Inductive position transducers such as resolvers and linear transformers have been the preferred choice for harsh environments and safety-critical applications for more than 50 years. However, a new generation of inductive transducers is now gaining in popularity. Mark Howard of Zettlex describes why inductive transducers are chosen for the toughest jobs, how they operate and why a new generation of inductive devices is taking over.

Traditional Approach

Linear inductive position transducers are normally referred to as variable reluctance or linearly variable differential transformers (LVDTs). Rotary forms are known as synchros or resolvers. They all work on the inductive or transformer principles first discovered by Michael Faraday in the 1830s. Both linear and rotary forms have been used in the industrial, aerospace, defence, oil and gas sectors since World War II.

Inductive position transducers have a well-deserved reputation for reliability in difficult environments, which might include harsh vibration, shock, extreme temperatures, extreme duty cycles, aggressive chemicals, electromagnetic noise, radiation or extended life. Many safety-related applications will only consider inductive transducers for this reason.

Typical applications include motor controls, actuator servos, rotary or linear displacement measurement in military equipment, aircraft, heavy industrial plant, agricultural vehicles, industrial process equipment and the like.

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Fig. 1 – Gimbal systems for weapon systems & antennae are typical applications for inductive transducers

How Traditional Inductive Transducers Work

A typical inductive position transducer has at least one – usually several – wire spools or windings. When the windings are energized with an AC signal they form a local electromagnetic field. The field is modified by either relative movement of the windings or movement of a magnetically permeable/conductive element usually referred to as an inductive target. The field change is proportional to displacement.

LVDTs use at least three wire spools: a primary and two secondaries arranged around a central rod. As the rod moves, it varies the electromagnetic coupling between the primary and secondary spools. The ratio of the signals induced in the secondary windings indicates the position of the rod relative to the spools. This ratiometric technique is an important aspect of the LVDT’s high stability and measurement performance. Resolvers work in a similar fashion but in a rotary rather than linear arrangement.

Why Inductive Transducers Work So Well

A key aspect of Inductive Transducer operation is that they are non-contact. This means that, unlike potentiometers, there are no electrical contact points to wear out or fail due to ingress of foreign matter such as grit or sand. The fundamental interaction between the sensing parts of a traditional inductive transducer is electromagnetic and so the sensor is highly insensitive to environmental conditions.

If one were to submerge most optical or capacitive transducers in muddy water they would pretty quickly fail. The optical sensors would be obscured by the foreign matter and the capacitive sensor would be unable to differentiate between foreign matter and the capacitive target. An inductive sensor would not notice and carry on working as if nothing had happened.

Whereas optical, capacitive & magnetic sensors require electronic circuitry adjacent to the sensing point, the electronics for an inductive transducer can be located remotely. This enables the transducer element to be located in harsh environments with the usually more delicate electronics in a more benign location.

So why aren’t traditional inductive sensors used everywhere?

A clue is given by the types of industry sectors that use them most – military, aerospace, heavy industry, oil and gas. Typically in these sectors, performance and reliability are the primary selection factors, with cost a secondary issue. These sectors are also technically sophisticated and the deployment of traditional inductive sensors requires some expertise because they need AC energization and signal processing – not exactly the kind of stuff you give to a rookie engineer.

Size and weight are important factors for many applications and most traditional inductive transducers are fairly bulky and heavy. For example, an LVDT to measure 100mm will be almost 300mm long at full stroke. On top of that is the space and cost required for the signal generation and processing circuitry. As a rule of thumb, for every £1 spent on the actual transducer, a further £1 will be needed for the signal processing.

The net result is that traditional inductive transducers are often too expensive, too large or too heavy for many mainstream applications.

The New Generation

Over the last six years, a new generation of inductive transducer has emerged, which is attracting business away from traditional inductive devices, potentiometers, optical encoders and magnetic encoders. This trend has been helped by a swing away from optical encoders now that many engineers have learned the hard way that optical devices are simply not robust enough for tough environments.

This new generation of inductive sensors uses the same fundamental physical principles as traditional inductive sensors, but rather than bulky spools of wire, these new sensors utilise printed circuits on flexible or rigid substrates. Using laminar constructions means the new sensors are less bulky, less costly and lighter – precisely the factors that have limited the use of traditional inductive transducers.

An example of this new generation of inductive devices is the IncOder (from ‘Inductive encoder’) range of angle encoders from Zettlex based in Cambridge. The design is simplicity itself: two flat rings, one that rotates and one that is stationary. The stationary part receives DC power and delivers absolute digital data of up to 16 million counts per revolution. There are no contacts, no bearings, no seals, no precision installation and no need for maintenance.

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Fig 2 – New Generation Rotary Transducers – just as rugged but flat & compact with digital outputs

From the design engineer’s perspective, this new generation means much greater flexibility as the designs can be readily tailored to match specific application requirements. Almost any shape can be accommodated, from miniature rotary devices to linear, curvilinear, 2D and 3D shapes. If specified early in the design phase, design engineers can eradicate the need for shafts, bearings, seals, couplings and housings – further minimising cost, weight and space.

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Fig 3 – Goodbye winding, hello printing – a new generation inductive sensing device

EMC performance is generally as good as that of resolvers or LVDTs. Indeed, new generation inductive devices are being selected for safety-critical aerospace and military applications, where the ability to design the sensor for tight space and weight constraints have proven key factors.

Also, because the new generation inductive devices have been developed in the 21st Century, the choice for their associated electronics is 21st Century. This means there is no need for separate signal conditioning electronics. Instead, simple on-board electronics is used to receive power and to generate high-speed position data in SSI, SPI, A/B pulses, CANbus, etc. Analogue outputs can also be generated including 0-5V, 0-10V, 4…20mA, etc.

The transition to printed, laminar forms brings other advantages:

  • Any shape you like: a huge range of form factors, from tiny rotary devices to long linear, 2D & 3D shapes.
  • Flexible and compliant – down to just 0.1mm thick.
  • Greater measuring range – from microns to metres.
  • Greater measurement performance by eliminating inaccurate winding processes.
  • High resolution and stability (e.g. 24 bits for a 2“ device).
  • Complex measurement geometries such as roll, pitch & yaw sensing.
  • Multiple sensors can be located in the same space by using multi-layer circuit boards (e.g. redundant sensors in safety-related applications).
  • Certification for intrinsically safe environments (ATEX) is readily achieved since all the electronics can be epoxy potted.

As with all emerging technologies, the change from old to new is a gradual one. Nevertheless, the change is already well underway with tens of thousands of new generation inductive position sensors already in use on land, sea and in the air – from industrial process controls and maritime radars, to aileron control for unmanned aerial vehicles.

For more information on Zettlex’s new generation of inductive position sensors, please email [email protected].