How to find the right encoder for your application.

Encoders are a diverse product category used in applications requiring position and/or speed feedback. Most applications can be satisfied by a few key selection criteria, such as measurement range, resolution, and mounting requirements.

For more challenging applications, many specialty encoder technologies are available. If your design project requires an encoder, you’ll need to choose one that meets your specific needs. Here are some key considerations to keep in mind to ensure the right encoder selection.

Incremental versus absolute feedback is the first criterion to be addressed when selecting a rotary encoder. Incremental encoders produce a pulsed signal as their position changes, resulting in fast and precise feedback. Absolute encoders are used to produce a unique position signal that describes the specific location of a system within its range of motion.

For systems where only speed feedback is necessary, incremental encoders are the easy choice as they are less costly, have a broader selection of designs, and are simpler to implement. When position feedback is necessary, it requires a deeper look at the pros and cons of incremental vs. absolute technologies.

Incremental vs. Absolute

There are five key features differentiating incremental and absolute technologies that must be considered, including range, resolution, calibration, availability and interchangeability, and cost.

Range: Incremental encoders can measure practically infinite movement ranges as they pulse per a fixed position change; the maximum pulse count is dictated not by the encoder but by the controller performing the counting. Absolute encoders have a fixed output range; when exceeded, they “roll over” to the start of a new signal level. Single-turn encoders roll over their output with each revolution, and multiturn encoders store a revolution count and include this data in addition to the single-turn data. Some absolute encoders require a battery to store this multiturn data through power loss. However, battery-less options rely on multi-staged gear sets, each with its own feedback tracks to retain multiturn data.

Resolution: Incremental encoders come in a variety of pulse per revolution (PPR) choices, with common choices ranging from single PPR (coarse resolution) to more than 16,000 ppr (fine). One of the most common PPRs used in industrial applications would be a 1024 ppr (10-bit) quadrature encoder (meaning two pulse tracks, A and B), which gives an approximate accuracy of +/- 0.1 degrees. Absolute encoders commonly come with single-turn resolutions higher than what is seen in incremental encoders. Common resolutions are in the 13+ bit range, often as high as 20 or 24 bit. This gives the ability to measure position changes of less than 0.0001 degrees.

Calibration: Incremental encoders can only provide pulses, so it is up to the connected controller to interpret where a system exists within a range of motion. A system that relies on an incremental encoder must have some method of homing or calibration, where the system will advance to a known location before it can “zero” its position data and proceed with normal operation. This routine can be unacceptably long for systems with a large range of movement coupled with slow motion (e.g., a large satellite dish that can take hours to home). Absolute encoders will output a unique signal anywhere within a system’s range of motion. This signal will still need to be referenced from a mechanical location; however, this is usually done only once during system construction, or later if some mechanisms are replaced and a new calibration is necessary.

Availability and Interchangeability: Incremental encoders are the most common style of encoder found in the industrial marketplace, with many brands to choose from and a huge breadth of shapes, sizes, and mounting styles. If a specific make or model becomes unavailable, they are usually easily interchangeable with another. Incremental encoders with special environmental ratings are also easier to find than similarly rated absolute encoders. Absolute encoders tend to be highly specialized and engineered for specific applications. Many combinations of signal types, communication protocols, single-turn and multiturn resolutions can be selected. Vendors often have limited pairings to select from, making finding interchanges impossible without significant re-engineering by the system integrator.

Magnetic encoders work via a sensor or sensors, such as the hall-effect type that passes over a series of magnets placed along a disc. (Image: Fouad A. Saad/

Cost: Incremental encoders are typically less expensive to use on simple applications; however, it is important to consider the total cost to implement when deciding on a technology. If a homing routine is necessary, the time it takes to design and implement these routines can be costly. There are also the costs of limit sensors, which may be expensive. Another often overlooked cost is the time needed to perform the homing routine if it takes time away from revenue-generating production. Absolute encoders tend to be more expensive as they typically have more complex components and embedded circuitry to handle the higher resolution, position data storing, and signaling protocol processing. It is important to note that the increased component cost can be offset by the benefits previously discussed.

A Comparison of Encoder Technologies

Figure 1. At-a-glance comparison of each encoder technology’s strengths and weaknesses. (Image: Motion)

Encoder selection consideration for your application requires a thorough understanding of the most common encoder technologies. These technologies have their own strengths and weaknesses (Figure 1).

Optical Encoders: Multiple categories of optical encoders are available, with both incremental and absolute options. However, they tend to share a few key characteristics. They measure position changes by detecting the existence and absence of light from an emitter (typically an LED or laser) as seen refracted through or reflected from a rotating disc with a predefined pattern of reflective or transparent lines. There can be multiple pattern tracks, each with its own sensor (such as with an incremental quadrature encoder that will have both A and B tracks) that are out of phase with each other by a half pulse, creating four times the resolution and allowing the interpretation of direction from the feedback.

For absolute options, a unique track with a non-repeating pattern can be referenced from a lookup table to determine where the shaft is located within a revolution. Once the initial lookup is completed after power-up, the encoder commonly uses the incremental track(s) to maintain faster update rates. Note that the absolute position is only available once movement has begun, so these are commonly referred to as pseudo-absolute.

Strengths: Low cost, high resolution, small sizes.

Weaknesses: Vulnerability to contamination, vibration, and condensation.

Magnetic Encoders: Magnetic encoders work via a sensor or sensors, such as the hall-effect type that passes over a series of magnets placed along a disc. These can have incremental and absolute varieties. The sensors detect oscillating magnetic field strength and interpret this into position changes. By implementing multiple tracks or “keyed” magnetic patterns, absolute position can be determined by the combination of sensor signals read. As the magnetic field can be read without motion, these can be truly absolute with no motion necessary before reporting the shaft position.

Strengths: Low cost, immunity to nonmetal contaminants, small sizes available.

Weaknesses: Low resolution, weak magnetic immunity, temperature variance that skews measurements.

Resolvers: Resolvers are a rotary feedback technology based upon an induced magnetic field generated by an alternating current in one conductor. The conductor then creates varying-strength alternating currents in secondary conductor windings that can be measured to interpret the rotor position. They are naturally absolute feedback devices; however, it’s important to note they are analog devices, so their accuracy will depend on the analog amplifier circuit they are connected to.

Strengths: Robust, reliable, no embedded electronics.

Incremental encoders can measure practically infinite movement ranges as they pulse per a fixed position change. (Image: pisitpong2017/

Weaknesses: Bulky, expensive, more complicated to interface with.

Inductive Encoders: Inductive encoders function like resolvers except that printed circuit boards (PCBs) are used instead of coil windings to generate and measure the induced currents. PCBs save space and allow more flexibility in shape and design. Due to printed circuits, there can be multiple tracks with varying winding counts per track in a small space. With the tracks having different winding counts without a common denominator, every position within a single rotation will have a unique output signal.

Strengths: Small and large sizes with special geometries, robust, reliable.

Weaknesses: Mounting requires precise alignment and more engineering to select and implement.

Making the Right Choice

Rotary encoders are a broad product category with innovative technologies regularly introduced to satisfy the ever-expanding needs of automation. A fundamental understanding of the different technologies available will help find the correct product for any application, ensuring reliability while remaining within the project’s budgetary constraints.

If you have an application with special encoder requirements to address, contact your in-house engineer or a third-party automation specialist for assistance with arriving at and implementing the best solutions.

This article was written by Kyle Coiner, Automation Specialist, Motion (Birmingham, AL). For more information, visit here .