CambridgeIC has added axial linear position sensing of rotating targets to the list of sensing capabilities available from its products.

“The sensors are unique in measuring the continuous position of a freely rotating wireless target, from the side, and with a big gap, and without interference from nearby magnets,” according to the company. “Typically 10 bits or more of noise free resolution are available.” Linearity is within ±0.2%.

Gaps of over 10mm are possible between sensor and target, allowing each to be housed separately. Maximum distance between the sensor and the rotation axis of the target increases with the size of the target used. For example, said the company: 12mm for a 20mm long ferrite rod based target, or 27mm for a 40mm diameter air cored target.

Substantial misalignment can be tolerated between the coil axis and the measurement axis, according to CambrigeIC: “In float level sensing this means the float can move side to side with big clearances, so there is no risk of it becoming stuck in the float tube. Tolerance to angular misalignment means the liquid can slosh around with an angled surface level, which is important for applications in moving vehicles.” (see above diagram)

CambridgeIC-sensor-and-rodIn a ferrite-cored example (diagram left) +/- 2mm of variation is available in the gap. “For example, with the rod style target illustrated, error is no more than 0.15mm per mm of gap change,” company MD David Ely told Electronics Weekly. “In the other direction, across the sensor PCB, it is much less sensitive to misalignment.”

Angular misalignment in the gap direction can be up to 5°, with the rod example having ≤0.15mm error per degree. Again, the system is less sensitive to angular misalignment in the other direction (horizontal yaw in the diagram).

When measuring the linear position of valves and pistons, the company argues that this tolerance to misalignment means the sensor and target do not require additional linear bearings to constrain motion.

The company’s sensing products are resonant inductive, using wires or PCB tracks for coupling. An IC at the sensing end drives sender coils and makes the measurement, while a capacitor is needed at the mobile end to resonate the moving target coil.

For this new application, CambrigeIC’s existing CAM312 chip has been upgraded.

“The coil designs and processing algorithms in the CAM312 are substantially different to our existing standard parts,” said Ely. “Those algorithms are an order of magnitude more complex than for previous sensors. The root cause of the extra complexity is that you can’t energise the target with a single sensor coil along its entire travel.  At least 2 coils are needed for energising the target. Then you also need to know very precisely how much each of those coils couple to the target, which means extra measurements and calculation steps.”

“In spite of their complexity, the calculations inside the CAM312 have beauty and symmetry,” he added.

Communication with a host controller is through an SPI bus, and users lay-out the necessary coils on their own PCBs – the inherent accuracy of modern PCB fabrication delivers high sensor accuracy, said the company. Samples of a standard 78mm sensor are available.

A development kit is available for evaluating the new sensing technique and enhanced CAM312.

Applications are foreseen in valve control and liquid level control as well as position sensing in hydraulic actuators, pneumatic actuators and syringe plungers.