Laser sensor takes position on crash test rig

Edited by the Engineeringtalk editorial team Sep 15, 2005

When Transport Research Laboratories wanted to update one of its test rigs it turned to JK Controls to provide the control and data capture equipment.

Transport Research Laboratories (TRL) in Crowthorne, Berkshire, performs a multitude of tests on vehicles of all shapes and sizes.

Offering testing services to both vehicle manufacturers and component manufacturers, many of the tests performed involve collisions to measure the forces when a vehicle is involved in an accident.

When TRL wanted to update one of the test rigs it turned to JK Controls to provide the control and data capture equipment.

JK Controls is a specialised control engineering company with a proven track record of supplying control and measurement systems to test laboratories.

The scope of supply included a PLC/Scada-based control system together with data acquisition and logging hardware and software.

The new test rig is designed to simulate the effects of a collision on vehicle components and attachments.

Consisting of a sled, which is accelerated down a short track (only 20m in length) and crashed into a dampened stop, the rig is designed to evaluate the effectiveness of vehicle subsystems in a typical vehicle impact.

Throughout the setup and performance of the test, the sled's position must be accurately monitored.

As the motion of the sled is produced by tensioned bungee cords, it was not convenient to use a mechanically coupled method to determine the position of the sled.

As a result, JK Controls needed to look for an alternative positioning solution - one that offered a noncontact method of operation, millimetre accuracy and, most importantly, a fast dynamic response.

The DME5000 laser distance measuring system from Sick (UK) proved to be the ideal solution.

This system, used extensively in the control of stacker cranes in automated warehouses, emits an eye-safe red laser beam, which is reflected by a reflector mounted on the sled.

The DME5000 uses the time-of-flight principle to determine the distance to the target.

As the DME5000 has a response time of less than 1ms, can operate at a speed of up to 10m/s and produce an absolute positional output with a resolution of 0.05mm, JK Controls found this measuring system to be ideal for its application at TRL.

In the TRL application, the DME5000 has two primary functions.

The bungee cords, which accelerate the sled down the track, are put under tension by pulling the sled up the track using a winch.

The position of the sled on the track at the point of release determines the speed the sled will be travelling at the point of impact.

This means that, during the setup of the test, accurate positioning of the sled using the distance measurement produced by the DME5000 is essential in ensuring that the required terminal speed is achieved.

When the sled is arrested one vital piece of information is the total stopping distance.

The stopping device will deform both plastically and elastically - the sum of these two being the total stopping distance.

The plastic deformation can easily be measured after the impact but not so the elastic deformation.

The depth of impact can be precisely recorded by the DME5000.

The test rig is capable of sending the sled down the track at speeds that exceed the DME5000's maximum speed of 10m/s.

However, in this application, the position of the sled whilst it travels down the track is not critical.

Of much greater importance is that when the sled speed is reduced by the collision, the DME5000 responds rapidly enough to provide the impact depth for recording.

Before utilising the DME5000 in this application, JK Controls performed extensive testing to simulate the speed reducing from high speed to less than 10m/s to determine how quickly the DME5000 would recover when the speed fell during impact.

JK Controls confirmed that within 2ms of the speed falling to within specification, the DME5000 produced the positional output.

As the impact duration is between 10 and 100ms this enables a number of readings to be recorded during impact.

The control equipment selects the value of deepest impact.

Now that the unit is installed and in operation, TRL is carrying out tests on various restraint systems, car seats and other vehicle subsystems to establish their behaviour during impacts.

The DME5000 is performing admirably and providing the vital data required to enable component manufacturers to improve the safety of their products.

So, although Sick's DME5000 cannot help the crash test dummies at the TRL, it is helping to save the lives of drivers and passengers in tomorrow's cars.