Using sensors to make grippers more productive

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:

• Direct - monitor the jaws themselves
• Indirect - monitor the mechanism that moves the jaws

Direct (inductive proximity sensor)

The open/closed status of 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 material).

 
Figure 1: Proximity sensors sense tabs on moving gripper jaw mechanism to indicate fully open or closed position.

Indirect (magnetic field sensor)

The position of the actuator that moves the gripper jaws can be monitored by a sensor, giving an indirect indication of whether the gripper jaw position is open or closed. Two magnetic field sensors installed on the exterior of the built-in pneumatic cylinder of the gripper give jaw position. The air piston inside the aluminium-walled cylinder of the gripper is manufactured with a factory-installed magnetic ring around its circumference. The 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 precisely positioned along the axis of motion to set the exact switch point required. Normally, two, or even three, sensors can be installed to indicate jaw fully open, jaw fully closed, or jaw closed on a part.

 
Figure 2: Magnetic field sensors sense magnets internally mounted on gripper mechanism to indicate open or closed position.

Analog signals can also be used to monitor the operating efficiency of the gripper mechanism - for example, if there is a supply line air leak or a failing jaw mechanism, the gripper will operate more slowly. This abnormally slow motion can be detected by analysing the rise and fall time of the real-time analog signal in the control software.

For grippers driven by linear-acting prime movers, there are again two primary approaches to determining gripper jaw actual (analog) position: direct and indirect.

Direct (analog inductive proximity sensor)

 
Figure 3: Analog inductive proximity sensor continuously tracks position of gripper jaws.

An analog inductive proximity sensor can be configured to directly sense the linear motion of the gripper jaws. The sensor outputs an absolute analog position signal that is directly proportional to the motion of the gripper jaws.

Another method is to fit an angled or sloping metal target that moves with the jaw. An analog inductive proximity sensor provides a high-resolution voltage or current output that is directly proportional to the target distance from the sensor, and thus its signal is directly proportional to absolute jaw position.

The sloping metal target is used to translate a 25-50 mm jaw motion into a smaller target displacement that is within the relatively short sensing range (typically one or two millimetres) of the analog inductive sensor.

Because the sensor is an absolute rather than incremental device, after a power interruption the analog inductive sensor turns on and supplies the correct actual jaw position signal without any need for rehoming the gripper jaws (which might be inconvenient if there is a part already held in the jaws).

 
Figure 4: Analog inductive proximity sensor continuously tracks position of gripper jaws by reading distance of wedge face from the face of the sensor.

Indirect (analog magnetoinductive linear position sensor)

Instead of discrete magnetic field sensors, an analog magnetoinductive linear position sensor is fitted to the gripper cylinder housing in order to continuously sense the internal magnet mounted on the pneumatic actuator piston. Rather than on/off position signals, the magnetoinductive linear position sensor outputs an absolute analog position signal (such as 4-20 mA or 0-10 V) that is fed back to the control system.

Part present or absent in jaw

Detecting the presence or absence of a part in a gripper jaw can provide key information about the status of an assembly or production process.

 
Figure 5: Magnetoinductive sensor tracks position of internally installed magnet to provide constant position information.

There are two convenient and reliable sensor technologies used for detecting parts in grippers - inductive proximity sensors and self-contained photoelectric through-beam sensors.

Part-in-jaw detection using inductive proximity sensors

If the part being handled is metallic such as sheet metal, machined parts or castings, inductive proximity sensors can be integrated into the gripper jaws to directly sense the metal part. Sensors used in this manner are typically low-profile, block-style housings commonly called ‘flat packs’. The flat pack housings can be either plastic or metal, but in any case, the sensor must be high quality and designed to withstand repeated shock and vibration. The sensor cable should also be flex rated for continuous flexing in the application without internal conductor fatigue and breakage.

 
Figure 6: Flat-pack inductive proximity sensor provides positive part-in-jaw verification.

Self-contained through-beam photoelectric sensors can detect part presence regardless of material. Certain versions can even detect clear glass or plastic. Various light sources can be employed (eg, infrared, visible red or visible red laser) depending on the characteristics of the part, the operating environment (clean or dirty), and the required precision of the part positioning in the gripper.

Self-contained through-beam sensors are available in U-slot or L-style configurations. U-slot types require the part to be introduced from one direction, while L-style sensors can accept part introduction and removal from multiple directions. Available in a variety of physical sizes, self-contained through-beam sensors can be fitted to virtually any size or style of gripper.

Key considerations for selecting gripper sensors

When selecting sensors for gripper applications, some key points to consider include:

Figure 7: U-shaped photoelectric sensor provides positive part-in-jaw verification.

Wear-prone versus wear-free

Electromechanical limit switches are prone to mechanical failure and contamination of their electrical contacts. Magnetically actuated reed switches are prone to contact sticking and welding. When applying sensors to grippers, it’s important not to introduce additional failure modes into the process. Unreliable sensors can be as bad as no sensors at all, and can derail your smart gripper initiative. Always look for wear-free, solid-state sensor technologies such as magnetoresistive, inductive, magnetoinductive, capacitive and photoelectric.

Temperature stability

Look for sensors that maintain stable sensing characteristics despite changes in temperature within their rated operational range. Sensors with switch points or signals that drift with temperature can create all kinds of set-up and adjustment headaches.

 
Figure 8: L-shaped photoelectric sensor provides positive part-in-jaw verification.