This month’s feature article is about position feedback devices for shaft-based automation.  These devices include optical encoders, resolvers, hall-effect encoders, and rotary cam limit switches. They are used on manufacturing equipment including compaction presses, punch presses, and assembly machines.  They are mechanically coupled to the shaft of rotating machinery.  The shaft could be a main line shaft or the output shaft of a motor, clutch/brake unit, or a gearbox.  Machine functions, like the actuation of a solenoid valve or the activation of a servo motor, depend on the shaft position angle (0 – 360 degrees) for proper timing.  The timing signal is sometimes referred to as a ‘cam window’ or a ‘cam time’.

Optical Encoders generate signal-level electrical pulses as they rotate.  They have an LED that shines light through a slotted disc attached to its shaft.  The light pulses are received by phototransistors and then counted electronically.  In order to determine shaft position angle, there is a marker pulse that resets the count to zero once per shaft rotation.  Therefore, rotary encoders are incremental devices in that they do not provide an absolute position.  Advantages of optical encoders are that they are less expensive, less complex and have fewer output signals to wire.  Disadvantages are that they will lose position after a power outage, they are not recommended for high-shock/high-vibration applications, and electrical noise will corrupt the position value until the next zero-marker pulse.  Resolvers look very much like optical encoders and are sometimes referred to as absolute encoders.  Basically, they are rotary transformers with one rotor winding and two stator windings.  As the shaft rotates, the output voltage of the stator windings is directly proportional to shaft angle position.  The output voltage is typically converted (by auxiliary electronics) to a format compatible with a PLC, for example BCD (binary-coded-decimal).  Resolvers were developed during World War II for military applications.  They have the advantages of being very durable for harsh environments where heavy shock and vibration occur.  They are also absolute devices, meaning they will not lose the position value after a power loss and during coast-to-stop after a power loss.  The main disadvantage is its higher cost compared to other rotary position devices. For optical encoders and resolvers, the ‘cam windows’ are written into the PLC logic with a compare function (GT, LT EQ, NE, etc.).  Rotary Cam Limit Switches are very simple devices but are limited to a small amount of ‘cam window’ signals.  They are an enclosed unit with a shaft that has mechanically adjustable switches wired to PLC inputs.  These devices are well-understood by experienced mechanics who can make timing adjustments without the need for PLC programming software.  Hall Effect Position Devices/Encoders are similar in concept to the optical encoder in that they count pulses and have a zero marker.  The pulses are the Hall Effect device usually detecting the teeth of a timing gear mounted to a shaft.  These devices are capable of very high rotating speeds.  They are relatively inexpensive to apply and can be used in severe conditions.  Another common application of Hall Effect Encoders is the input signal for the RPM control on high speed servos (for example ones used on a CNC machine’s spindle).

Tips on Being Creative

Creativity is very important in any organization; whether it is to find ways to reduce costs, increase sales, process a new part, or create new products.  In his book ‘Take the road to creativity and get off your dead end’, David Campbell (1977 Argus Communications) says that ‘You’ve got to combine experience, energy, and planning to turn on the idea light bulb’.  One chapter is dedicated to the phases of creativity.  1.) Preparation involves learning the complete background of the situation.  2.) Concentration requires long periods of total focus.  3.)  Incubation is taking time-out from the endeavor.  4.) Illumination is when you get the answer(s) or the ‘AHA!’  5.) Verification / Production are where the details are developed and the bugs are worked out.