CITROEN

ELECTRIC POWER STEERING (EPS)

TECHNOLOGY USED

              Electric Power Steering (EPS) or Electric Power-Assisted Steering (EPAS) completely replaces the hydraulic system that previously has always been associated with power steering. Electric power steering systems assist driver effort by the use of an electric motor which acts through a reversible gearbox and in some cases, also an electromagnetic clutch. An electronic control unit determines the degree of assistance that is rendered.The first electric power steering system appeared on the Suzuki Cervo in 1988.

            Electric power steering has some significant advantages over any form of conventional hydraulic steering, both for the owner of the car and its manufacturer. The reduction in engine load of an electric power steering system (it can be as low as 4 watts when the car is being driven in a straight line) means that the fuel economy of a car equipped with electric power steering is very similar to that of a car with no form of power steering. Analyses provided by manufacturers of electric power steering systems indicate potential fuel savings of 4-8 per cent over cars equipped with conventional hydraulic steering, with the lighter mass of an electric power steering also having an impact here. The independence of the system from engine operation also means that should the engine stall, steering assistance does not change.

              From a manufacturer's perspective, using electric power steering reduces assembly line time, allows the easy software tuning of the steering assistance characteristics to suit a variety of cars (a sports car or a limousine - more on this in a minute), and has the potential to improve reliability - 53 per cent of all power steering warranty claims are from pump and hose problems. Environmental gains are also possible from the decreased production and disposal of hydraulic fluid (worldwide, an estimated 40 million litres of power steering fluid was in use in 1995) and from the decreased requirement for the non-recyclable polymers used in hydraulic hoses.

SYSTEM COMPONENTS

1. Pinion Shaft 

• Transmits rotary motion to the rack, which converts this movement into linear, side-to-side motion.

2. Torque Sensor Steering

• The TSS (Torque Sensor Steering) was developed for vehicles with electric power steering EPS. The sensor measures the steering force applied by the driver and thus enables sensitive control of the electric steering support.

3 .Rack-and-Pinion Housing

• A rack-and-pinion gear sets is enclosed in a metal tube, with each end of the rack protruding from the tube. A rod, called a tie rod, connects to each end of the rack.The pinion gear is attached to the steering shaft. When you turn the steering wheel, the gear spins, moving the rack. The tie rod at each end of the rack connects to the steering arm on the spindle.It converts the rotational motion of the steering wheel into the linear motion needed to turn the wheels. It provides a gear reduction, making it easier to turn the wheels.

4. Electric Motor

• Electric motors convert electric current into a rotational torque.

5. Ball-screw Mechanism

• A ball screw is a mechanical linear actuator that translates rotational motion to linear motion with little friction. A threaded shaft provides a helical raceway for ball bearings which act as a precision screw. As well as being able to apply or withstand high thrust loads, they can do so with minimum internal friction.

6. Steering Rack

• The steering rack produces linear motion from an applied torque to the pinion.

7. Drive Belt

• A Drive Belt is used to drive accessories installed on a car engine, such as an alternator, water pump, power steering pump, air conditioner compressor, etc.

8. Electronic Circuit Unit 

• Inputs from the steering sensor are digested by a microprocessor control unit that also monitors input from the vehicle's speed sensor. Typically use inputs from the anti lock brake wheel speed sensors, steering angle and steering effort sensors, and other inputs to provide the proper amount of steering assist.

SYSTEM OPERATION

                This system is a position servo system, and its working process is: when the steering wheel turns, the torque sensor which is installed in the steering wheel axle sleeve detect the torque that the steering wheel produces. It will make the torque converter for voltage signal to form a torque signal. The A/D module which in the controller makes the tested torque signal converts for the digital quantity. The current flows through the power motor will be collected back, form the feedback current signal feedback to the controller. According to the torque signal and feedback current signal, the controller will calculate the size of the booster current and it will drive the power motor through the Pulse-Width Modulation (PWM) control mode. Speed signal detection is one of the most important parts. With different speed, the power curve will also change correspondingly: the higher the speed, the smaller the power. When it reaches a certain speed, the power will stop, which make the driver have good road feeling. This article focuses on the design of the controller.

BENEFIT OF TECHNOLOGY

Features	Benefits
Engine independence	Reduced engine power drain Improved fuel economy and acceleration Instant-on power steering Assistance available even should the engine stall
Elimination of pump, hoses, fluid, and pulley	Simplified packaging Environmental compatibility Reduced mass
Modular design and integrated controller	Reduced assembly time Design and packaging flexibility
Multi vehicle use	Design and packaging flexibility
Software tuning	Wide assistance range In-vehicle laptop PC tuning Tuning process reduced from months to hours
Cost-effective advanced features	Variable effort steering, Assisted return to centre Steering damping capability