Integration and industrial applications of position feedback

Editor’s note: Read about how the perspective from how the sensor works internally in hardwired fieldbus applications.

In the ensuing years from the early 1990s to 2010 or so, many programmable logic controller (PLC) manufacturers came out with instructions that automatically applied the Gray code by mapping the input points into an instruction that produced the Gray-coded value without hard-coding the process. Encoders still use Gray code to maintain accuracy and stability, but we are lucky, in that most encoders communicate on a fieldbus, and we can access registers to give us position and speed, as well as the ability to change the relationship between the encoder counts and the driven device, including considerations for gearbox and pulley ratios, if they exist, via the encoder interface on our programming device.

We can also change the resolution of the encoder and even change it from linear to angular. This last feature is very useful for applications with machinery where we traditionally relate one machine cycle to be 360° of rotation. Timing of the various machine functions can then be activated using a direct value or, more often, a window of activation during a machine cycle. This method can also be used to have a device activated more than once in a cycle, like lobes on a cam shaft.

Carrying the use of an encoder further, we can program an encoder to reset—pass through zero—at a different point than 360°. With a horizontal cartoner, for example, we could have four pouch-making machines feeding the cartoner. While the packing/closing cycle at the cartoner would happen within one machine cycle of 360°, the moving conveyor feeding the packages to the cartoner will be bringing product from four sources.

In order to make sure that each pouch machine only feeds into a single bucket, we would put an encoder on that infeed conveyor using pulleys to multiply the overall cycle count to four times 360°, or 1440° of revolution. With 1440° being four individual buckets of linear travel, each pouch-making machine would be prompted to insert product only during its own window of that 1440° total cycle. Pouch 1 at 0-359°, Pouch 2 at 360-719°, Pouch 3 at 720-1079° and Pouch 4 at 1080-1439°. 1440° would be the same as 0°.

Another suitable application for an encoder is to perform product tracking. Let’s say we have a sortation system where we have packages travelling down a conveyor belt. Depending on the identity of the package, we want to divert the package down a specific side spur for further processing. Each identified product would be assigned to a specific lane or one could assign several similar packages to sort to a particular spur for further processing.

In this example, each package has a barcode that needs to be read to determine the destination of the package on the sorting station. As one might imagine, it would get very expensive if we had to have a barcode reader before each of the sorting spurs to interrogate each package and decide if it needs to be diverted off the main line. This is where the encoder comes into consideration.

If we locate a scanning station before the sorting station, we can use the results of each inspection to assign that product to a corresponding divert station. The encoder would be mounted on the shaft of the drive roller for the main conveying line. The relationship between the rotation of the drive roller and the linear distance traveled by the belt being driven by the roller gives us a predictable value. Applying that fixed distance to the length of each divert station from the scanner station, we can determine how many encoder counts relate to that physical distance in linear feet.

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Each package, as it leaves the scanner station, will get an associated encoder count to represent the dropping point for the desired sorting spur relative to the starting point. When the main line encoder count reaches the proper value, the diverter at that relative position will turn on for a window of time to divert the package off into the sorting spur.

One prominent use of encoders these days is in collaborative robots. Each joint in a robot arm contains a motor, gearbox and an encoder in one package. This combination provides a high degree of accuracy allowing robots to perform precise motions to handle anything from large components down to extremely small chips in electronic devices.

These examples show some of the potential uses for an encoder. As with all technologies, encoders have gotten faster and smarter while fitting into increasingly smaller packages. The earlier optical encoders were of significant diameter and could only be used in situations where there was plenty of room around the device. Commonly available encoders for industrial automation are now just 1-2 inches in diameter and with M12 connectors that allow for easy installation pretty much anywhere. Offered in shaft and hollow-shaft versions, encoders are adaptable to most applications.

While we often think of encoders as a device about the size of your palm, encoders can also be incredibly small, as used in small robotics, consumer electronics and medical devices.

Some manufacturers of variable frequency drives (VFDs) have even made versions that allow for feedback via the connection of an encoder, essentially turning a VFD into a less expensive version of a servo drive without the additional cost. These, by nature, aren’t nearly as accurate as a full-on servo application, but they can add some sophistication to movement where fine accuracy isn’t needed.

The cost of encoders and the ease of use by way of fieldbus connections make them an excellent choice for inclusion in a controls package. The potential applications are seemingly endless.