Power steering

3.6 Power steering:      Introduction

As increasing traffic density, high axle loads and poor road conditions all are situations force the driver to operate the vehicle at slow speeds, increasing steering effort causing fatigue to drivers. Judgment of a fatigued driver will be poor. Poor judgment in driving means increased potential for accidents. That’s all led to the development next of automobile steering i.e. “Power Steering”.

If you’re a city slicker who regularly requires complex maneuvers to park your car, then power steering is your best friend. Nowadays, power steering is found on most automobiles. It generally consists of a hydraulic pump that amplifies the force you apply to the steering column to change directions. Without power steering, you’d have a hard time turning the wheels of a vehicle that’s stopped or travelling low speed. Interest in power steering increased during World War II, as it facilitated maneuvers for large military vehicles. In 1951, Chrysler released the Imperial, a vehicle equipped with power steering. The technology has come a long way since then, and it’s now considered an essential feature.

The flexibility and controllability of automobile steering system was innovated continuously with the automobile technique renovation, and they were improved better and better. Mechanical steering gearings trend to diversification such as pinion rack gearing, worm gearing, and circular ball gearing. However, mechanical steering gearings have disadvantage themselves. The disadvantage of pinion-rack gearing is that the precision of rack is hard to control and it tends to be wore and tore. For the worm gearing and circular ball gearing, moment output is large, but volume is even too large and configuration is too complex. Therefore power steering gearing break a new path avoiding the disadvantage of traditional mechanical steering system.

Fig 1: Manual Vs Power

Power steering has two types of power source for steering effort one type is a hydraulic device (hydraulic pump) utilizing engine power. The other type utilizes an electric motor. For the former, the engine is used to drive a pump. For the latter, an independent electric motor in the front luggage compartment is used the pump. Both develop fluid pressure, and this pressure acts on a piston within the power cylinder so that the pinion assists the rack effort. The amount of this assistance depends on the extent of pressure acting on the piston. Therefore, if more steering force is required, the pressure must be raised. The variation in the fluid pressure is accomplished by a control valve which is linked to the steering main shaft.

There are three basic types of power steering systems found in vehicles; the Hydraulic Power Steering (HPS), the fully Electric Power Steering (EPS) and the Electric Power Hydraulic Steering (EPHS). Electric and electronic power steering both refer to the same system.

1. Hydraulic Power Steering (HPS):

The major components of HPS system are steering wheel & column with fluid lines, power steering pump (positive-displacement type vane pump), steering gear (rack & pinion type), power steering pressure hoses, pressure relief valve, rotary control valve, double-acting hydraulic cylinder and hydraulic reservoir with fluid.

Fig 2: Power Steering (Hydraulic)

In a HPS system, the pump provides power steering fluid under pressure to the rack and pinion gear unit. A pump attached to the engine and driven by a belt, a pressure hose assembly, and a return line. Also, a rotary control valve is incorporated with steering column in the hydraulic circuit. All systems are constructed so that the car can be steered manually when even the engine is not running or if any failure occurs in the power source, due to steering booster arrangement, inclusion of steer torque detector, controlled pressure distributor case, hydraulic booster pump, and tank with a working fluid.

It is a hydraulic system for reducing the steering effort on vehicles by using hydraulic pressure to assist in turning the wheels. It is intended to provide for easier driving direction control of the car while preserving "feedback", stability and unambiguity of the trajectory specified.

HPS systems work by using a hydraulic system to multiply force applied to the steering wheel inputs to the vehicles steered (usually front) road wheels. The hydraulic pressure typically comes from a pump driven by the vehicle's engine. A double acting hydraulic cylinder applies a force to the steering gear, which in turn steers the road wheels. The steering wheel operates valves to control flow to the cylinder. The more torque the driver applies to the steering wheel and column, the more fluid the valves allow through to the cylinder, and so the more force is applied to steer the wheels.

Benefits or advantages of HPS:

Effortless steering

Precise & quick response

Absorbs road shocks easily

It provides greater safety and controllability under critical situations.

Limitations or disadvantages of HPS:

HPS provides many disadvantages. Since the power-steering pump equipped on most vehicles runs constantly and pumps fluid all the time, it wastes horsepower. This wasted power translates into wasted fuel and higher emissions. In addition, this system is susceptible to leaks and noises, and commonly results in failure due to a broken belt. Another major disadvantage is the addition of a belt-driven engine accessory, and several high-pressure hydraulic hoses between the hydraulic pump, mounted on the engine, and the steering gear, mounted on the chassis, this leads difficulties in manufacturing and maintenance. Loss of power steering can significantly affect the handling of a vehicle.

2. Electric power hydraulic steering (EPHS):

EHPS is a hybrid of hydraulic and electric. In this system, a hydraulic pump gets its energy from an electric motor instead of a belt driven by the engine. In EPHS the customary drive belts and pulleys that drive a power steering pump are replaced by a brushless motor. The power steering is driven by this electric motor, which reduces the amount of power that needs to be taken from the engine. The main components of an integral power steering system consist of a hydraulic pump assembly connected with electric motor. A rotary valve power steering gear for the integral system using recirculating ball type worm and wheel steering gear is most commonly used one.

Heavy duty commercial vehicles require a high-power steering system that used engine-driven hydraulic pump systems (17Liter/min) compared to passenger cars. In recent technical trend, In order to improve fuel efficiency and realize autonomous driving technology, engineers designed EHPS and Motor driven electric control actuator with the same structure as EPS.

Unlike the HPS system, the power used in the EHPS system is the motor actuator, which rotates the gear pump to generate flow and pressure, and the steering oil is delivered to the ball nut gearbox to operate the power steering system through the pitman arm. The advantage of EHPS is that it does not use the engine drive torque, so it improves fuel efficiency by about 1%, and when the pump is connected in parallel, it can secure the flow rate to the main steering. In this process, it is difficult to realize the active autonomous driving technology of the vehicle itself through the steering angle control separately from the driver's steering intention. Therefore, the motor control device directly connected to the steering wheel is essential, and engineers can design the actuator system by selecting the package and the required output according to the use conditions.

3. Electric power steering (EPS):

Electric power steering systems are becoming increasingly more common. Electric power steering systems consist of additional components including various sensors, wires, actuators, motors, and an electronic control unit.

In the EPS system, an electric motor replaces the hydraulic pump and a fully electric power steering system is established. The electric motor is either attached to the steering rack or to the steering column. The electronic control unit controls the steering dynamics. EPS is often a preferred system since it results in better fuel economy and lower emissions.

Fig 3: Electric Power Steering

Components of EPS:

1. Hydraulic pump or motor (DC)

2. Power cylinder and piston with control valve

3. A speed sensor and fluid pressure sensors

4. EPS CPU

5. Basic components of Steering Angle Sensor G85 are:

a) Absolute and increment ring with two codes

b) Photoelectric beam pairs, each with one

c) Light-Emitting Diode (LED) and one optical sensor.

6. Fluid bath steering gear-box – (recirculating ball type worm and worm wheel)

7. Electric and hydraulic connections.

Electric Power Steering (EPS) is designed to use an electric motor to reduce effort by providing steering assist to the driver of a vehicle. Sensors detect the motion and torque of the steering column, and a computer module applies assistive torque via an electric motor coupled directly to either the steering gear or steering column. This allows varying amounts of assistance to be applied depending on driving conditions. The system allows engineers to tailor steering-gear response to variable-rate and variable-damping suspension systems achieving an ideal blend of ride, handling, and steering for each vehicle. In the event of component failure, a mechanical linkage such as a rack and pinion serves as a back-up in a manner similar to that of hydraulic systems. It has also called as Electro-mechanical Power Steering.

The Advantages of EPS:

1. No hydraulic components, for example power steering oil pump, hoses, oil tank, filter.

2. No hydraulic fluid.

3. Space savings.

4. Reduction in noise.

5. Energy savings.

6. No complex hose and wiring system.

EPS provides many additional advantages. The amount of assistance provided by EPS is easily tunable to the vehicle type, road speed, and even driver preference. Another benefit is the elimination of environmental hazards posed by leakage and disposal of hydraulic power steering fluid. In addition, electrical assistance is not lost when the engine fails or stalls, whereas hydraulic assistance stops working if the engine stops.

It is an environment-friendly technology for environment-related regulations such as reducing emissions and improving fuel efficiency, for driving convenience using EPS, and safety technology to protect drivers, passengers, nearby vehicles and pedestrians.

EPS is more effective than the HPS, since the electric motor only needs to provide assistance when the steering wheels turned whereas the hydraulic pump must run constantly. Unlike HPS, which requires a permanent circuit flow, the electro mechanical power steering only draws energy when steering force is necessary. This input response performance leads to a reduction in fuel consumption.