Reliable gripper status signals can detect gripper faults when they occur, allowing automatic, orderly stopping of the process to so that problems can be corrected before scrap is produced or machinery is damaged. For grippers operated by linear-acting prime movers such as built-in pneumatic cylinders, there are two primary approaches to determining gripper jaw position open/closed: indirect and direct.

Indirect sensing with magnetic field sensors
The position of the actuator that moves the gripper jaws is monitored, giving an indirect indication of gripper jaw position open / closed.  This is accomplished by means of magnetic field sensors installed on the exterior of the built-in pneumatic cylinder of the gripper.  The air piston inside the aluminum-walled cylinder of the gripper is manufactured with a magnetic ring factory-installed around its circumference.  The magnetic field sensors detect the presence of the magnetic ring as it moves past them, causing them to change their outputs from low to high.  The magnetic sensors are typically mounted in extruded slots on the outside of the cylinder, so that they can be infinitely re-positioned along the axis of motion to set up the exact switch point required.  Normally, two or even three sensors are installed to indicate:

• Jaw fully open
• Jaw fully closed
• Jaw closed on a part

For miniature grippers, very compact magnetic field sensors are available that allow detection of extremely short piston strokes, for example half an inch or less.
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Direct detection with inductive proximity sensors
The open / closed status of the gripper jaws can also be monitored directly using miniature inductive sensors that detect the metal of the jaws themselves, or metal targets attached to the jaws, such as screw heads or brackets.  This technique is useful when the indirect method cannot be employed (for example when the piston has no magnet installed or when the cylinder walls are made from ferromagnetic steel instead of aluminum or non-magnetic stainless steel).

Miniaturization of inductive proximity sensors allows them to be successfully applied to today’s increasingly miniaturized grippers.  Designers can select from the latest inductive proximity sensors with diameters as small as three millimeters and overall lengths as short as six millimeters.  These devices are now fully integrated and do not require bulky external amplifiers.
Find more about inductive sensors