Resonant Target Encoders

Absolute encoders with the best technical specification;
Cambridge Encoders do not require tight mechanical installation tolerances;
Ordinary technicians can easily install kit encoders and bearingless sensors;
16-bit (20″) accuracy is guaranteed right out of the box.

Kit encoders:

19-bit resolution, 16-bit accuracy

Kit encoders are the most affordable choice when it comes to installing encoders in mechatronic systems. Our kit encoders go one step further by reducing installation costs and improving yields by eliminating the strict mechanical tolerances required by magnetic and optical encoders.

In most cases, you can use our kit encoders right away after installation. A set of electronics PCB, sensor PCB, and target PCB has been fully tested in production. After installation, you do not need to rotate the target to calibrate sensor performance. However, we offer an optional separate master adapter to perform full functional tests after sensor installation. A PC can read raw signals via USB. With special software you can check the circular shape of the signals recorded in the quadrature receive channels.

The electronics and the sensors have redundant measuring channels built in. The green LED lights up when the basic redundancy checks are successful. The functional checks are carried out continuously. A red LED indicates an error in the sensor. The digital output shows the error flag in that case.

20-bit resolution, 17-bit accuracy

Due to the increased number of fine scale periods in the receiving channels, larger diameter sensors can achieve a resolution of 20 bits (1.24″) and an accuracy of 17 bits (10″). The electronics board is mounted on the side of the sensor with a bottom entry connector. The electronics PCB is arranged in an arc section with an approximate length of 120 mm (65 degrees). Such an arc section has an outer diameter of 205 mm and an inner diameter of 165 mm. The top plate above the rotating target acts as a mechanical and electrical shield.

Bearingless encoders

The bearingless encoder is the preferred option for simplified installation. The raw signals and other debugging information were fully investigated after the encoders were assembled in production.

Although bearingless encoders can operate at 24,000 rpm, they are best suited for high-precision applications and provide uncompromised accuracy immediately after power-up.

The through-hole encoder offers 19-bit (2.5″) resolution and 16-bit (20″) accuracy. A 58 mm end-of-shaft encoder offers a resolution of 18 bits (5″) and an accuracy of 15 bits (40″).

Traditional form factors:

18-bit resolution for 58mm, 19-bit resolution for 25mm through-hole encoders

The ruggedised industrial encoders with integrated bearings are designed for demanding applications. These encoders are sealed to IP65 and can be used in a range of industrial test benches.

The rotational speed is only limited by the rubber seals in the bearings. A metal shielded bearing can be installed to allow a higher rotational speed.

Bearingless encoders offer more precise measurements as they do not generate mechanical vibrations related to the eccentricity of the shaft coupling. We recommend the bearingless options especially for the end-of-shaft sensors.

Why Resonant?

The use of a resonant target eliminates one of the significant error channels in the sensor and thus improves its linearity.

Inductive rotation coupling is utilised in robust angle sensors, so-called resolvers, used in many industries. One way to construct a cheaper and more accurate resolver version is to use two parallel circuit boards with an excitation coil on the stator PCB and either a coil or metal pattern on the rotor PCB. Such angular sensors have two sets of receiver coils on the stator PCB that are very similar but angularly offset to create sine and cosine receive channels. The rotor position is determined by calculating the arctangent function of the ratio of the signals generated by the target in the two receive channels of the stator PCB.

This technology has been referred to as a ratiometric planar inductive sensor. The coils on the rotor and stator PCB are inductively coupled along the entire ring of the rotating target. Because the coils are rotationally symmetrical, the absolute encoder can withstand small misalignments of up to a few hundred microns between the centres of the rotor and stator PCBs without degrading its linearity.

This PCB-based sensor technology has been developed independently in Germany and Cambridge over the last two decades.

The mutual inductance between the receiving coils and the excitation coil is zero in an ideal sensor. In actual absolute encoders, significant unwanted offset in the receive channels is caused by the remaining non-zero mutual inductance. The offset is added to the signals induced by the rotor PCB coils and degrades the sensor’s accuracy. Cambridge engineers added a capacitor to the rotor PCB coils to resonate them at the excitation frequency. The induced signals are boosted, and their electrical phase is shifted 90 degrees from the offset signals. This modification improved the ratio of target iduced signals to offset by at least fifty times. As a result, the unwanted offset makes only an insignificant contribution to the overall non-linearity budget.

This improvement is not present in absolute encoders with metal targets from German manufacturers, which limits their accuracy. Cambridge Encoders offers the most advanced rotary sensors based on resonant target technology. Our algorithms are optimised to provide the best possible update rate, resolution and accuracy.