Encoders in focus

To reference or not?

Perhaps the most obvious difference is that absolute varieties do away with need for datum (reference) homing. An absolute encoder does not require a reference return cycle prior to every run, which might be critical for certain applications. There is a time penalty associated with homing and if a machine has multiple axes then homing cycles may be both complex and time consuming. Also, certain types of robots such as hexapods and wafer-handling robot arms risk damage either to themselves or their payload if forced to reference return after a power outage, for example.

Discrete position requests vs. continuous output

A machine controller may typically request position from an absolute encoder every 65 µs (15 kHz), so there is a discrete time interval between each reading. This has implications for highly accurate velocity control, particularly in applications such as DDR motors. Incremental encoder systems produce a continuous sinusoidal output that helps to minimise velocity errors thereby reducing velocity ripple. The ripple effect partly results from the amplification of tiny errors in the encoder output by higher control gains for the purposes of controlling a high stiffness servomotor. In applications such as flexo-graphic printing, e.g. web handling, much emphasis is placed on achieving ‘perfect' velocity control as velocity ripple causes uneven coating and undesirable horizontal bars across the substrate.

Cyclic error

For incremental optical encoders, there are two main sources of cyclic error. One source is a fringe field, produced by a filtering optics scheme, that has harmonics and another comes from electronic sources, including detectors and ICs, that produce lissajous off-sets and ellipticity. Optical regimes can be designed which produce suitably harmonic-free fringe fields. In pure (single-track) absolute systems, the dominant source of cyclic error is aliasing. An alias is a sinusoidal signal that appears identical to another waveform when sampled at the same rate; aliasing becomes problematic whenever a large number of frequencies are passed to a detector. Optical filtering applied to incremental systems minimises this effect, however, absolute systems do not generally employ digital filtering due to increased latency. The aliasing phenomenon typically gives an error of approx. ≤10 µm periodicity in the case of absolute systems and is the result of discrete sampling of the scale period due to the periodic detector; careful design and some optical filtering can minimise this effect such that its related error amplitude is below 10 nm. Well-designed incremental encoders may feature marginally better cyclic error than equivalent absolute systems.

Jitter

In both incremental and absolute systems, the fundamental causes of jitter are various types of noise (e.g. Shot, Johnson and 1/f)* that occur over a broad spectrum of frequencies. It is possible to reduce the impact of these by filtering to reduce the range of frequencies (bandwidth) that are passed by the encoder to the motion control system. With incremental encoders, where position information is transmitted continuously, this is done by limiting the bandwidth of the (analogue) quadrature signals, which reduces the maximum speed of the encoder (see Speed). For example, the best possible jitter performance from a TONiC™ encoder is achieved by limiting the maximum speed to well below 1 m/s. In absolute systems where position information is acquired at discrete time intervals, position jitter is the uncertainty in each of those measurements. It is not possible to limit bandwidth in the same way, although some digital filtering is possible. The result is that absolute encoders have a slightly larger position jitter than well-tuned incremental systems. For high precision scientific stages requiring very stable position holding with higher stiffness, incremental encoders are generally preferred.

Substrate mastered scales

Substrate mastered scales use some method to rigidly fix the two ends of the scale to the substrate, e.g. epoxy bonded ‘end-clamps', thereby ensuring that the scale and substrate expand and contract axially as one. In other words, the coefficient of thermal expansion (CTE) of the scale is matched or ‘mastered' to that of the substrate.

Adhesion of scale to substrate between the end clamps is generally achieved using a compliant double-sided adhesive tape. This ensures that scale linearity performance is maintained as the scale expands or contracts due to temperature changes.

Mastering is only feasible if the scale cross-section and stiffness is significantly lower than that of the substrate so as not to affect its mechanical stability during predicted thermal movement.

The main advantage of this approach is that the thermal behaviour of the scale is easily understood as it matches that of the substrate.

Floating scales

In contrast, floating scales must be mounted in such a way that their thermal movement is independent of the substrate. To accommodate differential thermal expansion between scale and substrate, they may be rigidly fixed to the substrate at one point only, the remaining length being held in place with double-sided adhesive tape, multiple retaining clips or an appropriate scale carrier.

However, whichever fixing method is employed, the so called floating scale is never truly independent of the substrate with respect to thermal expansion. This is because frictional and other effects inevitably lead to positional disturbance and potential hysteresis so the effective CTE is not precisely equal to that of the scale material in its free state. This behaviour can be modelled for floating scale fixing methods (adhesive tape, clips and carrier) to provide design guidance, although in practice compensating for it is not straightforward. However, floating scales can often be made with high accuracy, particularly when made of materials that exhibit very low coefficients of thermal expansion.