Using Inductive Sensors to Measure Position and Speed in Harsh Environments

Inductive sensors are often used in harsh environments for the measurement of speed or position. This article explains the various types of inductive position sensors and their operating principles, together with their strengths and weaknesses.

Inductive speed and position sensors are available in various sizes, shapes, and designs. Inductive sensors operate using transformer principles and use a physical phenomenon based on alternating electrical currents.

Michael Faraday first discovered this phenomenon in the 1830s when he realized that a current-carrying conductor could ‘induce’ a flow of current in another conductor. Faraday’s discoveries helped in develop dynamos, electric motors and inductive sensors for speed and position measurement.

Inductive sensors include simple proximity switches, synchros, resolvers, variable reluctance sensors, variable inductance sensors, rotary and linearly variable differential transformers (RVDTs and LVDTs), and the new generation of inductive encoders (also known as incoders).

The Various Types of Inductive Sensors

In a simple proximity sensor (also referred to as a proximity switch or prox switch), electrical power is supplied to the sensor, to induce an alternating current to flow in a coil (also referred to as a spool, loop, or winding). When a magnetically permeable or conductive target, for example a steel disk, approaches the coil, the coil’s impedance is changed.

When a threshold is passed, a signal is generated that indicates that the target is present. The presence or absence of a metal target is detected by proximity sensors, and the output often behaves like a switch. These sensors find wide use in applications where the electrical contacts of a conventional switch would pose problems, particularly where lots of dirt or water exists. Many inductive proximity sensors are used in applications as varied as car wash machinery or in the landing gear of an airplane.

In variable reluctance and variable inductance sensors an electrical signal proportional to the displacement of a conductive or magnetically permeable object (normally a steel rod) relative to a coil is produced. In a similar fashion to proximity sensors, the impedance of a coil varies proportionately to the displacement of the target in relation to a coil energized with an alternating current.

The displacement of pistons in cylinders, for example in hydraulic or pneumatic systems, is measured by such devices. The piston can be arranged to pass over the outer diameter of the coil.

The inductive coupling between coils when they move in relation to each other is measured by synchros. Synchros are usually rotary devices that require electrical connections to both the stationary and moving parts (the stator and rotor).

Synchros are highly accurate and find applications in radar antennae, telescopes, and industrial metrology. However, they are expensive and increasing hard to find, having been mostly replaced by (brushless) resolvers. In this form of inductive detector, electrical connections are only made to the windings on the stator.

The change in inductive coupling between coils, referred to as the primary and secondary windings, are measured by LVDTs, RVDTs, and resolvers. The primary windings couple energy into the secondary windings but the ratio of energy coupled into the individual secondary windings varies proportionately to the relative displacement of a magnetically permeable target.

In an LVDT, the permeable target is typically a metal rod passing through the bore of the windings. In an RVDT or resolver, it is usually a shaped rotor or pole piece that rotates relative to the windings arranged around the circumference of the rotor. LVDTs and RVDTs find applications in hydraulic servos in aerospace aileron, engine and fuel system controls. Resolvers are used in brushless electric motor commutations.

The associated signal processing circuitry of the system does not need to be located close to the sensing coils, which is an important advantage of inductive sensors. This allows the sensing coils to be located even in harsh environments, which is advantageous when compared to other sensing techniques –  such as optical or magnetic – which need relatively delicate, silicon-based electronics to be located at the sensing point.

Applications

Inductive sensors provide reliable operation in difficult conditions, and they are often the natural choice for safety-critical, safety-related, or high reliability applications such as in the aerospace, military, rail and heavy industrial sectors.

This solid reputation is based on the basic physics and principles of operation, which are generally independent of:

  • Moving electrical contacts
  • Humidity, condensation, and water
  • Temperature
  • Foreign matter such as grease, dirt, grit, and sand

Inductive Sensors: Strengths and Weaknesses

Most inductive sensors are very robust due to the nature of the basic operating elements: metal parts and wound coils. An obvious question, given their good reputation, is ‘Why are inductive sensors not used more frequently?’ The reason is that their physical robustness of the sensors is its strength and weakness.

Inductive sensors tend to be reliable, robust, and accurate, but also big, heavy, and bulky. Additionally, they are expensive to produce, because of their material bulk and the requirement for precisely wound coils, especially in high accuracy devices.

Other than simple proximity sensors, sophisticated inductive sensors are expensive for many general-purpose, commercial or industrial applications.

Another reason for the relatively scarcity of induction sensors is that it is difficult for a design engineer to specify them. Each sensor often needs its associated AC generation and signal processing circuitry to be separately specified and individually purchased, which requires a substantial amount of skill and knowledge of analog electronics. Since younger engineers tend to focus on digital electronics, they regard such disciplines as an unwanted ‘black art' to be avoided.

A New Generation – Inductive Encoders or Incoders

In recent years, a new generation of inductive sensor has entered the market which has a growing reputation in the traditional and mainstream sectors: the inductive encoder or ‘incoder’ (a mix of inductive and encoder).

The method used in incoders is the same basic physics as used in traditional devices, but instead of the bulky transformer constructions and analog electronics, the new device uses printed circuit boards and modern digital electronics. This elegant approach opens up several applications for inductive sensors such as 2D and 3D sensors, curvilinear geometries, short throw (<1 mm) linear devices, and high precision angle encoders, including small and large rotary encoders.

PCBs enables sensors to be printed onto thin flexible substrates, which can also eliminate the need for traditional cables and connectors. The flexibility of this approach – both physically and from the ability to readily offer customized designs for OEMs – is a major advantage.

Similar to the conventional inductive sensing techniques, incoders also provide reliable and precision measurement in harsh working environments. Other important advantages are:

  • Reduced cost
  • Reduced weight
  • Increased accuracy
  • Compact size – notably in the stroke length when compared to conventional LVDTs
  • Simplified mechanical engineering, for example, the eradication of seals, bearings, and bushes
  • Simplification of the electrical interface – typically a DC supply with an absolute, digital signal

Image of traditional LVDT (top) and IncOder linear sensor (middle). Rule for scale below.

This is shown in the above image, which shows a conventional 150 mm stroke LVDT and its new generation incoder replacement, which has been produced for a manufacturer of linear actuators. The parallels to the ‘before’ and ‘after’ dieting photographs are obvious. This is reinforced as the new generation device also includes the associated signal generation and processing circuit (not shown with the conventional LVDT). The IncOder device offers:

  • >10 fold increase in accuracy
  • 95% savings in weight
  • 75% volume reduction
  • 50% savings in cost
  • Direct generation of digital data – eliminating the need for analog-to-digital conversion

For extreme environment applications, Celera Motion's Zettlex IncOder range of inductive encoders is the market-leading position sensing technology. The move by engineers to replace optical encoders and capacitive encoders with IncOder technology has seen a sharp increase. Celera Motion also regularly develops custom linear sensor and rotary sensors for OEM requirements.

Celera Motion

This information has been sourced, reviewed and adapted from materials provided by Celera Motion.

For more information on this source, please visit Celera Motion.

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