Magnetic grippers can be a very feasible means of handling ferrous materials. The stainless steel plate in example would not be an appropriate application for a magnetic gripper because 18∼8 stainless steel is not attracted by a magnet.
Other steels, however, including certain types of stainless steel, would be suitable candidates for this means of handling, especially when the materials are handled in sheet or plate form.
In general, magnetic grippers offer the following advantages in robotic-handling applications:
- Pick up times are very fast.
- Variations in part size can be tolerated. The gripper does not have to be designed for one particular workpart.
- They have the ability to handle metal parts with holes (not possible with vacuum grippers).
- They require only one surface for gripping.
Disadvantages with magnetic grippers include the residual magnetism remaining in the workpiece which may cause a problem in subsequent handling, and the possible side slippage and other errors which limit the precision of this means of handling.
Another potential disadvantage of a magnetic gripper is the problem of picking up only one sheet from a stack. The magnetic attraction tends to penetrate beyond the top sheet in the stack, resulting in the possibility that more than a single sheet will be lifted by the magnet.
This problem can be confronted in several ways. First, magnetic grippers can be designed to limit the effective penetration to the desired depth, which would correspond to the thickness of the top sheet.
Second, the stacking device used to hold the sheets can be designed to separate the sheets for pick up by the robot.
One such type of stacking device is called a ‘fanner,’ and it makes use of a magnetic field to induce a charge, in the ferrous sheets in the stack. Each sheet toward the top of the stack is given a magnetic charge, causing them to possess the same polarity and repel each other.
The sheet most affected is the one at the top of the stack. It tends to rise above the remainder of the stack, thus facilitating pick up by the robot gripper.
Magnetic grippers can be divided into two categories, those using electromagnets, and those using permanent magnets. Electromagnetic grippers are easier to control, but require a source of de power and an appropriate controller unit.
As with any other robotic gripping device, the pan must be released at the end of the handling cycle. This is easier to accomplish with an electromagnet than with a permanent magnet.
When the part is to be released, the controller unit reverses the polarity at a reduced power level before switching off the electromagnet.
This procedure acts to cancel the residual magnetism in the workpiece and ensures a positive release of the part.
Permanent magnets have the advantage of not requiring an external power source to operate the magnet. However, there is a loss of control that accompanies this apparent advantage.
For example, when the part is to be released at the end of the handling cycle, some means of separating the part from the magnet must be provided.
The device which accomplishes this is called a stripper or stripping device. Its function is to mechanically detach the part from the magnet. One possible stripper design is illustrated in fig.
Permanent magnets are often considered for handling tasks in hazardous environments requiring explosion proof apparatus. The fact that no electrical circuit is needed to operate the magnet reduces the danger of sparks which might cause ignition in such an environment.