2. Sled test facility and equipment overview

A crash sled facility is not a single machine but a family of propulsion and control architectures, each producing a different acceleration–time history at the fixture interface. Selecting the right architecture determines whether a given restraint system can be evaluated at all, and how faithfully the resulting pulse reproduces the target vehicle crash event. This section examines the three dominant architectures — deceleration sleds, acceleration sleds, and servo-hydraulic systems — and explains how each generates, shapes, and limits the pulse delivered to the test article.

Deceleration Sleds

A deceleration sled stores kinetic energy in the moving mass and then removes it through a dissipative medium. The test article is mounted on a carriage that is first accelerated to a target velocity, then decelerated by a programmable energy absorber. The pulse shape is therefore a function of the absorber's force–displacement characteristic, not of an active actuator.

The most common energy absorbers are:

• Metal tube or strip benders — a steel strip is drawn through a set of rollers or a die; the plastic bending work produces a nearly constant force plateau. Pulse duration is set by strip length and roller geometry.
• Hydraulic or pneumatic dashpots — orifice-controlled fluid flow gives a tunable force–velocity curve, allowing a more rounded pulse.
• Crushable honeycomb or foam blocks — cheap and repeatable, but the force profile is essentially rectangular and difficult to fine-tune.

Because the absorber is passive, the achievable pulse is bounded by the energy the carriage carries: E = ½mv². For a 1500 kg carriage at 50 km/h (13.9 m/s), the available kinetic energy is approximately 145 kJ. If the target pulse requires a 40 g plateau for 100 ms, the average deceleration force is F = m·a ≈ 1500 × 392 ≈ 588 kN, and the stopping distance is roughly v²/(2a) ≈ 0.25 m. These numbers illustrate a key design constraint: high-g, long-duration pulses demand very large forces and long absorber strokes, which is why deceleration sleds are often physically massive.

Deceleration sleds are widely used for full-vehicle-level occupant restraint evaluation because they naturally reproduce the "vehicle stops, occupant continues" kinematics. Their main limitation is pulse-shaping flexibility: changing the pulse usually means changing hardware.

Acceleration Sleds

An acceleration sled reverses the energy flow. The carriage starts at rest and is accelerated forward by an external energy source — typically a pneumatic piston, a spring, or a linear motor — while the test article experiences the acceleration pulse directly. The fixture is mounted so that the sled's acceleration vector corresponds to the vehicle's crash deceleration direction.

Acceleration sleds are preferred when:

• The test article is a component or subsystem (seat, steering column, airbag module) rather than a full body-in-white.
• The required pulse is short and high-g, which is easier to produce by impulsive acceleration than by dissipative deceleration.
• The laboratory needs rapid turnaround between tests, since pneumatic and spring systems can be recharged quickly.

The pulse in a pneumatic acceleration sled is governed by the piston force and the moving mass. A simplified model gives a(t) = F_piston(t)/m. By profiling the piston area or the gas release, engineers can approximate a half-sine or trapezoidal pulse. However, the achievable velocity change Δv is limited by the stroke and the pressure reservoir, so very long-duration pulses are difficult.

Servo-Hydraulic Systems

Servo-hydraulic sleds combine an actively controlled hydraulic actuator with a closed-loop controller. The actuator can both accelerate and decelerate the carriage, so the system is not restricted to a single energy-flow direction. This makes it the most flexible architecture, capable of reproducing arbitrary pulse shapes within its force and stroke envelope.

The control loop works as follows: a target acceleration–time history is stored as a reference signal. An accelerometer on the carriage provides feedback. The controller compares the measured acceleration with the reference and adjusts the servo valve command in real time. The result is a pulse that tracks the target within a specified tolerance, typically ±5 % of peak g and ±3 ms in timing for occupant-safety pulses.

Key parameters of a servo-hydraulic system include:

The trade-off is cost and complexity. Servo-hydraulic sleds require significant hydraulic power, precise calibration, and a controller tuned to the specific carriage mass and fixture stiffness. If the controller gain is too high, the system oscillates; if too low, the pulse lags the reference and the peak g is underestimated.

Comparing the Architectures

The following diagram summarizes the energy flow and control structure of the three architectures, and shows where the test article sits in each.

Practical Selection and Verification

Choosing an architecture is a trade study between pulse fidelity, cost, and throughput. A useful first step is to plot the required pulse on an acceleration–duration map. If the target lies in the high-g, short-duration region (e.g., 60 g for 30 ms), an acceleration sled or a servo-hydraulic system is appropriate. If the target is a long, moderate pulse (e.g., 20 g for 120 ms), a deceleration sled with a bender absorber is often the most economical choice.

Once a sled is selected, the pulse must be verified before the test article is installed. The standard procedure is:

1. Mount a calibrated accelerometer on the carriage at the fixture interface.
2. Run a dry pulse with the target mass and absorber or controller settings.
3. Record the acceleration–time history and compute the velocity change by integration: Δv = ∫a(t) dt.
4. Compare the measured pulse with the target in terms of peak g, rise time, duration, and Δv.
5. Adjust absorber geometry or controller gains and repeat until the pulse is within tolerance.

For a deceleration sled, the same verification applies, but the tuning variable is physical: strip thickness, roller diameter, or orifice size. A useful rule of thumb is that the absorber force should be sized so that the stopping distance is at least 60 % of the available stroke, leaving margin for pulse shaping and preventing hard bottoming.

Understanding these three architectures — and the energy-flow logic that distinguishes them — is the foundation for interpreting any sled test result. The next section builds on this by examining how the pulse is specified and how it relates to the real vehicle crash event.