DIY enthusiasts

A new type of test sled is set to improve the safety of everybody, from children in cars to wheelchair users to Formula 1 drivers. It has already influenced the way train passenger safety is being developed. The sled has been designed and built by the world-leading Transport Research Laboratory (TRL) at the company's new £40m research centre in Crowthorne, Berkshire.

The story behind the project is how self-sufficiency left TRL in control of its own destiny — and its own IP.

The firm's existing drop-weight sled was already working to capacity so TRL's safety test specialists made a list of the design improvements they'd like a new design to incorporate, including rapid turnaround between runs to deliver answers quickly and make best use of the facility. Then TRL tried to identify who could design and build it.

As with all businesses, finance is the bottom line for TRL, regulating its investment in facilities and activities. However, since it was privatised in 1996 the company has been wholly owned by the Transport Research Foundation, a not-for-profit distributing organisation overseen by 80 members from the transport industry. Profits from TRL are invested in its own research programmes which are selected by the TRF to enhance knowledge in the critical areas of safety, environmental impact and sustainable development. So developing the sled was a hard-nosed commercial project.

'It soon became clear that none of the commercially available designs provided the level of sophistication we want to offer our customers,' said Peter Bignell, TRL's engineering services manager. 'There are several highly accurate and consistent products but we felt that the market had not really taken advantage of the many technical advances now available for making them easier to set up and use, or to integrate the electronic gathering of data.'

So TRL decided that the best possible supplier was TRL itself because it knew how to get the best rig for the money. The traditional path to high accuracy and repeatability is to use either a pneumatic sled or a drop-weight sled similar to the one already installed at TRL. Yet for the new sled TRL wanted a much lower-cost solution that would allow its clients to conduct more tests more quickly yet without compromising accuracy or consistency. The answer was to take proven bungee sled technology a leap forward and integrate it with the latest electronic control and data collection systems.

Traditional bungee sleds use lengths of synthetic rubber that are tensioned as the sled is pulled along the rig to the propulsion end of the rails. The system then fires like a catapult, hurling the sled down the rails to the impact face at the far end. This is low cost compared with pneumatic and drop-weight rigs and convenient to use. But the nature of the bungees means that it has always, until now, been less accurate and consistent. 'Bungee repeatability can be a problem,' said Bignell. 'And repeatability is extremely important, particularly for the motorsport sector.'

Several factors that affect bungee performance cannot always be controlled easily. Temperature, humidity, the age of the bungee and how recently it was used all impact on consistency. Combined, these factors could lead to variations in impact speeds greater or less than one per cent, a figure that was way outside the research-quality accuracy needed by TRL's engineers. So they called on their collective experience of bungees.

Their new design uses similar bungees but in an innovative configuration that TRL is patenting. It allows the tension to be applied to both ends of a substantially longer length of bungee. It is, in fact, twice the length of traditional bungees for the same length of rig. The extra length and tensioning provide a much smoother release of energy, which cuts the risk of dislodging test pieces on the sled through rapid and uneven accelerations that can happen with less sophisticated bungee systems. Also, there are 12 bungees available and any number can be selected for tensioning to optimise sled acceleration.

Consistency is achieved through a closed-loop learning control system that uses load cells in the tensioning mechanism to calibrate the system during every pull-back. A laser system tracks the position of the sled during pull-back and during the impact, recording any elastic deformation of the test piece — a task traditionally performed with a piece of modelling clay, a spike and a ruler.

In-house expertise in bunjees allowed TRL to slash costs and develop technology it can sell to others

TRL developed a unique optical coupling system to replace traditional electro-mechanical systems for synchronising data gathering on and off the sled. It means that up to 94 channels of on-board data can be collected at 20kHz.

Speeding up the turnaround has been achieved by a computerised control system that manages the entire test procedure, including calculation of pull-back to minimise time-consuming pre-test calibration runs. 'With no set-up changes on a test piece, the minimum time between tests is 10 minutes — but that is extremely rare,' said Bignell cautiously. Other time-saving innovations include speed timers in the track and in-situ weighing via load cells in the rails.

Most of the design work was done in-house, as was the data-logging synchronisation system. But the rig's control system tendered out and contractors assisted with construction.

So how much did TRL save by designing and building its own solution? 'We came out at about 60 per cent of the commercial market costs,' said Bignell. The firm also has valuable IP in components of the 20m rig and is offering its design services and expertise to other test houses.

The new sled delivers a speed accuracy of plus or minus half a per cent with a maximum speed of 25m/s — and it is already in demand. Even before it is fully commissioned we have significant work in child seat testing, wheelchair tie-down testing, restraint system development, rail occupant protection and motorsport testing,' said Bignell.

Max Glaskin