1991 ACURA NSX Service Repair Manual

System Operation

Manual Steering Operation

Steering wheel input rotates the pinion of the gearbox through a universal joint. Due to the rack and pinion mechanism,

the rotation of pinion is converted into a transverse motion at the rack, which steers the front wheels throught the tie-rods

and knuckles the same as an ordinary rack and pinion system.

Assisting Operation

In addition to the manual steering operation, the steering sensor on the pinion shaft transmits a signal to the EPS control

unit when it receives a steering input. The EPS control unit calculates an adequate motor control with addition of vehicle

speed signals. Based on the signal from the sensors, the EPS control unit drives the motor inside the gearbox. The torque

generated by the motor is transmitted to a ball screw, through gears, and converted into an assisting thrust in the axial

direction of rack. The assisting thrust acts in the steering direction and lessens the steering force required at the steering

wheel.

EPS CONTROL UNIT

RECIRCULATING

BALL SCREW

STEERING WHEEL
RACK

STEERING SENSOR

(Build in the rotation sensor

and torque sensor)

MOTOR

STEERING GEARBOXProCarManuals.com

System Description

Function and Operation

Steering Sensor

The steering sensor consists of a torque sensor, an

interface, and the rotation sensor.

Torque Sensor

The torque sensor converts steering torque input and its

direction to voltage signals, in conjunction with the

interface.

A torsional force caused by steering operation is con-

verted to an axial movement of a slider core. A variable

differential transformer is installed around the slider

core.

Within the transformer are three coils, a primary coil on

the inside, and two secondary coils, one for right turns

and one for left turns.

Alternating current is passed through the primary coil

when the system is energized. The amount of mutual

induction from the primary coil to the secondary coils

changes depending on the position of the slider coil.

The input shaft and the slider are one piece. Rotation of

the shaft moves the slider in a circular direction. The

pinion is turned via the torsion bar. The slider core is

installed on the pinion shaft on grooves, and turns with

the pinion shaft.

When there is little resistance, the input shaft torsion

bar, pinion shaft, and the slider core turn together to the

same angle. The slider core does not move up/down.
TORSION BAR

INPUT

SHAFT

INTERFACE

CIRCUIT

SPIRAL

GROOVE

ROTATION

SENSOR

GUIDE

PIN

TRANSFORMER

PINION GEAR

WING

When there is resistance on the input shaft (hard to steer situation), and the slider pin is turned at an angle by the steering

wheel, the torsion bar is twisted due to the resistance. Accordingly, the pinion shaft turns at a smaller angle. The differ-

ence in the turning angles changes the position of the slider pin in the groove, forcing the slider core upward or down-

ward, depending on the direction of the direction of the turn.

PRIMARY COIL

UPPER SECONDARY COIL

LOWER SECONDARY COIL

SLIDER
SLIDERProCarManuals.com

Rotation Sensor

The rotation sensor is housed inside the steering gearbox near the pinion. It converts the rotation speed and direction of

steering into voltage signals. The rotation of the steering wheel is transmitted to the pinion, which drives the sensor by

accelerating its speed through the three pieces of transmission the gears. Acting as a kind of direct current generator, the

sensor generates direct current voltage proportionately with the rotational speed of the pinion. When the steering direc-

tion is reversed, the voltage is generated in the opposite direction. No power is generated when driving straight ahead

without turning the steering wheel.

Interface Circuit

The interface circuit is housed inside the steering gearbox near the pinion. It rectifies and amplifies the signals from the

torque sensor and the rotation sensor, and transmits the steering signals to the EPS control unit. A phase compensating

circuit is also built in, which changes the output signals in accordance with the drive signals input from the EPS control

unit.

TRANSMISSION GEARS

INTERFACE CIRCUIT

ROTATION SENSORProCarManuals.com

System Description

Function and Operation

EPS Control Unit

The control unit receives input signals from the car's speed sensors, torque sensor and rotation sensor. It performs a mul-

titude of control functions, including a circuit which can detect troubles in each functional part of the system and the con-

trol 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 (II) position.

Basic Control

(1) Travelling speeds are compiled into data, receiving input signals from the car'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 input signals from the rotation sensor.

(4) Determination of motor torque data from the prescribed assisting force map, based on the car speed data, steering

rotational speed data and the steering torque data.

(5) Changeover of control modes based on data on car 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 car 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.

Rotation sensor
Torque sensor

Steering rotation duty

Unloader duty

(Reduces over-current flow

at full-lock position)

Output duty
Torque duty

Mode changeover

• Normal mode

• Return mode

• Damper mode
Motor control

signal output
Base duty
Car speed sensorsProCarManuals.com

Self-Diagnosis Functio n
Th e EP S contro l uni t monitor s th e syste m input s an d outputs , an d th e drivin g curren t o f th e motor . I f ther e i s a proble m in
th e system , th e contro l uni t turn s th e syste m of f b y actuatin g th e relay . Powe r assis t stop s an d norma l manua l steerin g
operatio n resumes . Th e contro l uni t als o turn s th e EP S indicato r ligh t o n t o aler t th e driver , an d memorize s th e proble m in
th e for m o f a code . Connectin g th e terminal s o f th e servic e chec k connecto r wit h th e SC S servic e connecto r (specia l tool )
enable s th e EP S indicato r ligh t t o blin k th e proble m cod e whe n th e ignitio n switc h i s turne d o n (II) .
Unloade r Contro l
I f th e steerin g whee l i s turne d full y an d hel d in th e full-loc k position , th e steerin g torqu e reache s th e maximu m point , an d
a n over-curren t flow s t o th e motor . Th e contro l uni t detect s thi s an d reduce s th e curren t flo w to th e motor .
Averag e Movin g Curren t Contro l
Th e electri c curren t flo w to th e moto r i s estimate d fro m th e curren t value s detecte d b y th e curren t sensor , an d th e averag e
curren t i s obtaine d a t tw o secon d intervals . Th e moto r drivin g curren t i s suppresse d whe n th e averag e curren t valu e
exceed s a predetermine d margina l value . Th e contro l uni t regulate s th e moto r curren t durin g continuou s loadin g t o sup -
press any excessiv e temperatur e ris e in th e motor .
Over-Voltag e Contro l
I f ther e i s a n excessiv e increas e i n powe r sourc e voltag e du e t o a poo r batter y condition , a n alternato r voltag e regulato r
problem , etc. , th e moto r assistin g forc e increases , resultin g i n excessiv e control . T o preven t this , th e contro l signal s ar e
correcte d to ensur e tha t adequat e assistin g forc e is generated .
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System Description

Function and Operation

The 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 modu-

lation (PWM) method on receipt of an input of driving signals from the EPS 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 (V) is called the duty ratio (8). Its relationship is expressed by V = E x 8. 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.

(Motor rotational speed slow)

ONE CYCLE
(Motor rotational speed fast)

ONE CYCLE

AVERAGE /

VOLTAGE (40%)
VOLTAGE APPLIED RATIO

IN ONE CYCLE: 40%

RELAY

CONTROL

SIGNALS
VOLTAGE APPLIED RATIO

IN ONE CYCLE: 60%
AVERAGE

VOLTAGE (60%)

FROM

BATTERY
POWER

RELAY
CURRENT

SENSOR

Rotational Direction Control

Normal Mode Control:

The table below shows the normal control mode to con-

trol the flow of current from the battery:

CURRENT

FEED BACK

DRIVE

SIGNAL

FET DRIVE

CIRCUIT
GROUND

FAIL SAFE

RELAY

("PWM" in the table indicates PWM control based on

torque sensor data).

Return Control Mode:

Return control mode improves the steering return char-

acteristics. ("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 (1) and (2), and turning on

FET (3) and (4), 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 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 cur-

rent to switch to manual steering operation.
CONTROL UNITProCarManuals.com

Steering Gearbo x
Motor an d Powe r Assis t Mechanis m
A moto r i s house d insid e th e gearbo x housing . I t consist s o f a yok e wit h a permanen t magne t fixe d i n it , a roto r wit h a
fiel d coil , an d brushe s whic h pas s electricit y t o roto r commutators . Th e moto r i s o n th e rac k shaf t o f th e steerin g gearbox .
Th e driv e curren t fro m th e contro l uni t flow s throug h th e powe r rela y an d fail-saf e rela y t o th e brushe s o f th e motor .
Th e driv e curren t flow s t o roto r commutator s throug h brushes , excitin g th e fiel d coi l o f th e roto r an d rotate s th e roto r wit h
th e magneti c actio n o f th e permanen t magne t insid e th e yoke . Th e roto r rotate s i n th e opposit e directio n whe n th e direc -
tio n o f driv e curren t i s reversed .
The rotatio n torqu e of th e moto r is transmitte d to th e bal l scre w through the helica l gear . The rotatio n torqu e on th e bal l
scre w is converte d int o assis t thrus t i n th e directio n o f steerin g rac k b y th e recirculatin g bal l screw . Th e mechanica l advan -
tag e o f th e recirculatin g bal l screw , compaire d t o a conventiona l rac k an d pinion , act s t o reduc e th e steerin g forc e t o th e
steerin g rac k throug h th e joint , t o lesse n th e steerin g effor t require d a t th e steerin g wheel .
ROTORRACK
RECIRCULATIN GBALL SCRE W
BAL L SCRE W
HELICA L GEA R
Recirculatin g Bal l Scre w
Th e recirculatin g bal l scre w is constructe d s o tha t stee l ball s rol l betwee n th e scre w shaf t an d th e nut , an d thos e comin g
ou t o f th e scre w fac e ente r th e scre w groov e agai n throug h a tub e fo r recirculation . Th e rotatio n torqu e o n th e shaf t i s
converte d int o thrus t i n a n axia l directio n a s th e slop e o f th e scre w shaf t presse s agains t th e nu t throug h th e stee l balls .
Sinc e th e torqu e conversio n i s mad e i n thi s metho d throug h th e scre w shaf t an d th e stee l balls , th e frictio n i s small , hig h
transmissio n efficienc y i s obtaine d fo r bot h th e forwar d an d backwar d directions , an d th e steerin g feel s identica l t o th e
ordinar y manua l steering .
Furthermore , sinc e th e forc e fro m th e backwar d directio n (forc e pressin g th e steerin g rack ) i s converte d int o th e rotatio n
torqu e o f th e scre w shaft , instantaneou s kick-bac k fro m th e roa d surfac e wil l rapidl y rotat e th e motor . I n suc h a rapi d rota -
tion , th e inerti a o f th e moto r increases , whic h act s t o reduc e th e kick-back . An y grea t kick-bac k wil l b e transmitte d fro m th e
steerin g rac k t o th e torqu e senso r o n th e pinion , whic h act s t o twis t th e pinio n shaf t i n th e sam e directio n a s th e inpu t
directio n o f kick-back . A s a result , th e moto r operate s th e kick-bac k to reduc e assis t force .
BALL SCRE W
TUBESTEE L BAL L
NUT
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Component Location s
GAUGE ASSEMBL Y
UNDER-HOO D FUSE /
RELAY BO X
TRANSMISSIO N CONTRO L
MODUL E (TCM )
22 P CONNECTO R
VEHICL E SPEE D SENSO R
(VSS)
(M/T)PULS E UNI T 7 P CONNECTO R
TORQU E SENSO R 6 P CONNECTOR /
ROTATIO N SENSO R
4 P CONNECTO R
STEERING GEARBO X
(TORQU E SENSOR /
ROTATIO N SENSOR )
UNDER-DASH FUS E BO X
EP
S CONTRO L UNI T
ALTERNATO R4P CONNECTO R
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