Encoders – the basics

NUMERIK JENA’s LIKgo linear encoders are designed for the demands of production and inspection machines in the semiconductor industry. A new two-field scanning principle ensures that signals are produced free of offset errors and phase errors. The measuring standard is a SINGLEFLEX steel scale tape 8 mm wide with a grating period of 20 µm, making possible measuring steps down to 78.125 nm.

In automation and control systems using closed-loopcontrol, speed and position are two critical variables. And the mostcommon way to acquire position information is using an encoder.

Encoders can be classified in a number of different ways – whether they are rotary (the more common) or linear, by the sensing technique, whether the output is absolute or incremental, as well as by some common performance parameters.

SENSING TECHNIQUES

Common sensing techniques include optical, magnetic, and capacitive sensing. While optical encoders have historically been the only option for resolutions below 5 microns, improvements in magnetic scale technology now allow them to achieve resolutions down to 1 micron. Optical encoders use a light source and a photo-detector to determine position, but their use of light makes them sensitive to dirt and debris, which can disrupt the signal. The performance of optical encoders is influenced by the gap between the sensor and the scale, which must be properly set and maintained to ensure that signal integrity isn’t compromised. This means that mounting must be done carefully, and shocks and vibrations should be avoided.

Magnetic encoders use a magnetic reader head and a magnetic scale to determine position. Unlike optical encoders, magnetic encoders are mostly unaffected by dirt, debris, or liquid contamination. Shock and vibration are also less likely to affect magnetic encoders. They are, however, sensitive to magnetic chips, such as steel or iron, as they may interfere with the magnetic field.

Capacitive encoders, a relatively new technology, offer resolution comparable to optical devices, with the ruggedness of magnetic encoders. The basic principle behind capacitive encoders is that they detect changes in capacitance using a high-frequency reference signal. This is accomplished with the three main parts—a stationary transmitter, a rotor, and a stationary receiver. (Capacitive encoders can also be provided in a “two-part” configuration, with a rotor and a combined transmitter/receiver.) The rotor is etched with a sinusoidal pattern, and as it rotates, this pattern modulates the high-frequency signal of the transmitter in a predictable way. The receiver disk reads the modulations, and on-board electronics translate them into increments of rotary motion.

The primary concern when using capacitive encoders is their susceptibility to noise and electrical interference. To combat this, the ASIC circuitry must be carefully designed and the algorithms for de-modulation must be fine-tuned.

These kit encoders for stepper motors from POSITAL are modular devices designed to be integrated into a motor housing, measuring the rotary position directly from the drive shaft. The magnetic measurement module is compact measuring 37 mm in diameter and 23 mm deep, and highly resistant to dust, moisture and shock and vibration loading.

ENCODER OUTPUT

Encoder output can be either absolute or incremental. Absolute encoders have a unique code for each shaft position, so that every position is distinct. The absolute encoder interprets a system of coded tracks to create position information where no two positions are identical. Absolute encoders also don’t lose position if there is power loss. Because each position is distinct, true position is verified as soon as power is restored and so does not require a homing routine.

Absolute encoders can be either single-turn or multi-turn. Single-turn encoders are well suited to short-travel motion applications where position verification is needed within a single turn of the encoder shaft. On the other hand, multi-turn encoders are better for applications that involve complex or lengthy positioning requirements.

Incremental encoders generally supply square-wave signals in two channels, A and B, which are offset (or out-of-phase) by 90 degrees. This helps in determining the direction of rotation. The output signals of an incremental encoder only have information on relative position not absolute position like an absolute encoder. To provide useful position information, encoder position has to be referenced in some way, traditionally using an index pulse. So the incremental encoder sends incremental position changes to electronic circuits that perform the counting function.

A traditional limitation of an incremental encoder comes with a loss of power. For instance, if a machine with an encoder is turned off, the encoder will not know its position when switched on again. The encoder has to perform a homing routine in order to know its exact position. Then, a counter or buffer will be zeroed and the system will determine where it is relative to fixed positional points. One way around this issue of loss of power is to use a battery backup system. Such a solution ensures that the memory is backed up and can store the count information and provide an absolute count once power is restored.

These QUANTiC rotary (angle) encoders from Renishaw use a 40 µm-pitch ring scale, available in a range of sizes from 52 to 550 mm. The encoder system addresses applications with wide through-bore requirements and can operate at rotational axis speeds of up to 8,800 rpm with angular resolutions to 0.04 arc seconds.

PERFORMANCE PARAMETERS

Encoder resolution and accuracy are essential to the proper operation of a closedloop system. The ideal encoder has both the necessary resolution and is highly accurate.

Resolution is the distance over which a single encoder count takes place – it’s the smallest distance the encoder can measure. For rotary encoders, resolution is typically specified in terms of measuring units, or pulses, per revolution (PPR). Linear encoder resolution is most commonly specified as the distance over which the count takes place and is given in terms of microns (μm) or nanometers (nm). The resolution of an absolute encoder is specified in bits, since absolute encoders output binary “words” based on the encoder’s position.

Accuracy is the difference between the true position (or speed) of the device being measured and the position (or speed) reported by the encoder. For rotary encoders, it is specified in arcseconds or arcminutes, and for linear encoders accuracy is typically given in microns.

Note that higher resolution does not mean higher accuracy. Consider two encoders – one with 100 PPR resolution and one with 10,000 PPR resolution, but both with the same accuracy specification. The lower resolution (100 PPR) encoder can report a movement of 90 degrees just as accurately as the higher resolution (10,000 PPR) model. The higher resolution encoder just has the ability to break up that 90 degree movement into much smaller increments.

An encoder’s resolution is based on the number of lines (for an incremental encoder) or the pattern (for an absolute encoder) on the encoder disk or scale. Physically, resolution is fixed. Once an encoder is manufactured, there is no option to add more lines or patterns to the code disk. But the resolution of an incremental encoder can be increased through signal decoding. Incremental encoders output square wave signals, and by counting both the leading and trailing edges of one signal (signal

A), the resolution of the encoder is doubled. When the leading and trailing edges of both signals (A and B) are counted – referred to as quadrature decoding – resolution is increased by a factor of four.

While the number of lines or measuring units determines resolution, accuracy is affected by the width and spacing of these lines or units. Inconsistent width and/or spacing will cause errors in the timing of the pulses. For absolute encoders, accuracy is influenced by the precision with which the pattern is placed on the code disk.

Factors external to the encoder can also affect its accuracy. These include the rigidity of the assembly and mounting errors, such as a lack of concentricity between the encoder disc and the shaft to which it’s mounted. For linear encoders, thermal expansion of the scale and of the mounting surface can also degrade accuracy.