Inductive kit encoders

Kit Encoders with datasheet accuracy out of the box.

Screw-and-forget installation.

±9 arcsec across the full ±0.5 mm of axial tolerance. No shims. No gap gauges. No inspection jig. No calibration run after mounting.

Fit the target, bolt the stator, plug the cable. The encoder calibrates itself during the first partial turn, then holds full datasheet accuracy at standstill and on every power-up.

Measured performance — standard range
−40 °C to +115 °C

±9 arcsec

Measured position error

120 kHz

Internal position update

±0.5 mm

Air gap installation margin

400 Hz

Correction update even at standstill

Kit Encoders for direct drives, rotary stages, robot joints, indexing mechanisms and gimbals.

01 · The case for a different encoder

We know metal-target sensing inside out. That is why our Kit Encoders use a different approach.

Our metal-target sensor IP is licensed to Balluff, Continental and TT Electronics and ships inside their automotive and industrial linear sensor ranges.

For our Kit Encoders, we chose resonant-target technology to deliver much better accuracy and wider installation margins.

That choice turns a complex installation procedure into a simple screw-and-forget production step. No precision shimming. No air-gap inspection. Fit the target, bolt the stator, plug the cable. Screw-and-forget.

The encoder invoice is only the start.

The encoder invoice does not include the shim stack, gap gauge, inspection jig or calibration fixture.

Somebody has to write the work instruction for setting the tight air gap. Somebody has to follow it, on every unit, for the life of the product. Somebody has to own the inspection step, keep the fixture qualified, train the operators, retrain them, and take the call when a unit that passed the bench comes back from a customer.

None of that is on the encoder invoice. All of it is on your line. CamEncoders Kit Encoders remove that work.

CamEncoders Kit Encoders allow a nominal air gap of 1.4 mm ±0.5 mm and need no post-installation calibration run.

Inductive sensing relaxed radial alignment. We widened the air-gap tolerance.

Inductive encoders measure coupling around the full circumference, so rotor eccentricity averages out. Concentricity stops being an accuracy problem. It becomes a ±0.15 mm radial alignment tolerance. Inductive sensing also stands up to dust, oil, condensation, shock and vibration.

But inductive kit encoders are not yet screw-and-forget. The axial gap is still tight, with a typical nominal air gap around 0.35 mm and permitted variation typically ±0.125 mm, so shimming, inspection, and sometimes a calibration run remain part of assembly.

We eliminate the installation burden. Four times the air gap. Four times the tolerance margin.

Remove these steps from every build.

  •  No precision gap setting: no shims, gap gauges or inspection jigs.
  •  No post-installation calibration run or calibration fixture.
  •  No operator training for gap setting or post-installation calibration.

 

Every CamEncoders Kit Encoder assembled anywhere in the ±0.5 mm window meets specification after its first partial turn. Fewer rejects. No rework.

Before you commit to precision gap setting on every build, discuss your installation with us.

Margin that survives assembly

Has a batch of machined parts ever held up a build because the encoder’s air gap became out of tolerance?

An axial tolerance window is a budget. Housing stack-up, bearing end-float and deflection under load all spend it. At ±0.1 mm, the assembly stack alone consumes more than the window allows, and what remains after shimming must cover everything else.

At ±0.5 mm, the assembly stack takes a fraction, and the rest is margin. That is the difference between a tolerance you manage at every build and one you stop thinking about.

Why a wider gap tolerance needs a different encoder

A tight air gap is not an oversight waiting to be resolved in your supplier’s next product generation. It is a fundamental choice driven by sound engineering.

More fine-scale periods improve datasheet position accuracy and shorten the rotation span needed for self-calibration. However, more periods mean a shorter spatial period, with a proportionally smaller nominal air gap and tighter installation tolerances.

Our wider air-gap tolerance comes from lower error within each fine-scale period. That lets us use fewer periods without sacrificing datasheet accuracy.

02 · Standstill

Your axis holds position. Your encoder does not hold its accuracy.

How much does your axis move, and how often?

Inductive encoders correct their electrical zero while the rotor turns. On a shaft that keeps turning, a modern encoder keeps that correction fresh and holds its published accuracy. That is sound engineering, and it is the right answer for a motor shaft.

The limitation appears when an axis holds position or works within a small arc for prolonged periods: a direct drive under load, a tracking gimbal, or a robot arm whose workpiece is brought to it.

On those axes, the correction that depends on rotation has nothing to work with, so the drift accumulates as the motor warms. The controller follows the encoder’s drifting zero.

The axis moves while the reported position stays on target.

A temperature test that turns the shaft will not show it. Months later, the customer calls. Their machine has a fault nobody can reproduce.

CamEncoders Kit Encoders correct the analogue receive chain at a 400 Hz update rate even if the shaft stands still. Full ±9 arcsec at power-up, at zero speed, and after a dwell of any length, from −40 °C to +115 °C.

03 · For the engineer who signs it off

Main points for your design review.

Installation cost

Gap setting, inspection and calibration are paid for on every unit, for the life of the product, and none of it appears on the encoder invoice.

Standstill accuracy

A temperature test that turns the shaft will not show drift that only appears when the axis holds position.

Derived motion

The datasheet accuracy figure does not describe the error in derived motion such as calculated velocity and acceleration.

Find out why we use resonant-target technology when most manufacturers use a metal-target approach. See why inductive sensing relaxes radial alignment, why the axial air gap usually remains restrictive, and how our receive-chain self-calibration continues without rotation.

Use the information we provide in your design review. Ask production and purchasing to compare the cost of achieving datasheet accuracy on every unit. Our designers can join the call and answer your team’s questions directly.

04 · Start with your axis

No thermal drift to chase. No precision air-gap setting.

Tell us the outer diameter you can fit, the clear bore you need, and how far the axis travels in normal operation.

Nine standard sizes, with outer diameters from 70 to 200 mm and clear bores from 20 to 110 mm. Patented architecture. Designed in Cambridge.

Inductive vs optical encoders

The developers of mechatronic systems are aware that their products' quality depends mainly on the quality of the encoders integrated into the rotating joints.
Inductive rotary sensors measure signals along the entire circular path and therefore offer a degree of symmetry that optical encoders lack.
The optical sensor:
A target misalignment of 15 µm can cause an error of up to 250″. It is possible to compensate for this error by using several distributed sensors and combining their readings as shown in the figure.

The inductive sensor:
The rotational symmetry in the multi-period sin/cos channel system of the inductive sensor makes it immune to alignment errors of up to 150 µm, keeping the overall accuracy of the system at 20″. This is a far better technique for suppressing alignment errors than the distributed optical sensor approach. Consequently, the use of inductive encoders greatly simplifies the installation of sensors in mechatronic systems and helps achieve much higher accuracy.
Absolute optical encoders with multiple detectors for improved accuracy
Precise micron-level alignment of the optical scale to the centre of rotation is achieved by using a custom-built adjustment rig for optical encoders. However, in actual mechatronic systems, your rotor is used with deep groove ball bearings that have a standard radial clearance of 15 µm. This clearance is required to compensate for interference fittings and thermal expansion. Non-zero radial clearance makes claims of accuracy that can be achieved with optical or magnetic sensors unfeasible in most real-world applications. Compare this to the 150 µm required by an inductive encoder and you can see the value of inductive sensing technology!
Optical encoder error mode
A sinusoidal error Δϕ caused by the eccentricity e in the measurements made by an optical encoder, as explained in the Heidenhain tutorial.
M= centre of graduation; ϕ = “true” angle; ϕ’ = sampled angle.
Error plot in absolute optical encoder
Measurement error Δϕ as a function of the true angle ϕ. Eccentricity value e =15 µm, graduation diameter of optical encoder D = 24.85 mm .

Motor encoders

System engineers need to understand why certain encoder technologies are only suitable for motor control applications.
The sensor technologies used in encoders usually provide a pair of so-called sine/cosine channels. The ratiometric answer, ϕ= atan(Vsin/Vcos) , can be calculated from the voltage amplitudes measured in quadrature channels. Ratiometric sensors can be represented graphically by plotting the voltage in the cosine channel Vcos against the voltage in the sine channel Vsin.
Most ratiometric sensors are not error-free. As shown in the figure, the centre of the cosine/ sine curve can deviate significantly from zero. The deviation of the centre of the curve from zero implies non-zero breakthrough signals in both channels that are not related to the target rotation.
This significant error channel is typical of capacitive, Hall effect, and metal target inductive sensors. Resonant target technology shows a negligible shift from zero for the cosine versus sine curve’s centre. It makes a resonant target technology an exception and enables it to deliver much more accurate encoders.
All ASICs have implemented an algorithm that dynamically calibrates the breakthrough signal shown in the figure. They can also determine the difference in the gain of the sine and cosine channels to produce an accurate circular Vcos vs Vsin curve. Such calibration requires several revolutions of the target. The encoder output will have a significant linearity error until self-calibration is finished after power-up.
For many applications, self-calibration during target rotation is a good option. Such self-calibrated sensors have already been used successfully in motor control. Cambridge Encoders has developed proprietary solutions that extend the use of inductive sensors to other angle control applications.
Cosine vs Sine error plot of signals in an absolute encoder
A sinusoidal error Δϕ due to the breakthrough in the measurements of Vsin and Vcos. ϕ = “true” angle; ϕ’ = sampled angle. The shift of the received signals from zero, when represented graphically, is similar to the eccentricity e described for optical sensors. However, it is a completely different problem.

Metal Target

Cambridge Encoders offer subcontracted services to develop sensors using Renesas’ IPS2200 ASIC analogue front-end. The cutomers of Cambridge Encoders receive object code for the microcontroller so they can make their own sensors without any further input from us. We can always help you change the coil geometry if you need to.
An eddy current principle: Rotary Target has a metal pattern that shields inductive AC magnetic field generated by an excitation coil.

Advantages:
  • Encoder has a low installation profile.
  • Renesas’ analogue front-end ASIC simplifies electronics design
  • CamEncoders offers a non-exclusive licence for the sensor design and the microcontroller object code.

Disadvantages:
  • Direct breakthrough is in phase with sensor signals.
  • Four times lower resolution.
  • Target rotation is required for sensor self-calibration.
In inductive sensors, the mutual inductance between the excitation coils and the receiving coils is close to zero. However, zero is an elusive number; non-zero breakthrough is one of the main error source. When  target is rotated, the breakthrough signals can be measured as the deviation of the centre of the Vcos vs Vsin curve from zero.
In Cambridge Encoders solution, the last valid value of the calibrated breakthrough signals is read from the non-volatile memory at the start-up. The gain mismatch between the measurement channels is corrected during start-up using a method that does not require the target to be moved. Full accuracy is therefore available immediately after switch-on. As the target rotates, the shape of the receive signals is used to continuously update the gain mismatch and calibrate and write to the memory the updated value for the breakthrough signals.

Resonant Target

Cambridge Encoders offer kit encoders that consist of several PCBs and are installed in the mechatronic system on site. Our products include bearingless encoders and ruggedised encoders with integrated bearings. Customers can choose their form factor and we can develop customised solutions for them, for which we charge development fees.
A resonant circuit consists of a precisely shaped coil and a capacitor. It is tuned to resonate at the frequency of excitation coil drive.

Advantages:
  • The resonant coil signals are 90 degrees out of phase, so they are in quadrature with the direct breakdown signals.
  • Higher resolution and accuracy compared to metal targets.
  • Target does not need to be moved for sensor self-calibration.

Disadvantages:
  • Greater installation height
  • Commercial ASICs are not available; the analogue design is implemented in discrete, commercially available components.
Cambridge-based encoder companies have perfected resonant target technology; German encoder companies use metal targets. The resonant target technology has a unique advantage – it reduces unwanted breakthrough from excitation coil to receiving coils by a factor of about fifty, making it an insignificant source of linearity error.
With our inductive sensor electronics, we  continuously check and calibrate the gain mismatch between the measurement channels, regardless of whether the target is stationary or rotating rapidly. The receive channels are always self-calibrated, regardless of the target’s range of motion. Encoder applications benefit from this approach especially when the target movement is severely restricted most of the time. Resonant target technology is recommended by Cambridge Encoders for precise measurements following reduction gearboxes.