In vehicles using electric traction motors, alternating current (AC) motor drives are used to provide a requested torque to the motor shaft. In practice, the amount of torque produced by the motor is directly related (although not perfectly proportional) to the amount current provided to the motor. Therefore, by regulating and precisely controlling the input current to the electric motor, the amount of torque produced by the electric motor may be more accurately controlled. In response to a changing torque command, the motor torque response is smoother and/or faster when the amount of current provided to the electric motor is adjusted based on the torque command.
For purposes of efficiency, particularly in hybrid and/or electric vehicles, it is desirable to maximize the ratio of the output torque to the input motor current. However, in many systems, the input motor current is not directly controlled. For example, many electric motors are operated using pulse-width modulation (PWM) techniques in combination with an inverter (or another switched-mode power supply) to control the voltage across the motor windings, which in turn, produces the desired current in the motor. In response to a requested torque (or commanded torque), most prior art systems determine a desired input motor current for producing the requested amount of torque and utilize a closed loop control system to control the current through the motor windings and thereby regulate the amount of torque produced the motor. One or more sensors are used to measure the actual motor current, which is then compared to the desired input motor current. Based on the outcome of the comparison, the PWM commands for the inverter are adjusted to increase and/or decrease the voltage across the motor windings, such that the actual measured motor current tracks the desired input motor current.
However, when a current sensor does not accurately measure the motor current, these closed-loop control systems can no longer effectively control the motor torque. For example, without accurate motor current information, the control system may cause the motor to produce insufficient torque, excessive torque, or varying or oscillating amounts of torque. Furthermore, as a preventative measure in some prior systems, in response to a current sensor error, the control system may cease providing current and/or voltage to the electric motor, or drastically reduce the amount of current and/or voltage provided to the electric motor. As a result, in prior art systems, when a current sensor error occurs, the use and enjoyment of a particular vehicle is adversely affected.