Steering Control Module: Description and Operation
OPERATIONThe control unit receives input signals from the vehicle speed sensors, torque sensor. It performs a multitude of control functions, including a circuit which can detect troubles in each functional part of the system and the control unit itself. The control unit operates the system while the engine is in the process of starting, on receipt of voltage generated by the alternator. The system remains operational if the engine stalls with the ignition switch in ON position.
BASIC CONTROL
1. Travelling speeds are compiled into data, receiving input signals from the vehicle's speed sensors.
2. The magnitude and direction of torque are compiled into data, receiving input signals from the torque sensor.
3. The rotational speed and direction of steering are compiled into data, receiving calculated data.
4. Determination of motor torque data from the prescribed assisting force map, based on the vehicle speed data, steering rotational speed data and the steering torque data.
5. Changeover of control modes based on data on vehicle speed, direction of torque and rotational direction of steering.
Normal Control Mode:
The rotational direction of motor is determined after changing over to the left/right steering mode in accordance with torque direction data. The mode is switched to the straight ahead mode when the output data is zero.
Return Control Mode:
According to torque direction data and steering rotational direction data, the mode is changed over to the return control mode to improve the steering return characteristic.
Damper Control Mode:
According to vehicle speed data, torque value data and steering rotational speed data, the mode is changed over to the damper control mode to improve the convergence property of the steering.
SELF-DIAGNOSIS FUNCTION
The EPS control unit monitors the system inputs and outputs, and the driving current of the motor. If there is a problem in the system, the control unit turns the system off by actuating the relay. Power assist stops and normal manual steering operation resumes. The control unit also turns the EPS indicator light on to alert the driver, and memorizes the problem in the form of a code. Connecting the terminals of the service check connector with the SCS service connector (special tool) enables the EPS indicator light to blink the problem code when the ignition switch is turned on.
UNLOADER CONTROL
If the steering wheel is turned fully and held in the full-lock position, the steering torque reaches the maximum point, and an over-current flows to the motor. The control unit detects this and reduces the current flow to the motor.
AVERAGE MOVING CURRENT CONTROL
The electric current flow to the motor is estimated from the current values detected by the current sensor, and the average current is obtained at two second intervals. The motor driving current is suppressed when the average current value exceeds a predetermined marginal value. The control unit regulates the motor current during continuous loading to suppress any excessive temperature rise in the motor.
DESCRIPTION
The power module in the EPS control unit consists of a driving circuit, current sensor, field effect transistor (FET) bridge circuit, and two relays. It receives control signals from the central processing unit (CPU) and controls the driving current of the motor. The driving circuit controls the rotational direction and speed of the motor by driving the FET bridge circuit with a pulse width modulation (PWM) method on receipt of an input of driving signals from the CPU.
ROTATIONAL SPEED CONTROL
The PWM driving signal is a digital signal repeating the process of voltage ON/OFF at a constant frequency, which changes the ratio of ON time per one cycle of this signal. The ratio is called the duty ratio. When there is a change in duty ratio, the average voltage changes as smoothly as an analog type. The ratio of digital signal voltage (E) and the average voltage (V) is called the duty ratio (d). Its relationship is expressed by V = E x d. When the duty ratio is low, the rotational speed of motor is slow. As the duty ratio increases, the rotational speed increases to increase the torque.
ROTATIONAL DIRECTION CONTROL
Normal Mode Control:
The table shows the normal control mode to control the flow of current from the battery: ("PWM" in the table indicates PWM control based on torque sensor data).
Return Control Mode:
Return control mode improves the steering return characteristics. ("PWM" in the table denotes PWM control based on torque sensor data while "PWM-r" PWM control based on steering rotation speed data).
Damper Control Mode:
The damper mode control, which improves the convergence of steering, is performed with damper mode signals from the CPU.
MOTOR DRIVING CURRENT CONTROL
A current sensor, power relay and fail-safe relay are built into the control unit. The current sensor detects motor driving current. If there is a problem in the system, a cut-off signal is sent from the CPU to relay, then the relay cuts off motor current to switch to manual steering operation.