InSight sensor installed in tie-rod cylinder. The blue block houses prototype of sensor, which would fit into the cylinder’s end cap of production models. Control panel is mounted to cyliner, but can be mounted nearby or incorporated into automated controls.
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Brad Engstrand, president of Motion Controls LLC, Hartford, Wis., came up with a better idea. He knew that a sensor capable of providing continuous feedback of piston position would be a better solution because it would allow changing switching points without having to reposition a reed switch or make other physical adjustments. A sensor with continuous feedback (a linear displacement transducer, or LDT) could be used as a programmable switch, so different — or additional — switching points could be established simply be reprogramming a control.
New technology in sight
LDTs are, of course, used in pneumatic cylinders, but to a limited extent. Their main limitation is the challenge of achieving precise positioning with pneumatics due to the compressibility of air. Plus, non-contact devices add substantial cost to an otherwise economical cylinder. Linear potentiometers are more economical, but the physical contact inherent to their operation limits their life and reliability. Even if the application doesn’t require continuous feedback of position, the limitations of LDTs have kept them from gaining widespread use as programmable switches in air cylinders.
Engstrand developed InSight, a sensor that fits in the end cap of an air cylinder and requires little more than a drilled and tapped hole. The sensor contains a light emitting diode (LED) and light receptor. The LED transmits light into the cap end of the cylinder, and the reflected light picked up by the receptor indicates the relative position of the piston. As the piston rod extends, the reflected light picked up by the receptor decreases.
The complete sensor also consists of a control module that is external to the cylinder. This module contains the microprocessors necessary to regulate power to the LED and condition the feedback signal from the receptor. It also contains a small touchpad control panel for programming the sensor and an LED readout of piston position.
Programming is simple. The operator extends the cylinder’s piston rod to its first end position, then pushes a button on the sensor’s control module to lock in the position. Next, the operator retracts the piston rod to its second position, pushes a second button on the module, and locks in the second position. To program a new position, the operator simply extends or retracts the piston rod, then presses the appropriate button.
The InSight sensor can also be incorporated into controls that would automatically shift the cylinder’s directional control valve when the piston reaches a switching point. And because the sensor provides a continuous output signal, it could, conceivably, be used in a servopneumatic positioning system.