Gripping, Checking, and Measuring


Intelligent sensors transform actuators to sensitive helpers in automation

Sensors used in automation are essential organs of machines and plants. While gripping modules and other actuators performed pre-defined tasks in the past, today, they are intelligent and flexible helpers in the manufacturing process due to the use of intelligent sensors. They provide the plant with important information on the process, products or components as well as plant functions. This increases flexibility, shortens manufacturing time, ensures product quality, avoids production failures, and prevents the automation systems from damage.

The more quality requirements, variant diversity and cost pressure are increasing, the application of intelligent automation and sensor solutions becomes more important. Coordinated in accordance with the individual requirements and operational environment, they offer several possibilities for evaluation. They are becoming more integrated in the individual actuators and merge with the automation components to form a sensitive unit.

Inductive proximity switches withstand vibrations, dust, and water

Sensors in actuators are used for interrogating the condition of the component. In the simplest case an inductive sensor covers two possible conditions: "open" and "closed". By means of an oscillator coil, a high-frequency magnetic alternating field is produced. As soon as a metallic object enters this field, it detracts energy from the magnetic field. Thus, the oscillation amplitude diminishes. This change is recognized and the sensor switches automatically.

Electronic magnetic switches in the grooves do not protrude

Magnetic switches can be integrated completely into an actuator. Instead of interrogating the position of the gripper finger, the sensor interrogates the position of the piston inside the gripper, which is equipped with a small magnet. The sensor recognizes the approach of the magnets and depending on the type of contact, opens or closes automatically.

Magnetic switches of the latest generation (e.g. the MMS-P 22 from SCHUNK) are programmable and instead of recognizing just one switching point, they can recognize two. Moreover, the sensing positions can also be programmed and can be used for improving the process stability. Wireless sensor systems offer more safety for man, machine and process.

In certain surroundings or in case of special applications, sensor cables might be an obstacle. For machining centers, lathes or grinding machines, for crowded handling applications, and also for rough conditions the use of wireless radio sensor technology is a great option.

For modern high-dynamic machines, the cabling guide caused many problems. Cable disturbances are the most frequent reason for a malfunction of the production line. The trouble-shooting is complex and time-consuming. Particularly in manufacturing automation, the radio-based system for control of the fully automatic robotic loading is becoming more and more important. During this process, the sensors control the stroke of the gripper jaws and report the condition to a transmitter, which in turn transfers the information by radio communication to the receiver, which is connected to the control unit.

Instead of obtrusive sealings, cable shielding, strain reliefs, and the use of slip rings for rotational axes, the radio sensor systems overcome all mechanical obstacles.

A specialist for rough environments

Conventional sensor systems can be overburdened by high temperatures, a high risk of explosions, aggressive fluids or extremely high chip volumes. Air monitoring systems allow a precise and reliable statement on the present position or process status, if pneumatic actuators are used in extreme conditions. The actuator is merely equipped with two additional pneumatic connections. Via one single additional pneumatic measuring line and an AND-valve, the information "open", "gripped" or "closed" can be sent back to the control unit. This is done completely without the use of electronic sensors or additional cables.

Hall sensors for more than two switching points

If a sensor system provides between three and five independent switching points, the actuator or the gripping module gains additional functionality. The gripper can be used parallel for checking and sorting of components. Flexible positioning sensors determine the position of the gripper jaws and issue digitally in which of the five pre-defined position areas the jaws are located at the moment. This way, various components can be recognized and sorted automatically. The continuous quality control is also easy since good parts can be freely differentiated outside the given tolerance range.

Positioning system for miniaturized grippers

For miniaturized gripping modules the position is a special challenge. Due to their size, conventional sensor systems cannot be used. In the past, a detection of the gripper was only possible if a camera was available. In clean surroundings optical sensors can do this job now. They can send out light via a fiber optic cable that evaluates the quantity of reflected light. The sensor recognizes the distance to the reflection surface and thus the position for the gripper jaws.

Micron-precise precision sensors replace a complete measurement station

High resolution analog sensors are in the top class of positioning sensors. They transform a gripping module into a measurement station. With the precision of up to 0.003 mm, (0.0001 inches) the actuator measures the continuing process of each individual gripped component. By means of PLC, any number of switching points can be defined and thus any number of components or tolerance ranges can be differentiated. High resolution positioning sensors allow a 100% control of components and can replace a complete measurement station including the measurement of the necessary process steps.

The mechanical analog system, consisting of sensor and electronics, precisely registers the position of the gripper jaws. In case of long-stroke movements, the movement via an inclined plane is given over to the sensor, since its measuring path is short but has an extremely high resolution. Analog positioning sensors dispose of a considerably higher precision.

Balanced gripping force prevents the complete system from damage

While the sensor systems detect the individual positions of the actuators, force-measuring systems measure forces. On a gripper for example, one gripper finger is equipped with force measuring jaws. During the first teach-in process the gripping forces occurring at the fingers are determined. This data helps to avoid that the robot system is exposed to payloads that permanently occur from one-sided gripping and at each individual gripping operation. Even a minimum misalignment of the gripper of one to 200 mm would impact the robot arm at each gripping operation with the full gripping force in one direction. In the long run, the system will lose its precision and will wear out much faster as in case of a force-controlled access.

Moreover, with force measuring jaws, static as well as dynamic gripping forces can be precisely adjusted and controlled. This assures that the components themselves are safely gripped during high-dynamic movements. In regular precautionary maintenance with force-measuring systems, the proper function of the grippers can be inspected. Both increases the process reliability and avoids unexpected and expensive downtimes during operation.

Force measuring in every dimension

Automation components as well as measuring and test control units are becoming extremely sensitive when they are combined with force-moment sensors. These sensors determine which forces and moments are acting in all six directions. This way position misalignment of workpieces or tools can be compensated. Tools, workpieces and automation systems are also protected against overload. Force-moment sensors are suitable for rigid systems, and for pressing in components. They are used for quality control as well as in the medical technology such as control of denture, or the backbone load. Moreover, they are used in flexible systems for insertion, chamfering, or positioning.

SCHUNK offers the complete range of intelligent sensors

From magnetic switch MMS, radio sensors RSS to high resolution sensor system FPS, SCHUNK offers a sensor program adapted to your individual requirements. With state-of-the-art solutions such as pneumatic interrogation system PA3, the optical sensor ONS for miniature grippers, or the force-measuring system FMS, SCHUNK covers new fields of applications, where intelligent sensors can provide valuable help.

Conclusion:

In the future, integrated sensor systems will be standard of automation solutions. They open, with comparably low capital expenditures, a variety of possibilities for shortening manufacturing and assembly processes and make them flexible and reliable as well.

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