360° Rotary Resonant Inductive C Sensor


C sensors measure the absolute position of a target over a full 360° of rotation, using a thin rectangular PCB curved into a C shape around the rotation axis.

This sensor works in combination with a rotating target comprising twin, wound, ferrite rods.

The sensor and target combination measure over a full 360°, even though each only occupies just over 180° of arc.

C sensors operate using the resonant inductive principle - view >>

The C22mm sensor has two sets of sensor coils: one for taking fine incremental measurements at high accuracy and resolution and another for coarse, absolute measurements. The CambridgeIC CTU chip combines the information from both sets of coils to deliver an absolute, high accuracy and high resolution output to a host system.

C sensors are available complete with IP67 housing from TURCK, under their product name DSU35. Interfaces include 0-10V and 4-20mA, both teachable over its interface plug.

The technology is also available for embedded applications from CambridgeIC. CambridgeIC's CAM204 IC is used for processing, and includes an SPI interface for host communication.

Full absolute sensing over 360°
Sensor can be retrofitted to side of shaft
Target can be retrofitted to side of shaft

Figure 1 - CambridgeIC's C22mm Rotary Sensor PCB

Typical Performance
< ±0.5° error (aligned)
< ±2° error (0.5mm misalignment radial + axial)
> 13 bits of Noise Free Resolution with CAM204 IC
Up to 1mm radial misalignment
Up to 1mm axial misalignment

Through shaft retrofit without removing the shaft
Valve position sensing
Absolute optical encoder replacement

Figure 2 - Cross section views of CambridgeIC's C22mm sensor and matching target

1 - Principle of Operation

1.1 Overview
The sensor PCB comprises 5 printed coils: COSA, SINA, COSB, SINB and EX. Its equivalent circuit is illustrated in Figure 3. All 5 coils couple to the target's resonant circuit, which rotates relative to the sensor.

The EX coil is for exciting this resonator. The magnetic coupling between excitation coil and resonator is uniform with rotation angle, so that the excitation coil powers the resonator whatever the rotation angle.

The other 4 coils are sensor coils, and are patterned so that their coupling factors to the resonator vary sinusoidally, as shown in section 1.3. The CTU circuit connected to the sensor detects the coupling factors and uses them to determine position.

The resonator comprises 2 wound ferrite rods placed on opposite sides of the rotation axis, shown in Figure 2. These are distinguished by the relative placement of the coils on the rods, enabling the sensor to determine target angle over a full 360°.

Figure 3 - Equivalent Circuit of C22mm sensor and target

1.2 Electronic Interrogation
The sensor is connected to a CambridgeIC CTU chip (e.g. the CAM204) and its associated circuitry. To take a position measurement the CTU chip first generates a few cycles of AC current in the EX coil matching the resonant frequency of the resonator. This current forces the resonator to resonate. When the excitation current is removed the resonator continues to resonate, with its "envelope" decaying exponentially as shown in Figure 4. This decaying signal generates EMFs in the 4 sensor coils. The CTU chip detects the relative amplitude of the decaying resonator signal in each coil. It uses the amplitude information to determine position, as described below in section 1.3.

Figure 4 - Electronic Interrogation Process

1.3 Sensor Coils and Position Calculation
Figure 5 is a schematic representation of the sensor and target coils. Each coil is simplified and shown with one turn and without cross connections. To help with explanation, the coils are shown flat rather than curved. The Y-axis is upwards as drawn, and the circumferential direction is to the right.

The excitation coil is in two rectangular portions, one upper and one lower. These are connected together to form a single coil. When Ferrite Rod X is in its vicinity as drawn, field flows out of the upper excitation coil, through the Ferrite Rod X from Location C to Location A, and back into the lower excitation coil. When the angle changes so that Ferrite Rod Y is in its vicinity, field flows out of the upper excitation coil, through the Ferrite Rod X from Location A to Location C, and back into the lower excitation coil. When the measured angle is near 90° or -90°, the excitation coil couples with both ferrite rods, with some flowing downwards through each rod. This variation in coupling is illustrated at the top of Figure 6.

Since they are wound in the same direction, the excitation coil always couples with one or the other or both targets with a roughly uniform coupling factor. The resonant circuit formed by the series combination of the two windings and resonating capacitor is therefore powered by the excitation coil whatever the angle, and with uniform electrical phase. The upper portions of each ferrite rod, Ferrite Rod X Location C and Ferrite Rod Y Location A, have a magnetic potential in phase with the upper excitation coil, and their opposite ends have the opposite phase magnetic potential.

Figure 5 - Schematic representation of sensor and target coils, shown flat

The COSA and SINA coils are also each formed in two portions, one pair wound inside the upper excitation coil portion and one in the lower. The upper portion of the COSA coil is connected in series with the lower portion, and the same for the SINA coil. The winding directions flip between top and bottom portions, like the excitation coil, so that coupling contributions from the top and bottom locations of each ferrite rod reinforce each other.

The C22mm sensor has 3 sinusoidally patterned periods of COSA and SINA across 180° (6 half sine loops). As the measured angle changes from -90° to +90°, Ferrite Rod X couples with the SINA and COSA coils and induces EMFs whose amplitudes vary sinusoidally with angle, repeating 3 times across that 180° range. For the remaining 180° of rotation ferrite rod Y couples with the COSA and SINA coils, inducing a further 3 periods of sinusoidal amplitude variation. Near -90° and +90° both ferrite rods couple to the COSA and SINA coils. Since the ferrite rods are wound in the same direction, and the SINA and COSA coils are wound in the same direction near -90° and +90°, the EMFs induced in the COSA and SINA coils include contributions from each ferrite rod, with these contributions reinforcing one another. The result is that the coupling factor between the target and COSA and SINA coils varies continuously across a full 360° of rotation, with 6 sinusoidal periods and uniform amplitude. This is shown in the second graph from the top of Figure 6.

The CTU chip detects the EMF amplitudes in the COSA and SINA coils, and uses their ratio and signs to determine Fine Position. Fine Position is an accurate and high-resolution indication of angle, but is not absolute due to the repeating nature of the COSA and SINA coils.

Figure 6 - Sensor coil coupling factors and position calculation results plotted against measured angle

The COSB and SINB coils are used to determine absolute position. They are located between the upper and lower portions of the excitation coil, running through the middle of the sensor shown in Figure 5. When the measured angle is near 0°, ferrite rod X's location B couples with the COSB coil. This location is on the same side of ferrite rod X's winding as location C, whose magnetic potential is in phase with the excitation. The target therefore induces an EMF in the COSB coil with positive EMF amplitude. Conversely, when the measured angle is near 180°, ferrite rod Y's location B couples with the COSB coil. This location is on the same side of ferrite rod Y's winding as location C, whose magnetic potential is this time opposite that of the excitation. The target therefore induces an EMF in the COSB coil with negative EMF amplitude. The shape of the COSB coil is designed so that the coupling factor varies smoothly between these states with a sinusoidal patterning, as shown in Figure 6. The same reasoning applies to the SINB coil, except that this is phase shifted by 90° to yield sinusoidal variation in phase quadrature.

The CAM204 CTU chip detects the EMF amplitudes in the COSB and SINB coils, and uses their ratio and signs to determine Coarse Position. Coarse Position is a rough, low-resolution indication of angle, but unlike Fine Position it is Absolute.

The CTU chip combines fine and coarse position indications, so that its final output to the host has the accuracy and resolution of the "fine" reading and full absolute information from the "coarse".

2 - Document History

Revision Date Description
17 February 2014 - First draft, based on extracts of earlier sensor datasheet

3 - Contact Information

Cambridge Integrated Circuits Ltd

21 Sedley Taylor Road, Cambridge, CB2 8PW UK
Tel: +44 (0) 1223 413500
Email: info@cambridgeic.com

4 - Legal

This document is © 2012-2014 Cambridge Integrated Circuits Ltd (CambridgeIC). It may not be reproduced, in whole or part, either in written or electronic form, without the consent of CambridgeIC. This document is subject to change without notice. The publication of this document does not imply any license to use patents or other intellectual property rights.

The design of the sensor, comprising each of the patterned copper layers, drill locations, silk screens, assembly layers and board outline are protected by copyright.

The technology and parts described in this datasheet is subject to the following patents: US8570028, GB2461448. Other patents are pending.

February 2014


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