The PLS can consist of a remote keypad, and output modules in ac, dc, or analog configurations. The system can be ordered in a NEMA 4X or 12 enclosure for protection.

Programmable limit switches (PLSs) convert rotating shaft motion, or linear motion that was changed to rotary motion, into signals that control machine events or processes. These microprocessor- based devices have functions similar to those of programmable controllers (PLCs). Because their programming is simpler, though, they scan I/O faster, which lets them relieve PLCs of some motion control functions.

Cam evolution

PLSs have their origins in the mechanical cam switch. The mechanical cam helps synchronize process operations to the speed of the machinery. However, it is not dependable at high speeds because of switch contact bounce, mechanical wear, and switch arm deformation. In addition, the thin metal cams are often hard to adjust – if they are adjustable at all.

Replacing those mechanical components with electronic-based shutters and photocouplers improved the cam's responsiveness. The resulting electronic cam switches are better able to handle high-speed operations. But switching resolution is relatively coarse. The interaction of the shutter and photocoupler assembly limits the resolution to plus or minus a degree. Another drawback - adjustment of switching setpoints is not possible during operation.

Incorporating a microprocessor and a feedback device into the switch refines the resolution. Such a programmable limit switch meets the positioning accuracy needs of high-speed applications, such as metal cutting and assembly equipment, automated welders, lasers, and vision equipment. It is also used in material handling in palletizers, depalletizers, and conveyors, as well as related switching, turning, and transfer operations.

Finding its way

The PLS uses either a gray-code encoder or a resolver to give the microprocessor absolute position information of rotating motion, even if power to the sensor is interrupted. The microprocessor can mathematically manipulate these data, opening possibilities for offsetting position information to create a new "virtual" position. Through the encoder or resolver, the switch establishes the zero point anywhere in the shaft rotation. With a resolver, the scale factor, or number of "counts" in one full revolution, can be changed – from 1 to 4,096 counts per revolution – to suit the application.

Because of the microprocessor, the PLS offers capabilities not available on electronic cam switches. For example, you can adjust setpoints with the machine in motion, make program changes from a remote location, and adjust machine speeds during operation.

The switch to PLS

Though several of their functions are similar, programmable limit switches and PLCs solve different machinery control problems. PLCs provide a programmable method of relating the operation of sensors, limit switches, selector switches and other input devices to output components, such as solenoid valves, motors and pneumatic cylinders. Through ladder logic, sequential function charts, flow charts, and other languages, you change machine functions without rewiring the system.

During execution, the PLC performs a repetitive cycle of operations. It scans input devices and updates a memory table that indicates their status. Next, as it processes the control logic, it updates another memory table that indicates whether output devices should be on or off. Then it uses this table to change the condition of the output devices. The time it takes for this cycle varies from a few to a maximum of 10 msec, depending on the control program.

The programmable limit switch, on the other hand, performs accurate control (to ±0.09 deg in some resolverbased models) of repetitive high-speed machine functions. These functions often correlate with a rotating shaft, so the PLS microprocessor spends less time polling inputs. By nature of its firmware, it typically does its task in 300 μsec. Thus, with these shorter scan times, the PLS can more readily compensate for changing machine speeds than a PLC.

Like programmable controllers, the PLS executes machine logic functions by using inputs to initiate outputs. However, it does not translate programming code, such as ladder logic, into actions. Instead, it uses pre-programmed logic sequences, called modes, to direct action.

Each mode governs a number of output channels that must operate together within a framework of the machine process. One input bit initiates a mode. It's because each mode handles a limited number of outputs that scan times are fast.

The switch directly controls highspeed machine functions, while the PLC coordinates low-speed processes, as well as the operation of the PLS. Thus, many industrial control systems can benefit from combining a PLC with a PLS.

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