By Jim Kerr
Bang! It sounded like a shotgun going off beside my ear as I watched the occupants slam into the dash during a 25-mph collision. I could have been watching at any number of icy intersections. Instead, I was watching Ford’s state of the art Servo-Hydraulic Crash Simulator, usually referred to as the Servo Sled, simulate the impact, all in the quest for better occupant safety.
Part of Ford’s Safety Innovation Lab, the Servo Sled simulates vehicle collisions while sensors measure the effects on crash test dummies. In traditional crash testing, a complete hand built prototype vehicle costing several times that of a production vehicle is propelled into a barrier. Obviously, this type of testing is very expensive and takes a long time to set up each test. The Servo Sled reduces both the cost and time it takes to evaluate the effects of a crash.
The Servo Sled uses only part of the body structure. It may use just the dashboard, or a door structure, or the cab assembly. Crash test dummies loaded with sensors are placed on seats in the body structure and the sled is loaded onto the rails. The sled is slowly moved forward into position and then, Bang! — it’s over, faster than you can blink.
The servo sled does not crash into a barrier. Instead, it is moved up to a huge hydraulic cylinder that is part of the Servo-Hydraulic Crash Simulator. Oil is forced into the cylinder under high pressure (560,000 PSI) through computer-controlled solenoids, so the cylinder rapidly forces the Servo Sled backward. This reverse thrust accelerates the sled from a stop to 35 MPH in about two feet and about .060 seconds. In comparison, an eye blink takes about 0.1 second.
The unbelted crash test dummies slam into the dash under the never-blinking lens of high speed digital cameras that capture every movement at a thousand frames a second. After the initial thrust, the sled coasts to a stop down the long rails of the simulator. New components can be bolted into the sled and the sled used over again.
Now comes the real work, as scientists and engineers analyse all the data and pictures so they can recommend changes and improvements to the vehicle passenger compartment components.
We witnessed only one test. The Servo-Hydraulic Crash Simulator is used several times a day. Some tests are used for unbelted dummies. Other tests have belted in dummies. Side impact tests can be simulated too. The amount of data to analyse must be staggering.
Ford’s Servo-Hydraulic Crash Simulator is one of two in North America. Until 2005, the simulator was used for Ford’s internal testing only. All Government specified testing was done on an older nitrogen-propelled test sled simulator called the HYGE. In 2005, government agencies recognized the accuracy provided by the Servo-Hydraulic Crash Simulator and now accept that data.
Ford still uses the HYGE test sled as well. This analytical tool has been used for more than 28,000 tests since it’s installation in 1966. Bet you didn’t know they were doing simulated crash testing back then! It was still used for more than 1000 tests in 2005. The HYGE sled can take up to two weeks to set up for a test. To simulate the varying rates of deceleration during an impact (structural parts collapse at a slower rate than softer sheet metal and plastic), an aluminum plug is machined with varying diameters that control the flow of nitrogen gas into the HYGE’s cylinder. This aluminum plug is bolted onto the cylinder ram so it moves and changes the flow rate as the ram moves.
The Servo Sled can be set up much faster – two hours instead of two weeks. The varying rate of the vehicle deceleration is now controlled by a computer program through the hydraulic solenoids. This significantly shortened set up time has allowed more testing and quicker vehicle development times. Another advantage of the Servo sled is that it can simulate the pitching action that vehicles undergo during a collision. Separate hydraulic cylinders on the sides of the Servo-Hydraulic Crash Simulator can move the sled up and down at different angles during the test. This simulated testing is as close to real life as possible. Most people, even those in the auto industry, don’t realize all the work that goes on behind the scenes to develop a new vehicle.
Ford’s Servo Sled and HYGE simulators are examples of vehicle development that could potentially save your life one day.