WHY POWER STEERING WAS DEVELOPED
The term “power steering” isn’t quite right. In reality, the power for steering is provided by the driver’s arms.
Years ago, pure mechanical steering systems required varying levels of muscle power depending on the type of car, weight, tyre size and extent of wear on steering components.
With the growing size and mass of cars and indeed commercial vehicles, it became quite necessary to develop a system that could reduce steering effort required of the driver.
Hence, power-assisted steering was born.
POWER STEERING FOR CARS
It sounds quite simple – incorporate an electrical motor or hydraulic cylinder to work the linkages that steer the front wheels.
That’s fine for a fork-lift truck or an excavator where response and precision are neither priorities nor critical for operating.
Passenger cars on the other hand are driven over a very wide range of road speeds hence have very clearly defined steering requirements.
Most important of all is the linearity of response. This means the driver’s steering input must result in a proportional directional change of the front wheels.
At the same time, the relationship between steering wheel rotation and front wheel steer-angle must be delicately balanced for safety and driver comfort.
At the two extremes, twitchy steering (highly sensitive to steering input) can be nerve-racking and positively dangerous at speed.
On the other hand, a light and low-geared system requires plenty of wheel-twirling. That makes it difficult to manoeuvre on winding roads.
HOW IT WORKS
Power-assisted steering (PAS) provides the most comprehensive solution to the problems of steering system design.
Power steering is based on the concept of hydraulic oil pressure providing the “power”.
A pump driven by a belt off the crankshaft pulley or an electric motor, supplies the pressurised oil needed.
The way it provides assistance is dependent on a few other complex elements in the system. These include a mechanical torque sensor and a control valve.
To put it simply, any steering input by the driver activates the sensor which meters a proportional amount of fluid into the control valve.
Here, the fluid (pressurised, remember) is diverted to the appropriate section of the steering gear to provide a secondary force on the mechanism and hence amplifies the driver’s effort.
In other words, “assists” the basic hardware in its function. The components of the power steering system are manufactured to a high level of precision.
This enables the system to function without compromising feel, comfort or accuracy.
ELECTRIC POWER STEERING
Lately, car manufacturers are opting for fully electrical power steering setups instead of hydraulic ones.
All the mechanical links still exist, but there is no oil pump. There are also no hydraulic pipelines or complicated valve components.
In their place is an electrical motor that provides power assistance with control precision managed by microprocessor-based devices. A dedicated ECU controls the speed and output of the motor, based on the data received from electronic sensors on the steering gear.
ELECTRIC STEERING – THE ADVANTAGES
The electrical system is obviously smaller, lighter and more fuel efficient. But it also has far more significant advantages in being able to enhance safety and comfort.
The steering ECU can be linked to other ECUs in the car and provide a wide range of power assistance – not just a reduction in effort.
For example, the steering controller could communicate with the ESP. Together, they can provide active steering assistance to mitigate excessive steering inputs in a panic manoeuvre.
There is no danger in the event of an electrical motor failure because the basic mechanical connections are always there. You just need to exert a little more force than usual in such an event.
In theory, electric power steering can be further developed. They can offer feel, precision, comfort and active safety equal or better than the best hydraulic types.
Like the carburettor, hydraulic power steering will eventually become a technology of the past.