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Technology Comparison

The following tables illustrate how CambridgeIC's resonant inductive sensing compares with traditional technologies. The attributes are selected for their importance in typical high-volume product applications requiring a precise, built-in sensor.


Absolute Rotary Position Sensor Comparison

 
 PotentiometerHall Effect encoderOptical encoderRVDTCambridgeIC resonant inductive
Wear free alt alt alt alt alt
Works in dust and dirt without seals alt alt alt alt alt
Tolerant to misalignment alt alt alt alt alt
Operates at big gaps alt alt alt alt alt
Multi-Axis sensing alt alt alt alt alt

 

Absolute Linear Position Sensor Comparison

 
 PotentiometerMagnetostrictiveOptical encoderLVDTCambridgeIC resonant inductive
Wear free alt alt alt alt alt
Works in dust and dirt without seals alt alt alt alt alt
Tolerant to misalignment alt alt alt alt alt
Operates at big gaps alt alt alt alt alt
Short inactive zones alt alt alt alt alt
Multi-Axis sensing alt alt alt alt alt

 

Potentiometers have a simple operating principle and have been widely used for many decades. They require a moving electrical contact, which is a source of friction and ultimately limits their reliability in use. The contact must be made along a precise path with a uniform force, so potentiometers will usually be packaged as a separate device with their own bearings when integrated inside a product. This can make them bulky and expensive, especially when linear.

Optical encoders have also been in use for a long time, and their fast response and simple output interface makes them attractive for a wide range of motion control applications. However they are relatively expensive to build into products, especially where high resolution is required, since they require their own bearings to avoid any misalignment. They are not suited to dirty, dusty or potentially moist environments unless encapsulated with seals, adding further mechanical complexity and cost.

Hall Effect encoders are a relatively new class of rotary position sensor that integrate Hall Effect sensors and interpolation electronics on a single chip. Used with a rotating magnet, they deliver a precise measurement of angular position. However the point-like nature of the sensing elements makes them sensitive to misalignment, so they must usually be used as a packaged device including bearings, or they must be carefully aligned during manufacture. The technology has also been extended to linear sensing using a repeating N-S-N-S-N-S... magnetic track. This can deliver a precise linear position reading, but requires careful alignment and small and well controlled gaps.  Multiple tracks are required for absolute sensing, requiring additional space and complexity.  A similar principle is applied to off-axis rotary position sensing, but this lacks the balance of on-axis sensing and is therefore extremely sensitive to misalignment.  This creates particular sensitivity to installation tolerances, temperature change and vibration which are not present in a more balanced system.

Magnetostrictive sensors are for linear sensing only. They measure the position of a moving magnet. A current pulse in a fine magnetostrictive wire induces an acoustic wave which travels down the wire and is detected at one end. The time between pulse and detection is an indication of the magnet's position. This approach is widely used for industrially packaged sensors used in injection moulding machines, and is available in a cylindrical form factor suited to instrumenting hydraulic cylinders. The use of a magnet for positioning is paticurlarly important here, since it allows operation through stainless steel or aluminium housings and pistons. Magnetostrictive sensors require packaging to hold the delicate acoustic components and wire in position, and to protect them against mechanical forces. They are therefore not well suited to direct integration inside high-volume products. The magnet may also attract magnetic swarf over time, which can interfere with normal operation.

Linear Variable Differential Transformers (LVDTs), and their rotary conterparts RVDTs and resolvers, are widely used in aerospace and industrial applications. Their main attractions are mechanical robustness, precision and an inherently absolute output. However they must be accurately aligned inside heavy, expensive packaging and contain wound coils that are expensive to manufacture.

CambridgeIC's resonant inductive position sensing is a novel alternative for building rotary and linear position encoders. The underlying principle of operation is similar to an LVDT or resolver. However the sensing coils are implemented on a PCB, which makes them much simpler and easier to manufacture while actually improving precision. The extra design freedom available from PCB coils means CambridgeIC is able to design sensors that are highly tolerant of misalingment so rarely require additional bearings. Where Multi Axis Sensing is required, the technology is particularly cost effective.

Resonant inductive sensing has been applied to various custom products in the last 20 years. However a commercially available, single-chip processor has been lacking. Now, CambridgeIC's Central Tracking Unit (CTU) chips enable customers to build a simple, robust, cost effective position sensing solution into products.