Resonant Inductive Position Sensing technology is used to precisely measure the position of a target without mechanical or electrical contact. The target's position is measured using a sensor built with conventional Printed Circuit Board (PCB) technology. The target houses an electrical resonator comprising an inductor and capacitor. An electronic processing system interacts with the sensor to power the resonator and to detect the signals that it returns. The detected amplitudes of these signals are processed to calculate position.
CambridgeIC’s single-chip processing solutions are called Central Tracking Units (CTUs). These enable customers to embed non-contact position encoders into their products with the minimum of effort, and to deploy those solutions in high volume at low cost.
CambridgeIC has developed a range of standard sensors, including rotary and linear in different types and sizes.
They are built using conventional printed circuit board (PCB) technology. CambridgeIC supplies Sensor Blueprints to allow a customer's own PCB supplier to build sensors. This also allows a customer to design PCBs that combine one or more sensors with other electronic circuitry. Sensors are also available as finished PCBs assembled with modular connectors for development and low-volume production.
Type 1 sensors are the simplest and comprise 3 coils. The excitation coil (EX) is used to power the resonator, and the return signals are detected in the COS and SIN coils. The figure below shows an equivalent circuit for the sensor coupled to a resonator inside the target.
The figure below illustrates a simplified sensor coil design for a non-contact linear position sensor. Non-contact rotary sensors use a similar principle, with the coils arranged in a circle. The excitation coil is shown in black and surrounds the two sensor coils. As shown on the graph underneath, its coupling factor to the resonator is approximately constant, so that the resonator is always powered when the resonator is within the Measurement Range. The COS and SIN coils are patterned so that their coupling factors to the resonator kCOS and KSIN vary sinusoidally along the Measurement Range. This approach is also commonly used in LVDTs, RVDTs and resolvers. Sensor coils are balanced (they have portions wound in opposite directions) so that inductive interference cancels.
Type 2 and Type 6 sensors use more coils for precise measurement and/or over longer distances.
Type 3 sensors have a special coil arrangement that allows the target's magnetic axis to point along the measuring direction. This is for fluid level sensing, and other applications that require precise measurement while the target remains free to rotate.
Position Calculation is performed automatically inside CambridgeIC’s CTU chips. Resonant inductive position sensors use ratiometric calculations for immunity to resonator Q-factor, supply voltage and component variations.
The sensor coils of a Type 1 sensor are patterned to produce a SIN/COS variation of coupling factor with changes in position. The calculation of position (Pr in the figure below) is equivalent to the calculation of phase angle, and is performed with a 4-quadrant inverse tangent.
Changes in temperature, supply voltage, target Q-factor and component variations all tend to affect the SIN and COS signals equally. This means they remain in the same proportion, and the reported position Pr remains the same.
Many other sensor types use a similar underlying calculation.
CambridgeIC’s CTU chips generate a drive waveform that is used to inductively power the resonator inside a target, and detection circuitry to measure the levels of return signals from which coupling factors are determined.
CTU chips use a pulse echo approach, where the drive waveform is applied to power the resonator and is then removed for detection. This approach minimises errors due to direct coupling from the excitation to sensor coils, including any connecting leads that may be used. The sequence of events is illustrated below.
CTU chips detect resonator frequency, and they continuously adjust their operating frequency to match the resonator for optimum signal level.
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.
|Hall Effect encoder
|CambridgeIC resonant inductive
|Works in dust and dirt without seals
|Tolerant to misalignment
|Operates at big gaps
|CambridgeIC resonant inductive
|Works in dust and dirt without seals
|Tolerant to misalignment
|Operates at big gaps
|Short inactive zones
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.
CambridgeIC’s Arc Position Sensors are for embedded, non-contact measurement or angle inside machines. The angle to be measured is typically limited by the application, for example to 90°. This means an arc shaped sensor may be used, instead of a circular rotary sensor typical for full 360° angle measurement. The principle of operation is like a CambridgeIC linear sensor, except curved in an arc around the rotation axis.
Comparing CambridgeIC’s Arc Position Sensors to alternative technologies is difficult, because there aren’t really any other embeddable arc position sensors on the market. Instead, traditional approaches are to adapt linear or rotary position sensors to measure angle, as illustrated below.
Adapting a linear sensor requires angular motion to be converted to linear with bearings and sliders, as illustrated above. Typical linear sensors are potentiometers or LVDTs. The resulting measurement is indirect, and linkages can introduce errors and backlash. Some form of linear slider is needed, either inherent in the linear sensor or external, and is difficult to seal against moisture and dirt ingress.
Rotary sensors can be adapted to measure arc angle using a link arm. Alternatively, the sensor must be placed along the rotation axis and carefully coupled to the sensor. Both require mechanical complexity and careful mountings. Using a rotary sensor with a link arm makes the measurement indirect and subject to additional errors and backlash. Adding it along the rotation axis is often impossible due to the product’s mechanical constraints. In many cases, a 360° sensor is only used across a small fraction of this total measuring range, so that errors become a much greater fraction of the measuring range when that range is small. Typical rotary sensors are potentiometers, Hall encoders or optical encoders. All typically require sealing, and careful alignment between sensing elements and their rotation axis. This means the rotary sensor is usually encapsulated, and includes bearings and seals. This is again mechanically complex and adds weight. In some cases and in clean environments a hall encoder chip may be positioned on-axis without a housing for lower weight, but it requires very careful alignment and linearity error and resolution as a fraction of the angle range used can be poor.
A CambridgeIC arc sensor directly measures angle, and is positioned off-axis so it does not interfere with mechanical parts required along the rotation axis. The sensor and its target are mounted on separate parts for relative movement, and there can be a big gap between them. The system is therefore truly non-contact. Both parts are lightweight and do not add any mechanical linkages or bearings. Operation with big gap means a CambridgeIC arc sensor can be encapsulated inside its own enclosure for operation in extreme environments, for example operation under sea water. The enclosure does not require openings for seals or bearings, only electronic connections. In most embedded sensor applications, the PCB is left bare without housing.