Jaguar senses danger

Jaguar Cars are using a new long distance sensor to measure impact speed in pedestrian crash simulations at their engineering centre in Coventry. The Micro-Epsilon laser triangulation sensor promises to lead to more accurate tests and better pedestrian safety, which is now an increasingly important aspect of new car design and safety protocols under Europe’s NCAP (New Car Assessment Programme).

The NCAP legislation is focusing on reducing lower leg injuries, concentrated around the knee joint, and head injuries. As part of testing at Jaguar's Pedestrian Test Facility, bio-mechanic models are propelled towards a static test car at pre-defined speeds to replicate real-world impact conditions. 'The impact speed must be within a very tight tolerance band,' explained Miles Dadson, project engineer at Jaguar Cars in Coventry. 'Non-attainment of the required speed results in incorrect contact energies and therefore incorrect injury calculations. If the tolerances are not achieved, the implications on vehicle design can be enormous.'

Testing standards recommend measuring speed with two lasers mounted in parallel, 50mm apart, acting as light gates, but Dadson said that arrangement had several drawbacks.

'First, there are difficulties in laser positioning for true parallel lines. Second, there are difficulties in laser positioning at first point of contact. Then there’s the susceptibility of lasers to airborne vibration and they are vulnerable to impact damage from bio-mechanic models. Also, inaccuracies are created by measuring very high velocities [up to 40kph] over very small displacements [50mm].'

An alternative measurement principle was explored at Jaguar Cars, with preference being given to non-contact measurement. The device decided on was the Micro-Epsilon displacement sensor. The laser-optical sensor uses triangulation to determine displacement and post-test differentiation analysis to determine speed.

Dadson said the Micro-Epsilon laser triangulation sensor was the only device in the marketplace that could meet the criteria laid down by Jaguar Cars. The sensor focuses a laser spot onto a charged coupled device element, where the focal position determines the displacement with an accuracy of 0.01 per cent full-scale deflection. This system enabled Jaguar to take a measurement along the axis of travel as a linear analogue rather than as a digital measurement, which would have been the case with the two-beam laser system.

The sensor type selected by Jaguar Cars was Micro-Epsilon’s long distance sensor, the ‘ILD 1800-500’, with a displacement of 500mm, operating at ± 250mm from a central reference point. The laser device has a ± 5VDC output at ± full-scale output, and a frequency response of 2,500Hz. 'At the time, this was the fastest frequency response laser available on the market for the displacement range we required,' Dadson said.

Chris Jones, managing director at Micro-Epsilon UK explained how the sensor uses optical triangulation as the measuring principle. The visible laser used is rated as Class 2, he said, and the optical displacement sensors measure with a large reference distance and a small measuring spot diameter. From there, a digital charged coupled device array is used as the position-sensitive measuring element.

As part of the guidelines observed by vehicle safety testing, all data, which includes the acceleration, free flight and impact phases of the test, has to be collected at 10,000Hz.

Dadson explained that initially the laser sensor was tested at Coventry using a linear quasi-static ram, with a conventional, linear potentiometer mounted to provide ram displacement and the laser sensor mounted in parallel. To test the suitability of the laser for dynamic testing, he said, the pedestrian facility ram was setup with a target located on the planar end of the ram, normal to the axial beam of the laser, with the laser located at the forward end of the ram stroke, effectively firing the ram at the laser. The actuator was set to deploy at 11.11m/s and a sample of 30 deployments carried out over three test programmes. The actual ram deployment speed was recorded as a single attribute value from the rig control system, whilst the laser displacement was recorded as a time history triggered from rig deployment.

The speed was produced by differentiating the displacement. One benefit of using differentiated speed, Dadson explained, is that only a few values at the end of the ram stroke are required, whereas an integrated speed would require the complete time history.

When filtered with a two-pole, Butterworth 250Hz low pass filter, over the three test programmes, all distributions from the ram accelerometer were mirrored by the laser sensor response.

According to Dadson, the laser sensor measured the displacement of the ram as 36.1cm during fire phase; the actual ram displacement was 36.0cm. From the tests carried out, the error indicated by the laser was 0.28 per cent from desired deployment speed, the error indicated by the ram control system being 4.17per cent.

Over the three test programmes, the calculated difference between the ram control system and Micro-Epsilon’s laser sensor was 96 per cent of ram value, the ram overstating the propulsion speed by four per cent, whereas the actual error was less than 0.5 per cent. When using an adult, lower leg form of mass 13.8kg, an error of four per cent with an overstated impact speed of 11.11m/s, results in an undershoot error of 66.8J.

Whilst the laser sensor showed the ram deployed slightly lower than the required tolerance, the feedback from the ram control system would induce the operator to reduce the deployment speed still further by a nominal four per cent.

Dadson said it is not intended that the laser sensor is used for all deployments, but rather as a preventative maintenance tool used every 20 deployments to check the ram and any drift in the accelerometer control loop. 'More importantly,' he said, 'it serves as an independent verification tool for both accelerometers and for accelerometer-derived velocity algorithms. This will be used between launch rigs, not only for pedestrian impact testing, but also as interior head impact testing for effective impact damping of interior vehicle components.'