A Switch in Time

The First Programmable Limit Switch (PLS) Was Resolver-Based because of Encoder Limitations in High-Vibration Environments

By Loren Shaum, Contributing Editor 

The configurable switch might have evolved from the mechanical press industry. Adjustable switches were needed to manage functions during a press cycle. This was done with a relatively crude set of adjustable cams set to activate switches for various functions on the press or associated mechanisms such as dies, feeders, clamps and safety gates at a specific point in the press cycle.

As press speeds increased, more precision and a more-easily configured switch was needed.

The first programmable limit switch (PLS) was resolver-based because of encoder limitations in high-vibration environments. These early devices were somewhat costly given the electronics required to convert resolver signals to a meaningful data stream.

Early programming of these devices was complicated, but once configured, they operated faster and more precisely—with resolutions better than one degree of the machine cycle with quick-responding, solid-state switch outputs.

As encoder reliability improved from glass disks to more durable metal or plastic encoding disks, more-easily configurable digital switches were introduced for applications that required more than a 1:1 ratio— one revolution of the resolver equals one product-cycle— for each machine cycle. Today, the PLS is a staple for many types of highly cyclic machines that require synchronization of various functions.

Cieco offers a CPU-based K2000 PLS-Plus switch as a low-cost replacement for both rotary cam and fixed limit switches. A heavy-duty absolute resolver retains position location even on power loss. It has eight output switches and serial and Ethernet interfaces. “We focus on metal stamping and powder metal machines, so our PLSs have to be particularly rugged,” states Ron Cejer, president. Allegheny Electric Services, St. Marys, Pa. Allegheny uses PLSs to update and automate presses.

Ametek also makes a resolver-based PLS, the 2500 high-speed PLS. The traditional button faceplate is replaced by a rotary knob, a 2x24-character display and four pushbutton keys, to reduce setup time. For higher speed and accuracy, Ametek has the CPU-based Gemco 7500 UHS. “The 7500 was designed specifically for the unique requirements of high-speed diaper, napkin and other converting machines,” says Blake Cawley, sales manager.

Of its 32 outputs, 16 operate at 5 µsec scan time and are designed for the highest-speed applications. The remaining 16 outputs frequently are used in lower-speed applications that consume valuable I/O and processor time in the PLC. Parameters such as position, velocity and input and output status are passed to the host PLC. An input-capture stores the location where the inputs transition in every cycle for potential diagnostic purposes. The eight hardware and eight software inputs can be configured for 10 trigger modes to create registration and eliminate drift from occasional random triggering of input sensors.

The 7500 accommodates resolver or encoder input. “Where the switch cycle must match the machine cycle on a 1:1 basis, the resolver is the best choice,” continues Cawley. If the host machine does not have a 1:1 machine-cycle-to-product ratio, the 7500 includes quadrature encoder input.

Advanced Micro Controls has several versions of resolver-based, programmable limit switch modules that interface directly with PLCs.

Its 8213 PLS module for ControlLogix systems has up to 16 programmable outputs and 16 inputs that can be used via an external relay board and allows the device’s software features to become an integral part the controller functionality. “The most common applications are cartoning and case packing machines,” says Lou Brennan, business development manager. “In each of these applications, the PLS module controls the gluing cycle during carton pass-through, while the remainder of the PLC controls the line.” A normal update of the PLS module by the PLC is 1 msec.

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