Shaping the servo system

A CLOSED-loop system can only correct the position it believes the motor has reached, so errors in the feedback signal inevitably affect the resulting motion. Of course, while this makes feedback selection important, it does not mean machine builders should specify the highest possible resolution or accuracy on every single axis. The mechanics between motor and load introduce their own backlash, compliance and cumulative errors. In many applications, extremely precise motor-mounted feedback adds little if the transmission cannot preserve that accuracy.

The question builders must ask, then, is what the axis actually needs, and which feedback features can simplify the machine around it?

Beyond basic specification

Resolution and accuracy remain the obvious starting points. Resolution describes the smallest movement the device can distinguish, while accuracy describes how closely its reported position matches the true position. The two are related but not interchangeable, and more resolution eventually reaches a point of diminishing return. Most guidance suggests at least 16 bits per revolution for a typical industrial rotary servo, with 24 bits preferred where greater resolution is justified, such as in a direct-drive rotary axis used for semiconductor processing or precision inspection.

Other factors matter too. Feedback bandwidth must support the desired servo loop, while excessive latency can affect stability. Temperature, electrical noise, shock and vibration can also dictate which technology suits the application. A resolver may still be the right choice in a severe environment, while a high-end encoder can earn its place where exceptional precision is required.

For many mainstream axes, however, the biggest gains now come from features that do more than simply measure position.

Knowing where you are after power-off

Multi-turn absolute feedback is a good example. Incremental feedback can measure movement once the system is running, but after power is lost it no longer knows the axis’s true position. The usual solution is a homing routine, where the machine moves to a reference point before production can restart.

In contrast, multi-turn absolute feedback tracks both shaft angle and the number of complete revolutions, retaining this information when power is shut off. This allows the machine to know its position immediately at power-on. While this may seem minor, in applications where multiple motor turns translate into limited linear travel this can remove homing sensors and shorten startup time.

There are several ways to preserve that information. Battery-backed systems retain the position count while power is off but introduce maintenance and a potential point of failure. Mechanical gearing avoids the battery, but adds size, cost and complexity. In recent years, integrated energy-harvesting devices have started to offer another route, generating enough power from shaft movement to update a turn counter in non-volatile memory even when the machine is switched off.

Making feedback do more

Digital feedback can also carry information that once required separate connections. Modern devices may include non-volatile memory containing motor identification and setup data. When connected to a compatible drive, the motor can identify itself and allow the correct parameters to load automatically. This reduces commissioning work and helps prevent errors where similar-looking motor variants need different settings.

The same communications link can carry motor temperature data, removing another dedicated connection. Low wire-count digital feedback also makes it possible to combine power and feedback in a single hybrid cable, reducing connectors, installation work and cable-routing demands.

Functional safety can also be integrated into certain certified feedback devices, but this is a good example of where right-sizing still matters. If an application genuinely requires safety-rated position feedback, the added capability can simplify the wider safety architecture. If it does not, there is little benefit in paying for a more specialised device.

Choosing the useful default

Kollmorgen’s SFD-M is designed around this general-purpose middle ground. It combines digital feedback, 16-bit multi-turn absolute position through energy harvesting, motor identification and single-cable capability in a compact package. For many industrial servo axes, that mix covers the functions designers increasingly value without moving into the cost and size of more specialized technologies.

It is not the right answer for every application. Certified functional safety, the highest direct-drive positioning accuracy, or compatibility with a different drive platform may point to another feedback type. That is precisely the point.

The feedback device may be small, but its effect on the system is not. The best choice is not simply the sensor with the most impressive specification. It is the one that delivers the performance the axis needs while making the rest of the machine easier to build, commission and keep running.

Dustin Horning is global market manager for industrial automation at Kollmorgen

www.kollmorgen.com