Angle encoder is best option for steering tests

Robots used to test power steering systems incorporate Renishaw's RESR angle encoder technology.

Although modern vehicles handle far better than their predecessors, a great deal of work is still carried out by vehicle dynamicists and manufacturers of testing equipment to continually improve today's cars and trucks.

Lateral acceleration, roll angle, yaw rate and other factors are measured and analysed to enable improvements in road handling.

The result is better and safer vehicles.

One such company is AB Dynamics, which has supplied "blue chip" companies worldwide with the very latest testing equipment.

A recent project for the Bradford-on-Avon based organisation has seen the delivery of steering robots to leading automotive and truck OEMs, test houses and tyre manufacturers.

Today, these incorporate Renishaw's RESR angle encoder technology.

"In the area of transient testing, people in the automotive industry want to have better control of dynamic inputs", states Managing Director Tony Best.

"There are certain inputs that are difficult to replicate by hand, for instance those that are gradually increasing in frequency but maintain the same amplitude".

"It is also very difficult to produce nice sinusoidal shapes when you are going very slowly; the tendency is to produce triangular shapes instead".

"When you're trying to get good results in order to compare one car with another, or a slightly modified car, you need to have accurate, repeatable, well-defined inputs".

The test bed steering robot at ABD contains a powerful servomotor which is attached to the vehicle's steering column, together with a steering wheel which allows the car to be steered manually when not under computer control.

The robot's electronics include the necessary motor drivers and amplifiers, as well as the means of recording the inputs and processing the data.

"At first we used a rotary encoder from another supplier to control the inputs because it was one of the few encoders then on the market with a sufficiently large central bore to allow it to be used with truck steering systems", added Tony Best.

"However, we discovered that they were ceasing the manufacture of the encoder we were using and so we had to look for a new supplier".

"Although Renishaw did not manufacture a specific ring encoder to meet our needs, they offered to work with us to find a solution".

"We started to employ their highly successful RGR ring encoder system, which is an encoder tape scale fixed to a stainless steel ring, and our initial testing gave good results".

Shortly after ABD tested the RGR, Renishaw approached the company with its latest angle encoder, the RESR.

Here, the graduations are marked directly onto the low-profile stainless steel ring, increasing the accuracy and maximum speed capabilities, and extending the storage temperature range of the encoder ring.

Cars are often tested in "hot-house" conditions, for example in Spain or the Arizona desert, so the RESR was perfect for use within these applications due to its ability to withstand high temperatures.

"Fortunately the RESR and the RGH20 read head were easily interchangeable with the existing RGR and RGH24, and we fitted the prototype without any problems", commented Tony Best.

"The correct alignment of the ring and the read head is vital".

"On our robot we have clearance holes in the bracket that hold the read head in position that must be aligned in x, y and z rotationally".

"Here Renishaw has built in an LED that tells us when correct position is achieved, making setting up the system very straight forward indeed".

The steering robot is capable of a wide range of inputs, sinusoidal, steps and ramps at many different frequencies.

The company's latest version is more powerful than its predecessors, pushing up the speed and the torque, and offers a peak velocity of 1800 degrees per second.

Increasing the torque still further is an avenue currently under development.

In track-based testing with a power-steering-equipped car, you don't need more than 5 or 10Nm of torque to conduct most manoeuvres.

However, if the power assistance is disabled or a "catch up" test is conducted - spinning the wheel sufficiently fast to outrun the power-assistance - in effect running out of hydraulic push, then much higher torques are needed.

In "learn mode" the steering robot can be used to record a complete series of steering inputs from the driver, which can then be replayed as many times as needed under the robot's control to take measurements.

"We do a lot of testing and proving on an airfield, and installing the robot in a vehicle to take the measurements is a simple task", continued Best.

"A recent development is the addition of a path-following capability for the steering robot".

"In combining the robot with a 'motion pack' - a gyro and three accelerometers coupled with GPS - the robot can be made to follow a path or manoeuvre on the road".

"In doing so, we can ensure that the vehicle follows the required path to within a few centimetres".

"As the speed is gradually increased, very high lateral accelerations are generated allowing us to find the limits of the car".

Path following is already being used to establish criteria for "rollover", ensuring that cars driven by the public are not susceptible to the rollover problem that has hit the headlines over recent years.

In dynamic test measurement, the repeatable input and resultant data taken from a specific test requires the most reliable technology.

In some instances, as is the case of the RESR ring encoder, it results from a partnership between leading OEMs.

"Today's steering robots are testimony to the working relationship we have with Renishaw", concluded Tony Best.

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