Measuring Speed and Position in Harsh Environments

Inductive sensors are frequently used in harsh environments to measure position or speed. Many engineers find inductive position sensor techniques and terminology confusing. This article explains the different types of inductive position sensor and their operating principles, together with their weaknesses and strengths.

Inductive speed and position sensors are available in many different shapes, sizes and designs. Inductive sensors function by using transformer principles, i.e. using alternating electrical currents.

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

Michael Faraday

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

The Various Types of Inductive Sensors

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

When a threshold is passed, this acts as a signal which indicates that the target is present. The absence or presence of a metal target is detected by proximity sensors, and the output frequently behaves like a switch. These sensors find extensive use in applications where electrical contacts with a standard switch would pose problems, principally where lots of water or dirt exists. Lots of inductive proximity sensors are employed in car wash applications or in the landing gear of airplanes.

An electrical signal proportional to the displacement of a magnetically or conductive permeable object (normally a steel rod) relative to a coil is produced in variable inductance and variable reluctance sensors. Similar to a proximity sensor, the impedance of the coil differs proportionately to the displacement of the target in relation to the other coil (with an alternating current).

The displacement of pistons in cylinders, for instance in pneumatic or hydraulic 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 generally rotary devices, and require electrical connections to both the moving and stationary parts (the rotor and stator).

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

Inductive Sensors

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

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

A significant advantage of inductive sensors is that their signal processing circuitry does not need to be placed close to the sensing coils. This allows the sensing coils to be located even in extreme environments, which can preclude other sensing techniques – such as magnetic or optical – as they need comparatively delicate, silicon-based electronics to be placed at the sensing point.

Applications

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

Inductive Sensors

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

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

Inductive Sensors: Strengths and Weaknesses

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

Inductive sensors tend to be robust, reliable and accurate, but also big, bulky and heavy. Additionally, they are expensive to develop, due to their material bulk and the need for carefully wound coils, particularly in high accuracy devices that require precision winding.

Other than simple proximity sensors, sophisticated inductive sensors are fairly expensive for a number of general-purpose, industrial or commercial applications.

It is difficult for a design engineer to identify inductive sensors, and this is another cause for their relative scarcity. Each sensor often requires the associated AC generation and signal processing circuitry to be distinctly specified and has to be individually purchased, which requires a substantial amount of skill and knowledge of analog electronics. Since younger engineers tend to focus on digital electronics, they look at 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 sensors has been launched into the market, which has a growing reputation in both mainstream and traditional sectors: inductive encoders or ‘incoders’.

The technique employed here is the same basic physics as used in standard devices, but instead of the bulky transformer constructions and analog electronics, the new device makes use of modern digital electronics and printed circuit boards.

This sophisticated approach opens up several applications for inductive sensors such as 3D and 2D sensors, curvilinear geometries, short throw (<1 mm) linear devices, and high precision angle encoders, including large and small rotary encoders.

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

Similar to the standard inductive sensing techniques, the new approach also offers precision and reliable measurement in extreme working environments. Other important advantages include:

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

Image of traditional LVDT (top) and Celera Motion

Image of traditional LVDT (top) and Celera Motion's zettlex linear sensor (middle). Rule for scale below.

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

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

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

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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