How sensors enable information-based decisions

Optimize production, eliminate manual intervention and incorporate quality checks.

By Gary Highley, contributing editor

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Information is power. And it can come from data gathered with any number of sensing devices.

These three applications demonstrate the variety of sensors and the equally broad palette of uses for them. From replacing a linear variable differential transformer (LVDT) with a laser distance sensor in a nut-welding, error-proofing application to swapping above-the-conveyor reflector sytems for integrated light-bar sensors, as well as applying magnetic sensors to verify pneumatic-cylinder stroke length, these uses of sensors are a clear indication that machine-building innovation is alive and well.

Laser distance sensors

Paslin, a system integrator based in Warren, Michigan, designs and builds automated welding systems, gauges, tooling and fixtures. To support body construction operations in the automotive industry, Paslin builds a line of nut-welder machines. This particular machine is responsible for welding the nuts onto the front rail of the automobile frame.

During the operation, a flanged nut is placed into position at the position to be welded. A welding gun ram is then lowered onto the nut and the position is checked just prior to welding to ensure the correct orientation. On occasion, the nuts would get placed upside down, with the flange facing upward (Figure 1). Historically the method that had been used to verify the nut orientation was to measure the stroke distance of the ram when it lowered onto the nut. If the nut was inverted with the flange up, the distance traveled would be reduced by approximately ¼ in to 3/8 in, and therefore the control system would know to reject and replace that particular nut prior to welding.

“The machine as a whole uses a variety of sensors: proximity sensors for detecting the base part, air pressure and water flow sensors for validating the correct conditions prior to welding, as well as proximity for detecting the nut feeding cylinder and the status of the bowl feeder,” says Ron Pomaville, director of controls for Paslin. “The typical failure modes for the automatic nut feeder would be a missing nut, an upside down nut, a double nut, or a nut out of position. Using the stroke distance, the control system can determine if the gun is in the correct calibrated depth.”

Also read: How to get a better 'sense' with sensing

Historically, an LVDT had been attached to a fixed position on the machine frame and onto the ram of the machine in order to measure the stroke length. An LVDT is an electromechanical device used to convert the rectilinear motion of an object to which it is coupled mechanically into a corresponding electrical signal. LVDT devices typically use a nickel-iron core surrounded by primary and secondary windings to enable measurements as small as a few millionths of an inch.

While LVDT devices can achieve very high accuracy of measurement, they are mechanical devices and require a connection to both the fixed and moving portions of the machine. Paslin wanted to reduce the maintenance required on its machines due to environmental contamination and make it easier for both assembly workers and maintenance personnel to work on the machine. Obtaining this improvement, in part, meant transitioning to a different method for sensing the ram position and therefore the orientation of the nut.

Several alternatives were considered. Paslin decided to eliminate the electromechanical LVDT device and replace it with a Balluff BOD 21M laser distance sensor (Figure 2).

“The new application of the sensor is for using an analog laser sensor for detecting the distance of travel the weld gun makes prior to the weld,” explains Pomaville. “It is also helpful in non-operator applications to signal the robot that the gun is clear and the robot can re-position the part. This sensor was chosen over previous versions with LVDTs. The sensor was cheaper, easier to mount, easier to set up and easier to maintain, and it could still protect against all failure modes.”

Advantages of the laser sensor over the previously used LVDT device was that it only needed to be mounted on one fixed point of the machine—mechanical adjustment was no longer necessary—and it was resistant to any environmental dust and contamination that may occur. These sensors are wear-free and are available with sensing resolutions down to 30 µm.

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