Resolution and accuracy

Resolution is the smallest increment an encoder can express.

A linear encoder's accuracy is relative to scale length, the measurement's length, and characteristics of the part of the encoder involved in measurement. Accuracy is usually expressed as ±X µm over any one meter of scale length, where X is the maximum allowable error deviation from the mean over the given length.

Comparing linear encoders

At one time, designers had few options in linear encoders. Now, there are three distinct technologies from which to choose — optical, magnetic, and inductive. How they differ determines where each is likely to work best.

Optical: For the last several decades, linear encoders have been almost exclusively optically based. Optical encoders operate on the principle of light passing through or reflecting off a glass or metallic scale. A sensor collects the light and interprets position by measuring incident angles.

Optical encoders shine when it comes to measurements made under ideal conditions. Owing to the precision with which gratings can be applied to glass scales, optical encoders typically achieve high resolutions and accuracies. There is a downside, however. These finely crafted scales are susceptible to damage from shock and vibration, and the optics themselves are vulnerable to contamination, especially in an industrial environment.

Another consideration when using optical linear encoders is what's known as the “critical gap” — the distance between the optical sensor and scale. The angles of light coming off the scale determine position, so it's imperative that the scale-to-sensor distance remains constant. As traverse speeds increase, however, maintaining this critical gap becomes increasingly difficult.

Installation requirements for optical linear encoders vary. Some systems use reflective scales bonded to adhesive tape. Other styles use rigid scales encapsulated in larger guides or sheaths. If contamination is an issue, it may be necessary to pressurize the encoder, using an air compressor and appropriate filters and tubing.

Magnetic: Magnetic encoders tend to look like their optical cousins. However, rather than manipulating light beams, magnetic linear encoders monitor magnetic charges along a flexible tape scale. As opposed to optical scales, which have physical gratings, magnetic scales are patterned with magnetic poles. The read head senses these variations as it passes over them and uses this information to determine position.

Without the need for light paths and optics, magnetic linear encoders are less vulnerable to contamination than optical linears. This increased ruggedness comes at a price, however. Magnetic scale pitch (or charge spacing) is usually greater than the pitch achievable on an optical scale, which in turn limits resolution and accuracy.

Linear technology comparison

	Optical	Magnetic	Inductive
Resolution	0.5 to 10 µm	1 to 10 µm	0.5 to 100 µm
Accuracy	±3 to ±15 µm/m	±35 µm	±10 µm
Traverse rate	5 m/sec	10 m/sec	20 m/sec
IP rating	IP53 or IP64 (requires compressed air)	IP67	IP67
Shock	10 to 30 G	100 G	100 G
Vibration	3 to 20 G	15 G	30 G
Operating temperature	0 to 55°C	-10° to 70°C	0 to 70°C
Scale mounting	Adhesive tape or bolt-on carrier	Adhesive tape	Universal mounting brackets
Typical values for key parameters of optical and inductive linear encoders.

Like opticals, magnetic linears must maintain a critical gap between the sensor and scale. This is necessary because position measurements are based on the strength of the magnetic charge, which is a function of the distance between the read head and tape. The mechanical requirement of the critical gap limits traverse rates, just as it does with optical encoders.

Installing a magnetic linear scale requires a clean, dry mounting substrate. An adhesive on the underside of the flexible tape holds it down, while a cover strip protects it from above. Any irregularities such as bumps or kinks can cause measurement errors, as does misalignment. The read head must be perpendicular to the scale within a couple of degrees, and the ride height (or critical gap) must remain constant to within a fraction of a millimeter over the entire length of the scale.

Inductive: Inductive encoders, the newcomer, work by monitoring the effect of a metal scale on an electromagnetic field. Instead of a read head gliding a fraction of a millimeter above a reference plane, the scale itself (the reference) passes right through the sensor. The elimination of critical gaps translates to traverse rates an order of magnitude faster than other encoder types.

Inductive linears are also inherently robust. Their scales are made entirely of steel. And the lines of electromagnetic flux coming from the scales and the wire coils monitoring them are impervious to contamination. As for accuracy, the signals generated by the inductive encoding technique can be analyzed to such a degree as to rival the accuracy and resolution of optical encoders.

Installing an inductive encoder is basically a matter of mounting its scale. This is done by bolting a set of universal mounting brackets to the host machine via drilled and tapped holes. A dial indicator can verify that the scale — held in the brackets — is parallel to the axis of travel. Bracket height and alignment are adjustable, providing additional leeway during installation.

Inductive sensing's operating principle

Understanding the inner workings of an inductive sensing read head starts with an appreciation of how its sensing coils are positioned, and why. In the read head image, notice that each turn of coil A is positioned precisely one ball pitch apart, placing it at the same relative position for all six balls. The same holds true for coils B, C, and D. This configuration causes each of the four coils to sense each sphere's effect on the electromagnetic field from a different “angle.”

When the scale moves through the read head, its nickel-chrome balls interfere with the electromagnetic field generated by the drive coil. This interference varies the amplitude of sinusoidal signals induced in the sensing coils, causing phase imbalances. For example, A and C are 180° out of phase, as are D and B. A is 90° out of phase with both D and B, and so on.

Phase angle differences result from the sensing coils' orientation with respect to the ball bearings and are carefully orchestrated to allow trigonometric relationships to be used when calculating the read head's position. First, complementary signals are electronically summed: A with C and D with B. Then, an a/d converter digitizes each complement. Since A-C and D-B are sine and cosine waveforms, the ratio of sine to cosine, or tangent, represents the read head's absolute position over one sphere. To find the total distance traveled, a linear encoder's on-board processor adds the read head's current position to the number of spheres traversed since the last sample.