Power-Steering Systems Hydraulic, Electro-Hydraulic, Electric

Power steering systems can produce annoying tonal noise (whines). The frequencies of the noise are typically synchronous to shafts contained within key components of the system e.g. electric motors and pumps.

Plato is a successful power-steering NVH measurement & analysis tool for:

► end-of-line
► in-vehicle
► laboratory/R&D

When operating in its precision order-tracking mode measuring dynamic signals such as:

► air-borne noise
► structure-borne vibration
► fluid pressure fluctuation

it can assess these signals against pseudo-static parameters such as:

► pump/motor speed
► fluid static pressure

to form an operational “map”.

As a multi-channel measurement & analysis system, Plato can measure combinations of dynamic parameters simultaneously; invaluable for investigations of cause and effect etc.

The dynamic and pseudo-static parameter(s) chosen for testing depend to a large extent on where the power steering system is being tested and the main objective of the testing.

In a vehicle for example, when torque is slowly applied to the steering wheel the load on the motor and pump is gradually changed. With some engine speed controllers, this can result in the speed of the motor and pump reducing, so motor/pump speed can be a convenient variable against which to map the dynamic signals.

When the power steering system is being tested on a test stand (at the end-of-line or in a laboratory/R&D setting), there is often more control available on how test conditions can be set and varied to produce a meaningful test. For example, it is common to vary static pressure of fluid based systems by programming a variable dump valve.

ABD Plato systems have established good correlation between test stand results and in-vehicle results. However the test is constructed, ABD experience is that the noise/vibration signature of all modes of operation can typically be assessed within a 10 to15 second period.

Electric Power Assisted Steering (EPAS)

EPAS is direct electric motor powering of the steering. The motor drives the steering column or rack through a directly geared connection. A torque transducer in the steering column signals when and how much assistance is required from the motor, under control of an ECU.

The motor can be located just behind the dashboard, at the bottom of the steering column, or on the steering rack. The noise generators (motor and reduction gearing) are connected directly to the steering wheel shaft on all except the rack mounted version, hence there is a high potential for noise emission into the passenger cabin from the dashboard structure and the steering wheel. The motor only operates when the steering wheel is turned (in contrast to EHPS, where it is operating all the time). Motor and gear noise occurs at specific orders of rotation, but the frequencies depend on how fast the steering wheel is turned. There can also be constant frequency noise components caused by the ECU.

Plato can test an EPAS system in a similar way to an EHPS, using:

► Constant speed rotation of the steering wheel (around 3 or 4 turns, left and right are typically possible between the lock stops)

► Speed ramps (from zero to max to zero) of the steering wheel.

Conventional Hydraulic Power Steering

Steering pump noise is a well known source of problems. A multi-channel system is necessary for successful noise measurement in a vehicle. The Plato precision order-tracking analysis techniques used for gear noise measurement and analysis in the vehicle powertrain industry are equally effective for steering noise.