or more to the wheel motors (the Toyota Prius motors operate at 200 V), and these motors must be carefully controlled to meet performance requirements.
Motor control, however, is more than just the obvious MOSFETs that switch power to these motors, and the processors that execute the embedded motor control algorithms and energize/de-energize the MOSFETs. While active components are obviously critical, they are only a small part of the total picture. The need to power and control these numerous, widely disbursed motors, ranging from moderate to higher-power units, are placing difficult demands on two groups of passive components:
1. Sense resistors for the motion-control loop, which allow the motor controller to know precisely what the motor is doing and how well it is doing it. The challenge of doing so increases with higher currents and voltages.
2. Connectors that can handle the voltage and power, and are compact, cost-efficient, safe in use, easily disconnected/reconnected for factory test and field repair, and reliable despite the harsh automotive environment.
Sensing the Current Flow
When it is necessary to know what the motor is doing, to be able to compare with the commanded motor action (position, speed), some sort of feedback is needed. This can be obtained by using a shaft encoder (Hall-effect, optical, magnetic) or sensing current flow through the motor windings. Generally, designers like to start with the current-sensing approach alone, because it is less expensive and physically easier to implement than a shaft encoder.
For current sensing, the topology is simple at first appearance: a resistor is placed in series with each motor winding and the voltage across it is sensed and monitored by the motor controller (Fig. 1). This resistor is often called a “shunt,” but this is a misnomer, as it is in series with the winding and not shunting the current flow. (Note that there are other ways to sense, including noncontact approaches such as a Hall-effect device or sensing coil on the motor lead, but these are beyond the scope of this article.)
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