Better Sensor Testing

Gems Sensors & Controls, of Plainville, Connecticut, designs and manufactures a broad portfolio of liquid level, flow, and pressure sensors, miniature solenoid valves, and pre- assembled fluidic systems to exact customer requirements. The large number of configurable products, and the company’s high production volumes, create complex testing requirements. For example, many AC and DC voltages and resistance measurements need to be performed on the large number of different liquid level sensors that are built on a flexible production line.

Jeff Neuner, senior test engineer for Gems, overcame this challenge by developing an innovative testing application that scans a barcode to identify the part number and product configuration information. The test application software uses this product information to create a custom test profile. The application takes advantage of the versatility and speed of the 8845A precision multimeter from Fluke Corp., located in Everett, Washington, to handle many different part numbers and easily keep up with the production pace. The new multimeter has replaced three instruments that were required in the past, which saves floor space, simplifies test system architecture, and reduces maintenance expenses.

The production line supports Gems’ Series 16 general purpose and Series 26 low-water cutoff liquid-level sensors. Series 16 sensors are engineered for general purpose single-level or differential applications, while Series 26 are for boiler low-water product cutoff applications.

In the past, Gems used separate dedicated devices to measure resistance, AC voltage, and DC voltage. These devices occupied a considerable amount of space on the production floor and had to be separately maintained and calibrated. In addition, the operators needed to learn how to operate several different measurement systems. Gems has improved production testing by switching to the Fluke 8845A multimeter, which handles all measurements required to test the many different types and configurations of sensors produced on this line. Production capacity and productivity have doubled as a result of the new automated testing strategy.

“The extremely wide measurement ranges provided by the 8845A multimeter enable it to perform all required measurements for all of our Series 16 and Series 26 products as well as many of our other families of products,” says Neuner. “A few of our products require resistance measurements as small as a few milliohms. The wide measurement range of the 8845A handles them with ease.”

There are many advantages to working with a single instrument that can handle all electrical measurements on a production line. “We save space, and the operator just has to learn and monitor a single instrument,” says Neuner. “The programmer’s job is also simplified by working with a single instrument. We program in a modular way, which frequently makes it possible to save programming time by re-using modules. For example, once we have created our main instrument driver, we can use it anywhere.”

The sensors are designed to run at a number of different power sources, including 24 V AC, 120 V AC, 208 V AC, and 240 V AC. The various power source options are supported by jumper selection of the appropriate primary winding on a multitap transformer. The 8845A measures the resistance of the transformer’s primary winding during the pretest to make sure the operator has made the right cut.

Prior to the final test, the barcode on the sensor is scanned to determine the part number and product configuration. A pictorial representation of the sensor’s PC board displays on a monitor, informing the operator which jumpers to remove for proper power-source selection. The operator is then graphically prompted to place the sensor in the appropriate bed-of-nails fixture. The operator inserts the sensor and closes the lid. The bed-of-nails tester contains resistors and relays that are used to apply voltages and resistances required by all part numbers produced on the line.

To avoid damage to the sensor, a resistance measurement is first taken on the primary winding to verify proper jumper configuration for power source selection. The proper voltage is then applied to power up the sensor. The 8845A measures the voltage to be sure that the device is powered up. Then an AC voltage is applied across the relay. The AC power across the relay contacts is measured throughout the next segment of the test to determine when the contacts close and open. The relays used in some sensors are designed to switch to DC power, so they are tested with DC.

Next, a fixed resistance is applied across the probe to simulate a conductive liquid. Some sensors are designed to close the relay contacts immediately. Others are designed to close the contacts only after a certain specified delay. This avoids tripping the relay in the situation where the liquid is below the level of the sensor but occasionally impinges on the probe as it sloshes around. Finally, the resistance is removed from the relay and another test is performed to be sure that the relay contacts open.

“The 8845A multimeters have been rock-solid performers,” says Neuner. “We have used them on this line for about two years. This is just one of a number of very successful applications of the 8845A in this plant.”

Fluke Corp. 8845A Precision Multimeter

Benefits:

• Fast and versatile
• Handles many different part numbers
• Replaces multiple instruments, saving floor space
• Wide measurement range

http://us.fluke.com [1]