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Arc Position Sensor Comparison

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.

ArcAdaptedLinearA

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.

ArcAdapterdRotarySensorLinkArmA

ArcAdaptedEndShaftRotaryA

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.

NativeArcSensorA

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.

 
 Arc Position Sensor Comparison(a) Linear Sensor(b) Rotary Sensor, On-Axis(c) Rotary Sensor, Link Arm(d) CambridgeIC Arc Position Sensor
Directly measures arc angle alt alt alt alt
Eliminates mechanisms, bearings and couplings alt alt alt alt
Off-axis sensing - rotation axis clear of sensor parts alt alt alt alt
Truly non-contact - big gap sensor to target alt alt alt alt
Lightweight alt ? depends alt alt
Sensor can be completely encapsulated against dirt, dust, moisture alt   alt  alt  alt