Spin, look, and measure
Martin Rowe, Senior Technical Editor- May 1, 2005
|Read other articles from this issue:Table of contents, May 2005
Built for the long haul
Test fast clocks on the cheap
Spin, look, and measure
DEVICE UNDER TEST
Dimmer controls for automotive dashboards. Each control consists of a knob with an embedded light bulb. The knob attaches to a potentiometer. A 2-cm x 3-cm area in the center of the knob illuminates for night use.
Develop a test system that performs a visual inspection of each knob, measures torque and electrical resistance and current, and measures light intensity of the backlit area in the center of the knob. The system must turn the knob and keep track of its position over a 200° arc.
Galil Motion Control: motion-control card (PCI bus). www.galilmotion.
Kistler: torque transducer and amplifier. www.kistler.com.
Kollmorgen: motor. www.danahermotion.com
National Instruments: PCI bus data-acquisition and frame-grabber cards, graphical programming language, and vision software. www.ni.com.
Pentax: camera lens. www.pentax.com
Renishaw: motor-position encoder. www.renishaw.com
Sony: industrial camera. www.sony.com
When a manufacturer of automotive dashboard controls needed an automated manufacturing and test system, the company hired Meikle Automation (Kitchener, ON, www.meikleautomation.com) to develop and deliver it. The system consists of a manufacturing portion run by a programmable-logic controller (PLC) and a test portion controlled by a PC. The test portion uses a camera, a frame grabber, and vision software to capture and analyze multiple images of the control under test. A 2-cm x 3-cm area in the center of the knob lets light from a bulb pass through to indicate function to a driver. Data-acquisition and motion cards measure the torque, resistance, and angle of each control.
|An automated system performs a visual inspection and measures electrical parameters of automotive dimmer controls.|
The system starts in calibration mode where it monitors the control's torque through 200° of rotation. "We calibrate out the torque induced by the mechanical components of the system," said software team leader Dean Mills. "The encoder gives us enough pulses to produce a relatively constant velocity because it removes any jitter from the motor."
An analog-input channel in the data-acquisition card digitizes the torque sensor's signal, which the software continuously monitors. When the control reaches the end of its arc, torque rapidly increases. The system halts the motor and reads the encoder's value, thus finding the position of the control's minimum electrical resistance. The system then rotates the control clockwise until it detects the other end of the arc. From those two points, the system knows how many steps it needs to traverse the range (the system tests several models of control, which have varying numbers of steps).
Throughout the rotation, the system continuously monitors torque, resistance, and angle. The system decides if the part passes or fails based on the torque reading being between a minimum and maximum range throughout the curve, the potentiometer resistance linearity compared to a master resistance curve, and the minimum and maximum resistance at the extreme ends of travel. In addition, the overall angle of rotation must fall within a minimum and maximum range.
The PC-based system turns the DUT at 60°/s, and it tests a part in 4.5 s to 5 s. The previous system, which used a PLC for testing and for manufacturing, turned the DUT at 20°/s. The new system, in operation for over 18 months, uses a PC for its test portion, which is far better suited for the testing tasks. The PC also performs the visual inspection in parallel with the electrical measurements, something the old test system couldn't do with a PLC. "With the PC, we can take images without waiting for other events to conclude," said Mills. "The previous system performed one measurement at a time."
"The hardware works very well, with torque measurements accurate to within 0.001 N-m. Our software lets us tie data acquisition, digital I/O, instrument I/O, and interfacing with a programmable-logic controller that drives the assembly line," said software engineer Ben Zimmer. "The system has run 10 hours a day for over 18 months, testing a new part every 10 seconds."