Honda Acura NSX 1991 Service Manual page 607

System Description
Power Unit
The power unit consists of a driving circuit, current sensor, field effect transistor (FET) bridge circuit, and two relays.
It receives control signals from the 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)
on receipt of an input of driving signals from the control unit.
<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
of motor is slow. As the duty ratio increases, the rotational speed increases to increase the torque.
(Motor
<Rotational Direction Control >
Normal Mode Control:
The table below shows the normal control mode to con-
trol the flow of current from the battery:
Steering FET FET FET FET
condition
Straight
OFF OFF OFF OFF Stops
ahead
torque sensor data).
Return Control Mode:
Return control mode improves the steering return charac-
teristics. ("PWM" in the table denotes PWM control
based on torque sensor data while "PWM-r" PWM con-
trol based on rotation sensor).
Damper Control Mode:
The damper mode control which improves the convergence of steering, is performed with damper mode signals from
the control unit. In this mode, a short current circuit is formed on the motor side by turning off FET (I) and (2), and
on FET (3) and (41, which suppresses the returning speed of the steering.
<Motor Driving Current Control >
A current sensor, power relay and fail-safe relay are built into the power unit. The current sensor detects motor driving
current and transmits data to the control unit. If there is a problem in the system, a cut-off signal is sent from the control
unit to relay, then the relay cuts off motor current to switch to manual steering operation.
17-38
rotational speed slow)
Motor operation
(Motor rotational speed fast)
RELAY
FROM POWER
CONTROL
B T Y /
S I G N A L S A T E R R E L A Y
CURRENT
FEED BACK I
I
,
FET ISRIVE G R O N D
CIRCUIT
Steering condition
Return from right steering PWM-r oFF
to straight ahead
Return from left steering
to straight ahead
method
CURRENT
S E N S O R
I
. . ..- -_ . .
F A I L S A F E
RELAY
FET (I) FET (2) FET (3) FET (4)
OFF PWM
OFF PWM-r PWM OFF