1-5 Introduction
Mechanical Cam Switches
Mechanical Cams
The PS-6244 Programmable Limit Switch electronically simulates mechanical cam
switches. A cam switch consists of a roller limit switch whose arm rides on a cam as
shown in Figure 1. The cam shaft is driven by a machine at a 1:1 ratio, so that the cam
switch turns on and off at specific positions in the machine cycle. Cam limit switches
have the following disadvantages:
• The roller, the cam, and the limit switch wear out.
• The machine must be stopped during adjustment.
• On/off patterns are limited, and changing the pattern may require replacement of one
cam with another. For example, a cam that switches on and off twice in one revolu-
tion would need to be replaced with a different cam if three on/off pulses per revolu-
tion were required.
• They cannot run at high speeds because of contact bounce and excessive mechani-
cal wear.
Figure 1—Basic Cam Switch
Programmable Limit Switches
PS-6244’s & Encoders
The PS-6244 Programmable Limit Switch uses a quadrature encoder (Figure 2) in-
stead of a cam to indicate machine position. A quadrature encoder uses optical discs to
generate streams of pulses that can be processed by the PS-6244. From the encoder
signals, the PS-6244 can determine shaft position, direction, and speed. The encoder
is usually coupled to a machine shaft at a 1:1 ratio so that one encoder shaft rotation
corresponds to one machine cycle. Encoders have no brushes, contacts, or any fric-
tional moving parts to wear out.
Based on the encoder signal, the PS-6244 Programmable Limit Switch turns electrical
circuits, or “Outputs,” on and off, simulating the mechanical roller limit switch. Because
the combination PS-6244/encoder system is completely electronic and has no frictional
parts, it offers several advantages over mechanical cam switches:
• Long service life with no parts to wear out.
• “On” and “off” points can be adjusted instantly from the keypad; there are no cams to
rotate or replace.
• Adjustment is possible with the machine running or stopped.
• Programmable logic allows complex switching functions that are impossible with
mechanical cams.
• Operation at speeds up to 1500 RPM.
