Why an encoder can make your machine or process better.

Editor's Note: Jeff Christensen is director of feedback business for Danaher's Industrial Controls Group. Prior to joining Danaher he spent 16 years with General Electric, the last ten with GE Fanuc Automation — serving as president of GE Fanuc Automation's General Motion Control Business. He has a degree in electrical engineering from Baylor University.

While rotary and linear encoders are far from unusual, some designers do not consider them. Often designers don't realize that their application can benefit from an encoder, which may enable equipment to perform with better speed, accuracy, and efficiency, and do it at less cost.

This article will explore the different ways in which an encoder can lead to a better design than the available alternatives and do it at less cost. It also includes examples of encoders being used in places they might not be expected.

Expanding Uses For Encoders

The vast majority of encoders are used for feedback purposes. They're familiar components of servo systems, for example, where they provide highly accurate data on both position and velocity, but as prices have declined encoders are finding uses that before were handled by less-precise devices. Take a look at some examples of applications now being done with encoders that previously used other devices.

High-speed doors— In order to keep weather out and air conditioning and heating dollars in, factories often use strip doors so forklift trucks can enter and leave easily. A high-speed door operated by an actuator with an encoder to control its speed and position allows for better weather-tightness and much better security. Such doors can swing open, slide, or roll up.

Traveling saws— The manufacture of roof trusses for houses involves saws on high-speed gantries to cut the various components of the roof trusses. Encoders provide the feedback to control saw position.

Controlling table position— The position of the table in a magnetic resonance imaging (MRI) machine must be known exactly for proper results. An encoder measures that position and feeds it in digital form to the computer controlling the machine.

Controlling position of an overhead crane— An encoder provides precise feedback and requires no maintenance.

Kinematic analysis— Encoders on a weight training machine make it possible to analyze motion profiles for better training data.

Measuring and controlling speed of a camera— A Japanese company builds a remote-control video camera that can keep up with, and photograph, ski racers as they go down the slope at high speeds. The camera slides down a wire that runs down the slope at speeds up to 130 km/h (81 mph). An encoder provides the feedback to control the speed of the camera.

Robotics— Each wheel on the Mars Rover has an encoder to control its speed and monitor how far it has gone or what surface it is on — smooth rock or sliding sand and dust. In an earth-bound but no less exciting application, encoders are used to keep track of the position of an experimental mine-clearing robot.

Aligning optics— Linear encoders are used in the LIGO (Laser Interferometer Gravitational Wave Observatory.)

Operator Input

Encoders are finding increasing use as front panel controls, because most modern electronic equipment contains an internal computer. A unit that costs less then $5 can replace a front-panel potentiometer and provide anywhere from four to 36 steps per revolution. Many 32-position optical encoders are used as jog inputs for machine tools, as setpoint inputs on factory automation equipment, as tuning and volume controls on stereo equipment, and as operator inputs for medical equipment, test and measurement equipment, and onboard GPS navigation systems.

Encoders In Step Motor Systems

Step motor systems usually operate open loop, but under conditions of hard acceleration or heavy loads they can miss steps or even stall, creating an unrecoverable position error. An encoder attached to the stepper will keep track of true position and send corrective commands to the control equipment.

Alternatives

The main alternatives to encoders are potentiometers and resolvers, while in linear motion systems limit switches can be used.

Potentiometers have been used for many years as feedback elements in servo systems. Since a potentiometer is an analog element, it interfaces well with old-style analog servo systems. Unfortunately, this is also one of the potentiometer's major drawbacks. It has no digital output, so it cannot be used in a digital servo system. Its other drawback is that it uses a mechanical contact sliding on a resistive element, which is subject to wear and contamination.

The other main alternative to an encoder is a resolver, which is a motor-like device that produces two ac output voltages proportional to the sine and cosine, respectively, of the shaft angle. Resolvers got their start during World War II, where they were used in the aiming systems of naval guns and other artillery. Resolvers are still used as antenna position sensors in some military radar systems, partly because they are extremely rugged and partly, perhaps, because of tradition.

A resolver gives a highly-accurate absolute position output, but it requires a resolver-to-digital converter than may cost more than the resolver itself. In addition, a resolver is more expensive than many encoders. Finally, a resolver does not provide as great a speed range as an encoder.

There are, of course, other ways to control machine motion that do not involve an encoder. The simplest is a limit switch. Put a limit switch at the desired stopping point; when the machine reaches it its state changes and the drive shuts off. While inexpensive and reliable, a limit switch has two major drawbacks: it's not especially precise, and its reprogramming interface is a wrench.

A somewhat more flexible (and non-contact) alternative to a limit switch is a photoelectric sensor, which, like the limit switch, is essentially a one-bit device (on or off). Changing setpoints requires mechanical repositioning, and photoelectric sensors also tend to be struck by moving objects and knocked out of adjustment.

It's better to let the photoelectric spot items whose position cannot be predicted exactly, like boxes on a conveyor belt, and let an encoder keep track of machine elements. The encoder makes it possible to enter equipment stopping points with digital precision and change them quickly and easily, which makes for easier and faster equipment setup.

As equipment of all kinds has become more digital, it is natural to choose an inherently digital device to input position and speed data. Fortunately, today's rotary and linear encoders have become so inexpensive that they can be the first choice for most applications, where they can do a better job than the available alternatives, and do it at surprisingly low cost.