Resonant renaissance

A new concept in resonant inductive position sensing aims to give the technology a cost-effective revival.

Resonant inductive position sensing isn’t new, as CambridgeIC’s managing director David Ely would be the first to admit. But until now, developing systems based on the technology required specialised design skills, and because of that, it has only been deployed where environmental demands or price and performance requirements could justify a custom solution.

CambridgeIC’s mission is to change that, and to do so, engineers at the company have developed a chip that will allow design engineers who might not be skilled in resonant inductive sensor design to rapidly integrate linear, rotary, or both types of sensing into their new products.

The operating principles behind a resonant inductive position sensor are not totally dissimilar to those used by another group of linear sensors called linear variable differential transformers (LVDTs), which have been used in aerospace and military applications for more than 40 years.

CambridgeIC’s position-sensing technology is built onto a PCB

Widely available as off-the-shelf integrated units from a variety of manufacturers, these transducers comprise a core of magnetically permeable material that is free to move inside a hollow tube. Around the periphery of the tube are a primary coil and two secondary coils placed on either side of the primary.

In operation, the primary winding is energised by an alternating current that creates a magnetic flux. This induces an alternating voltage into the two secondary windings, through the core of magnetically permeable material. As the differential voltages induced into the two secondary windings vary as a result of the position of the core, dedicated circuitry on the sensor can be used to determine its position within the device – and hence any actuator attached to it.

Using PCB-based coils means that sensors can be designed that are highly tolerant of misalignment

But while such sensors might be mechanically robust, precise and capable of producing an inherently absolute output, they must be accurately aligned inside heavy and expensive packaging. What is more, they contain wound coils that are expensive to make, as well as custom electronic circuitry to perform the required signal conditioning.

The same cannot be said for CambridgeIC’s resonant inductive position-sensing technology. While the concept also makes use of a primary coil, two secondary coils and a target, it differs from the LVDT in that the coil windings – both the driver and the sensor coils – are not found inside an integrated transducer but on a sensor board, built using conventional printed-circuit-board (PCB) technology.

The other key difference is that the target is an electrical resonator. This consists of a custom-built resonant circuit, including an inductor and capacitor that are housed in a miniature sealed device, the movement of which is tracked by the system.

CambridgeIC’s David Ely said that the approach of using such PCB-based coils with a resonant target means that sensors can be designed that are highly tolerant of misalignment and capable of operating at large gaps.

To help engineers integrate resonant inductive position-sensing systems, the company has developed a range of standard linear and rotary sensor designs of different types and sizes. For volume applications, customers normally build the sensors themselves using conventional PCB technology. The company can also supply finished PCBs assembled with modular connectors for development and low-volume production.

“The system is unaffected by variations in supply voltage, coil resistancesand temperature”

CambridgeIC has developed a proprietary Integrated Circuit (IC) that uses what is known as a pulse-echo method to drive the target at its resonant frequency from the primary winding on the PCB, as well as to detect and interpret the signals received from the secondary windings.

To do so, the IC excites an alternating current into a primary coil on the PCB, which in turn causes the target to oscillate at its resonant frequency. After a specific period of time, the IC cuts off the driving pulse and the oscillation in the resonator starts to decay. The IC then detects the electromagnetic fields induced by the decaying oscillation in the two secondary sensor coils on the PCB, and uses their relative amplitude to calculate the position of the target.

During operation, the IC also continuously adjusts its operating frequency to match the resonant frequency of the target, to produce the optimum signal level from the detector coils.

’Since the calculation used to determine position depends on the relative value of two decaying amplitudes, the system is immune to variations in supply voltage, coil resistances, temperature and the sensitivity of the processing electronics,’ claimed Ely.

He said that the approach has several advantages. First, the toolkit of PCBs, resonant target and processor IC make it a simpler task for engineers to build industrial resonant inductive sensors. Second, because the system is PCB based, designers can integrate other circuitry onto the PCB to customise their designs. Third, by using the company’s single processor chip in a design, design engineers can create systems that can sense multiple axes at low cost.

Mounting holes are accurately aligned with the resonator

Indeed, the first of the company’s ICs can support up to four such sensors, which can be linear and/or rotary. The linear PCB-based sensors are currently available with measuring lengths from 25 to 200mm, while the rotary sensors are available in diameters from 25 to50mm, with the larger size capable of sensing at a gap of up to 11mm.

Nevertheless, CambridgeIC’s Ely does not see the technology replacing such other sensing technologies entirely. Magnetostrictive sensors, he said, which measure the position of a moving magnet, still have the advantage that they can operate through stainless steel or aluminium housings and pistons. And for the very highest speed and/or resolution, optical encoders still have the edge.

While Ely admits that his technology isn’t suitable for applications that demand the very highest speed or accuracy, he said that cost effectiveness and robustness are more important factors in the majority of electromechanical products built in high volumes.

Testifying to that fact, CambridgeIC has already established a relationship with Turck – a developer of industrial automation equipment – which has used the company’s processor IC inside its Li series of industrially housed and wear-free position sensors, which the company launched last year.

These have linear measuring ranges between 100 and 1,000mm and are targeted at industrial applications such as metal processing and injection moulding. Here, use of the CambridgeIC technology means they are unaffected by metal debris and magnetic fields that would impair the performance of traditional magnetostrictive devices in such an application.

But it’s not just low-to-medium volume applications that could benefit from using the IC. This year, it will be used in higher-volume applications as well.

design essentials

The key facts to take away from this article

  • CambridgeIC aimed to make resonant inductive sensing easier to integrate
  • As opposed to LVDTs, the company’s technology uses PCB-based coils
  • The company’s IC uses a pulse-echo method that benefits sensor engineers
  • While not the fastest or most accurate, the IC is cost effective and robust