Putting sensors in their place

12/01/1998


The conveyor winds through a maze of processing machinery. The challenge is knowing that a pallet is in place before activating machinery. What is the right sensor? What is the appropriate installation?

Typical sensors for verifying presence and position of objects include limit switches, proximity switches, and photoelectric switches. Selection of the proper sensor depends upon the nature of the object to be sensed and mounting constraints.

Limit switches, the oldest type of sensor, are electromechanical devices consisting of a switch body and actuator arm. They must be positioned close enough to the part for the actuator to touch. The target pushes the actuator arm until internal contacts are made. Drawbacks to limit switches include repeatability and wear.

Noncontact sensing
Proximity switches are noncontact sensors, usually not built to withstand repeated contact with the target. Their small size allows mounting very close to the target, which is good because they have a limited sensing range. A parts handling system that limits target movement is best.

Manufacturers provide a nominal sensing range specification. Actual sensing range may vary byrrous only, and nonferrous only.

Photoelectric sensors use light as the detecting medium and are capable of a wide variety of applications. Because of this, allowance for the environment must be made when specifying range. A chart showing margin versus distance sensed is a useful tool to assure satisfactory operation.

Three kinds of photoelectrics
Photoelectric sensors are available in retroreflective, transmitted beam, and diffuse modes.

Retroreflective sensors are most commonly used. This system includes the device, which includes both the light source and receiver, and a reflector. Position the device aligning with the reflector. When the target breaks the light beam the contact changes state. This sensor is ideal for wide conveyor applications where the targets may vary in size.

If the distance to be sensed is large, or if more power is necessary due to a dirty environment, then the sensor of choice is transmitted beam. This system has the light source and receiver in different devices adding power and light margin.

Diffuse photoelectrics are similar to proximity switches. Since the target serves as the reflector, these are useful where mounting a reflector or second unit would be difficult. Wide angle diffuse sensors detect small objects over a wide area. Fixed focus diffuse sensors detect a target at a fixed point--useful for detecting precisely placed objects, while ignoring other parts or background.

Whatever the application, there is a sensor to assure products are in the proper position. Just pick the right sensor and put it in its place.

Sensor Types

  • Limit switches--electromechanical, actuator arm must contact the target.

  • Proximity switches--noncontact sensor, detects ferrous, nonferrous, or all metals.

  • Retroreflective photoelectric--has light source and sensor in one unit, uses a reflector.

  • Diffuse photoelectric--light source and sensor in one unit, uses target as reflector.

  • Transmitted Beam photoelectric--light source and sensor are in different units, most powerful, longest range of this type sensor.





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