Crashing success in system control

Checking out car safety systems needs careful control, MIRA chose a system that has increased the reliability and efficiency of its facility.

The HyGe facility at MIRA (Motor Industry Research Association) is used to test airbags and seatbelts in crash situations and to analyse other safety characteristics by observing crash dummy behaviour. To do this, a vehicle bodyshell on a sled is shot down a track at high speed, propelled from a standing start by a large airgun, and the resulting data is collected for analysis.

MIRA decided to upgrade the facility control system. They chose to use the Opto 22 Mistic 200 industrial controller from System Devices to act as the brains of the system.

Accelerometers detect the rate of speed change on the sled and the facility control system provides a pulse at the desired point in the acceleration profile to deploy the airbag. In a crash, rapid deceleration causes the airbag to inflate but since it is the change in velocity that is important a rapid acceleration gives the same effect. The testers are interested primarily in the first few milliseconds after the gun has fired, when the sled reaches a speed of approximately 50km/h within a metre.

The pneumatically controlled gun which propels the sled consists of a load cylinder, a set cylinder and a hydraulic system. The hydraulic system moves a floating piston which provides varying volumes of air to the load side of the gun, while the set side contains different amounts of oxygen free nitrogen. Between the compartments is a plate that remains in place as long as the pressure on each side is in a 6:1 ratio; a small reservoir of air is used as trigger pressure to break the seal and unbalance the system. When this happens, air shoots out through an orifice controlled by a pin: the size and shape of which determine the flow characteristics of the air as it pushes the piston which propels the sled. Different pins allow different types of crashes to be simulated. It is also possible to perform static deployment of the airbag to evaluate the performance of the airbag system.

Upgrading the system to Mistic, which cost some £250,000, has brought a number of improvements to HyGe: greater utilisation, sophisticated environmental logging, ease of operation and improved safety. Joe Jurado, a higher project engineer at MIRA and responsible for the upgrade says: ‘The technical assistance we received from the engineers at System Devices was immeasurably helpful in getting system implemented.’

Because the old system leaked frequently, operators were continually tweaking settings up to the firing time. Mistic controls the demand and also checks the existing pressure in the gun, adjusting it accordingly. By setting the pressure and maintaining it, Mistic ensures a greater repeatability, and removes the need for so much operator intervention, so high pressure equipment is kept away from the operators for improved safety. Mistic also controls the audible warnings, and the interlocks on the doors leading into the firing room.

An MMI (man machine interface) – an Opto 22 package – is a practical way of communicating with the Mistic controller. Joe Jurado has written a custom front end for it using Visual Basic. Mistic can launch external applications that blend in seamlessly with the overall control functions.

All tests are measured from T0, a point at which the test has begun but there is no velocity. This point is established by a velocity unit that consists of two laser beams controlled through an RS232 link. What is actually measured is the speed between the two beams. Mistic handles all key aspects of the test area: volumes and load, set, trigger and brake pressures. Mistic also controls the high speed photography, and the lights for this are run through four sets of I/O, and are turned on just prior to firing the gun. The lights are positioned on the left and right of the camera, as well as overhead.

Although MIRA fires the airbag, companies do use their own sensors to perform various tests. But they often make use of data from MIRA: humidity and temperature are monitored using Mistic and stored in a historical log so that customers can use it for their own purposes and to verify that the airbags or crash dummies were tested in the correct environment.

System Devices. Tel: 01462 670633.