If you want to know where you're going, it helps to know where you are. For machines engaged in motion control, satisfying that truism starts with resolvers and encoders. These devices measure position and velocity, information that's fed into the controller to output commands needed to control machine movement. Selecting the right solution for a particular problem involves answering questions about the environment, type of position information needed, and mounting and connectivity options.
The environment is the first consideration, as can be seen by the basic technology used in a medical setting. Such locations often require any system be immune to and produce as little electromagnetic interference as possible. That, in turn, can dictate what type of encoder to use.
"The absolute optical encoders are typically used in medical applications, such as bariatric beds and scissors lifts, as well as in the vicinity of the CAT scan that rotates around you," says Cory Mahn, senior product engineer with encoder and resolver supplier Dynapar.
Dynapar explains that a resolver is "a special type of rotary transformer that consists of a stationary stator and a rotor that moves with the load. Voltage from the input winding couples to the output winding with a magnitude that varies as a function of angular position."
This construction makes a resolver a simple, inherently absolute, highly reliable position feedback device. There are no onboard electronics", making resolvers the position-information device of choice for harsh conditions, such as temperatures up to 200 °C, elevated radiation levels or a high degree of contamination, as well as settings with high shock and vibration.
A downside of resolvers is that translating their output into useful position information is left up to the machine maker or end user. That can be challenging, which is why encoders are often used.
Also Read: Infographic: What Is Your Primary Machine Motion Source?
An encoder provides position information by capturing the passage of an optical or magnetic mark past a point. The mark could be on a shaft that turns in response to movement, so it can be used to track motion. Optical encoders tend to offer higher resolution, with more pulses generated, for instance, per revolution.
"Higher resolution is beneficial in speed control, especially at lower speeds," Mahn says. "If there's a lot of rotation between edges of pulses, you can get velocity ripple between edges, and it's difficult for a drive or motion controller to keep that speed steady."
For an encoder, the first basic choice is between optical and magnetic methods, with the selection often dictated by the environment. If strong magnetic fields are present, or if it's necessary to minimize any electromagnetic interference, then an optical method might be the best choice. On the other hand, in dusty settings, an optical system could quickly fail, while a magnetic encoder continues to function.
Beyond environment, the next consideration is the type of motion to be monitored and what the manner of feedback should be. The motion is classified as either linear or rotary. The second has two basic categories: absolute and incremental. An absolute feedback device gives independent position information, while an incremental one provides current information relative to a previous position. Selecting between these alternatives comes down to what information is required for motion control.
"Do you need to know the actual position of the encoder when you power on and power back off again? Or are you just looking for speed information?" asks Mandee Liberty, encoder product specialist at Sick. The company offers many encoder and motor feedback products.
The type of location information needed varies with different applications. It might be important to know how fast a conveyor belt is moving, but not the position of a particular point. On the other hand, a robot using an absolute encoder can pay dividends in shortening start-up times and reducing power consumption. By knowing where it is in a motion cycle, a robot can stop in mid-movement, and avoid having to rehome at start-up and shutdown.
It's also the case that knowing absolute position over time allows calculation of speed. The most basic absolute-position encoders only provide position information. In contrast, the latest generation of these devices, particularly those using some sort of fieldbus, are incorporating velocity calculations, adds Liberty.
With regard to price, absolute encoders are generally more expensive than the incremental variety. That's because the absolute encoders are a bit more complex than the equivalent incremental device.
The final set of considerations involves mechanical and networking options. With regard to networking, the output can be a standard incremental signal over HTL/TTL, or it can be transmitted over Ethernet, a newer approach that is gaining favor. The choice of interface also sets the refresh rate of the position data.
As for mechanical options, there are a number of choices for shaft and flange size. Other elements of the mechanical package determine how well the encoder operates in settings that are vibration-prone, dirty or wet. "There are varying degrees of robustness within different families," Liberty says. "Some are considered heavy-duty, and some are considered light-duty."
At Dynapar, this distinction shows up in how well encoders are sealed against the elements. A heavy-duty encoder has NEMA- or IP67-certified protection against such environmental assaults as washdown by water jets, according to Dynapar's Mahn.
In general, the higher the IP rating, the greater the attention paid to seals and coatings. That's the only way to bolster ingress protection and, therefore, the IP value. For instance, instead of a simple gasket, the system might use multiple levels of interlocking flanges. These are often partnered with complex seals. The combination can drive up cost and increase size.
In some situations, there's little choice because the motion control being tracked is taking place in a particularly demanding location. Dynapar, for instance, offers products that it classifies as light-, industrial- and mil-duty in terms of their ability to withstand harsh conditions. The last category can be used outdoors, illustrating how robust these systems can be.
Speaking of this, Mahn says, "I know of an application where an SL series is installed on a drawbridge. We're tracking the motor speed and possibly the angular position of the drawbridge."