Experts at the Center for Advanced Product Evaluation (CAPE), a unit of advanced vehicular safety systems manufacturer IMMI, design and build test rigs to evaluate how fire trucks, ambulances, on-road heavy trucks, and school buses survive crash test scenarios. The test scenarios are designed to determine whether there is survivable space inside the vehicle, and whether the vehicle’s mounting system to its frame is sufficiently strong to withstand a rollover incident. The tests that are performed are typically designed to prove that vehicle manufacturing processes comply with standards set by organizations such as the National Fire Protection Association (NFPA) or the Society of Automotive Engineers (SAE).
The engineering team at CAPE recently completed the development of a test rig for emergency vehicles and school buses that can provide vehicle manufacturers with roof crush testing up to 100 tons. The unit can also be used to test off-road vehicle roll cages and race car chassis. At the core of the new test system Delta Computer’s eight-axis RMC150 electrohydraulic motion controller (Fig. 1).
1. The RMC150 can control up to eight motion axes simultaneously, plus it can serve as a data acquisition subsystem in test applications. A built-in Ethernet interface is provided for uploading test data from registers inside the controller.
Machine Elements Operate in Harmony
CAPE integrated Delta motion controllers into two other test rigs before the new roof crush system was developed. “We use Delta RMC motion controllers because of their performance for servo-hydraulic control,” offers Ryan Hoover, CAPE technical director. “Delta RMCs exhibit superior stability for test applications, and the software is very professionally developed and finished. It has been our experience that other suppliers too often provide buggy software.”
The test system uses four hydraulic cylinders mounted at the four corners of a heavy-gauge pressure plate (Fig. 2) and controlled as four separate motion axes. Cylinders—each with a 6-in. bore and 48-in. stroke—from Parker Hannifin were chosen and are operated by Parker D1FHE80 proportional directional valves rated for flow to 20 lpm and each mounted to a manifold right on each cylinder. “The valves feature spool-bushing construction with zero overlap and fairly high frequency response, making them an excellent fit for this project,” says Hoover.
2. The CAPE roof crush test rig operates with hydraulics cylinders at each corner of a pressure plate. The photo at top shows a school bus in the test rig, and at bottom the test rig set up for the body of a wildfire-fighting apparatus.
These are standard valve models used at CAPE. They provide about 100 Hz frequency response in the critical force modulating region of spool motion, more than sufficient for these load tests. CAPE incorporated best hydraulic practice by mounting the valves on the cylinders. This optimizing hydraulic stiffness by minimizing fluid volume between the valve and the cylinder. Doing so permits higher gains in the Delta RMC motion controller, maximizing our test capability.
Position feedback comes from Balluff linear variable displacement transducers (LVDTs) with SSI serial connections to the motion controller. The LVDTs were mounted external to the Parker cylinders. “We would have used position transducers internal to the cylinders, but they would have conflicted with the cylinders’ rear clevis mounts,” explains Hoover.
CAPE engineers used a special function of the RMC151 motion controller called “Virtual Gearing” to cause all four axes to move in precise synchronization to ensure that the pressure plate is kept completely level during a compression operation. The four slave axes follow a virtual master axis that is set up to control the position of the pressure plate and the cumulative force being applied. The motion controller controls the cylinders’ compression force using a load cell mounted on each cylinder rod end.
Besides the motion axes, the Delta controller gathers information on the deflection of the vehicle under test by tracking four reference axis inputs, which are connected to string potentiometers mounted to the body and test frame. In this way, the RMC functions as a multi-channel data acquisition device in addition to a motion controller.
The test machine’s hydraulic power unit (Fig. 3) uses a 2.75 in.3/rev piston pump and a 0.85 in.3/rev gear pump driven by a 15-hp electric motor at 1,800 rpm. Maximum operating pressure is set at 2,500 psi.
3. The hydraulic power unit of the testing system uses a variable-displacement axial-piston to generate pressures to 2,500 psi in a work cycle.
A Typical Test Cycle
The typical compression cycle starts with the hydraulic pump powering and the transducers initialized to zero values. The four compression cylinders are then set up to be geared together, and the system is given a command to move the steel pressure plate up and out of the way. The vehicle cab is placed in the rig, and the pressure plate is lowered until it reaches a position just above the cab but not touching it. The command is then given to preload the rig to 500 lb, followed by the command to apply the full load, a process that takes 1 to 5 min.
Full load capacity at 2,500 psi system pressure setting approaches 39,000 lb. The system is allowed to rest under load for 30 sec., and is then unloaded to 0 lb on the load cells. Finally, the pressure plate is moved completely off the cab. The test data is downloaded from the motion controller to a network drive over the RMC’s Ethernet interface.
Application development and tuning. Programming the motion steps was done using Delta’s RMCTools software. RMCTools is provided free with Delta’s motion controllers and enables programming the controllers using high-level commands, such as the Virtual Gearing arrangement mentioned before. Figure 4 is an RMCTools screen showing how programming the operation of the four corner cylinders is done by filling in boxes and selecting options from pull-down menus. As the figure shows, the velocity, acceleration, and deceleration rates can be set to cause the axes to start, stop, and move smoothly. RMC hydraulic systems use proportional servo valves to enable precise control over the closed-loop motion parameters.
4. RMCTools software can be used for both developing motion programs and setting up and monitoring tests in process.
Following programming comes tuning. “Initially, the test rig was shaking,” reveals Hoover. “Then we used Delta’s Tuning Wizard, another part of RMCTools, to get the system pretty close to where we wanted it to be.” After that, the CAPE team conducted fine tuning by operating the press plate up and down at various speeds. “We validated the tuning process by testing different cabs with different amounts of crushing force,” Hoover adds.
RMCTools as an Operator Interface
The motion controller in the CAPE test rig performs the data acquisition during testing operations and maintains all the test data internally. RMCTools software can do more than just motion program development; it can also handle test system operator interface functions and data transfer to an attached PC. “The RMCTools package is very capable for developing and running a vast array of tests,” says Hoover.
Figure 5 shows how the various test parameters can be tracked and displayed in real time. The black plot line is the total force being applied, and the upper red line shows that some roof crushing has occurred. The relationship between crushing force and amount of crush at all points in time is clear, and the plot serves as documentation of the complete test cycle.
5. The RMCTools Plot Manager enables the test operator to visually track the values of all transducers and the values of the geared axis parameters during a test.
“A Delta controller can serve as a powerful core of a flexible electrohydraulic test system,” concludes Hoover. “It is easy to program and tune for engineers, plus it offers the best performance once the system is implemented. The big factors are stability and eliminating steady-state error and overshoot. I’ve used a lot of alternatives and the Delta is by far the best controller I’ve used.”
Bruce Coons is regional applications specialist at Delta Computer Systems Inc., Battle Ground, Wash. and based in Cincinnati. For more information on Delta’s products and services, click here.