Edited by Leland Teschler
It is now possible to monitor the actions of pneumatic grippers through use of analog proximity sensors. Analog sensors that monitor the position of gripper fingers can report the position of the fingers over their entire range of motion. This contrasts with the information available from digital sensors, which generally can detect only when the fingers are fully open or fully closed.
The analog sensors used in such settings are generally either inductive-proximity or magnetoinductive types. Analog inductive-proximity sensors produce an output signal that is proportional to the amount of metal in their sensing field. A metal target can either approach the sensor axially to generate this signal or it can move perpendicular to the sensor axis. In the latter mode the sensor output is proportional to the amount of the sensor face that the metal target covers.
Magnetoinductive sensors use the change in inductance caused by an applied magnetic field as a sensing mechanism. They contain coils and an alloy material whose permeability changes linearly over the sensor's useful range when a magnetic field is applied. The coils are part of a resonant circuit. As the strength of the applied magnetic field changes so does the resonant frequency of the circuit. The change in frequency is converted to an output proportional to the change in position of a magnet attached to some target object.
Analog feedback from either of these two kinds of sensors can provide valuable information about gripper actions. For example, sensor readings can verify that the gripper has indeed picked up the right part. In the case of sheeted material, the sensor signal can tell whether the gripper has accidentally grabbed more than one sheet. Analog sensing can also note that the gripper is holding the part correctly, or that the fingers are applying the right amount of pressure. Finally, sensors can confirm that the gripper is in good mechanical shape, opening and closing to the right extent and at the proper speed.
There are three main ways analog sensors can provide this sort of information. The first involves the use of an inductive-proximity sensor and a ferrous metal tab mounted to the gripper finger. In operation, the tab traverses across the face of the analog sensor as the finger moves. Sensor output, usually a 0-to-10-V signal, is then proportional to the portion of the sensor face that the tab covers. Thus, the diameter of the sensor face determines the measurable range of finger motion.
Finger position applications typically employ high-resolution proximity sensors, those able to resolve finger position down to about 30 m or better. In practical use, electrical noise generally determines the minimum position change that the sensor can resolve.
The second method involves mounting a magnetoinductive sensor head in a T-slot on the gripper housing. The T-slot is normally reserved for a Hall-effect sensor. The analog sensor detects the movement of a magnet ring that is on the pneumatic-cylinder piston. The sensor output is proportional to the location of the magnet ring as the piston makes a stroke. In this manner the analog sensor can gauge absolute gripper-jaw position throughout the gripper's whole range of motion.
This method works best for stroke lengths ranging from 10 to about 50 mm. A point to note is that different commercial grippers contain magnet rings of varying strengths. Some magnetoinductive sensors have built-in teach functions that permit tailoring their response to the magnetic qualities of individual grippers.
The final method of sensing is to measure the amount of jaw travel directly. It is generally applied where stroke lengths are on the order of an inch to 1.5 in. Here a wedge-shaped target is attached to one of the gripper jaws. Its angled surface is in the field of an analog proximity sensor. The sensor detects gripper-jaw travel by noting the change in distance between its face and that of the wedge moving in front of it.
The metallic wedge is typically cut with a length and slope that lets the sensor output swing over its full range as the gripper jaws go from completely closed to full open. For example, a sensor able to detect a differential sensing distance of 1 mm would use a wedge with a 1-mm slope to produce a full-range output.