An example test system
A recent test system Sinclair developed using the Delta RMC151 was for MGA Research, a test engineering company. The RMC controller in the MGA test system was fitted with a digital I/O expander card that was multiplexed to control 128 relays for configuring and selecting external feedback inputs (Figure 2) enabling the RMC to coordinate all I/O controls.
"The Delta RMC is very configurable and flexible," said Sinclair. "With minimal external hardware, we were able to enable this test system to support a huge array of external inputs."
Different feedback options can be used depending on the sensor type: ±10V, 4-20mA, magnetostrictive linear displacement transducer (MLDT), or strain gauge. Each test unit also has an external in/out trigger to support daisy chaining of sensor networks such that the sensor data is synchronized to within a control loop interval.
The channel switching logic and analog signal conditioners are all contained on a special printed circuit board.
Figure 2. Schematic diagram showing how the sensor inputs are multiplexed into the Delta controller.
Figure 3. Plot of testing force versus time produced by MGA’s application software on the PC. The data was obtained from the motion controller.
"This PCB allowed the combination of all of the point-to-point wiring between the RMC, power supplies, input and output connectors signal conditioners to be reduced to a few simple 1:1 wiring harnesses," said Sinclair. "This dramatically reduced the labor needed to build these systems."
To configure the system to perform a seatbelt test, the motion controller is connected directly to load cells, measuring actual force on the end of the hydraulic cylinders, and MLDTs to measure cylinder position (one system uses string potentiometers for this purpose). Flow to the cylinders is controlled precisely by servo quality proportional valves. The typical test involves establishing a load profile via instructions that are sent from the PC to the motion controller, which causes the hydraulics to pull on the shoulder and lap belts at the same time to a defined poundage for a period of time and then relax.
During a test sequence, software in the PC sends a series of force ramp commands to the Delta controller at specific times and collects data on both the force and displacement during the test. "Certain tests we perform will have a deflection limit," said Rodrick Pierce, an engineer with MGA Research.
"A fixture that moves more than a certain distance away from its starting location when the test load is applied is considered to have failed."
The RMC controller has a finite capacity for test data storage inside the unit, and test application software on the PC can read this data over the Ethernet link with the motion controller and generate a plot (Figure 3). The plots produced by the system document the amount of force that is applied to the belt or the belt’s displacement over the test cycle, and can be printed or exported as Excel spreadsheets.
Reducing costs, increasing flexibility
Test system designers should consider coupling a motion controller to their preferred PC software platform. "Using a PC platform along with a motion controller gives test system manufacturers the ability to hit their performance targets while reducing costs and increasing flexibility," said Sinclair.