Inductive encoders offer an alternative to traditional position sensors for the new wave of robotics

Thirty years ago pundits were heralding a new age of industrial automation, forecasting huge numbers of robots in our factories and workplaces. These forecasts have proved to be overly optimistic because they were based on an assumption that robots would quickly replace large numbers of humans in repetitive, low skill jobs. Instead, a more accessible and cost effective solution was found in low-cost labour in the Far East. Although many forecasts have been wrong, we are now seeing very real signs of a massive, wide ranging, new era for robotics which will have a profound impact on industry and society. The leading edge of this new era of robotics is not in applications aimed at replacing people, but rather with robots cooperating with humans to do jobs that are either, difficult, impossible or dangerous. Examples include space exploration, bomb disposal, remotely operated underwater vehicles, fire-fighting, nuclear plant maintenance/decommissioning, chemical spill clear-up and the rapidly emerging areas of defence and security automation where robots are already playing an important role in intrinsically hazardous operations.

This new wave of robotic applications has become ripe for commercial exploitation because key technologies now have viable price-performance points. These include high-performance, compact batteries and motors; compact, low-cost, high-resolution electro-optics; powerful but inexpensive microprocessors; easy to use wireless communications and, last but not least, accurate and reliable non-contact position sensors.

A common theme in these new applications is that the operating environments are harsh.Generally, a harsh environment is one which has one or more of the following: high or low temperatures, thermal cycling, conducted or radiated electromagnetic noise, aggressive chemicals, submersion in liquids, ionizing radiation, extreme shock, prolonged or severe vibration, extreme pressures, pressure cycling or potentially explosive atmospheres.

Such environments are not new.Indeed, for many years engineers have developed systems for use in the aerospace, military, and oil and gas sectors that operate reliably in such conditions. The traditional choice for position measurement in these industries has been inductive sensors such as linearly and rotary variable differential transformers (LVDTs & RVDTs), resolvers and synchros. This type of position sensor typically uses transformer principles, constructed with precision wound spools of copper wire which are influenced by a moving inductive target such as a magnetically permeable rod or rotor. They are generally unaffected by foreign matter, shock, vibration or extreme temperatures. These traditional inductive sensors have a long and successful track record of reliable and safe operation in harsh environments, but they are often too heavy, bulky or expensive for robotic applications. Space constraints are common in modern robotics and although the total volume of a traditional inductive sensor may be acceptable they are often the wrong shape to fit in and around other robotic components such as motors, gears, slip-rings and wiring harnesses.

Compared to price insensitive industries, the traditional choice of position sensor for robotics has been the optical encoder. These devices work by shining a light on to or through an optical disk or grating and determining position from the resulting light signal. Optical encoders have many advantages and in benign, indoor conditions, optical encoders offer a compact, low-cost, easy to use and accurate option. However, they are often not sufficiently robust or reliable in harsh, outdoor environments because of their susceptibility to foreign matter, limited operating temperatures and relatively low resilience to shock or vibration. Foreign matter such as dirt, swarf, grease (even greasy thumbprints from maintenance or service operations), oil, fluff or water – notably condensation – can cause failure because it interferes with the encoder’s optical path. This can cause the sensor to stop working or, worse, report its position incorrectly. An incorrect position signal can be much more problematic than no signal as it may cause the robot to move in unexpected, dangerous or damaging ways.

New generation robotics in harsh environments is increasingly turning to a new breed of position sensor. They are based on the same basic physics as the traditional aerospace and military inductive sensors but combine many of the benefits of optical encoders. They are generally referred to as ‘incoders’ for obvious reasons. Rather than expensive and bulky transformer constructions, incoder technology uses laminar constructions whose printed, conductive tracks take the place of spools of copper windings to form the transformer field constructions. This new generation of encoder is generally as robust as their traditional, inductive counterparts whilst offering the compactness, ease of use and accuracy of optical encoders.

An inductive encoder has three main components: an electronics module, a moving target and a stationary antenna.The electronics module receives power from the host and supplies an AC signal to the antenna so that it forms a local electromagnetic field. The target disturbs this field, depending on its position relative to the antenna. In turn, the disturbance is sensed by the antenna and the electronics module outputs a signal proportional to the target’s position relative to the antenna. The target and antenna are both made from printed, laminar components, typically <1mm thick. Importantly, the electronics module need not be adjacent to the antenna, allowing the electronics to be placed in a relatively benign environment compared to the antenna and target which are able to withstand the harshest of environments.

The new inductive encoder technology is lightweight, non-contact, low power, shock and vibration resistant and, once encapsulated or conformably coated, is unaffected by immersion in liquids. Also key to the growing success of this new technology is that it can be formed in to a wide variety of shapes and sizes. Since the main components are printed, the tooling and engineering cost for a new, custom shape or size is modest. Sensing formats include rotary, linear, 2D and even 3D geometries to suit the host’s requirements and space constraints. For example, bearings, drive shafts, slip-rings or cables may need to be passed right through the space where traditional sensors might sit.

Sensing distances typically range from 1mm to 2m in either length or diameter. For example, the image below shows an unusually shaped, 50mm linear position sensor which has been designed to fit within the space constraints of an existing mechanical housing and shaft arrangement.Since these new inductive encoders are non-contact devices, they do not wear out and so need no periodic service or maintenance.

Inductive encoders are already in service in a wide range of demanding and harsh environment applications including military and civil aerospace, petrochemical process controls, marine controls, industrial automation and medical equipment.