Metal Target Encoders

Need a compact design but can not afford the $250+ for Heidenhain metal target sensors? Why not build your own absolute encoders with a BOM cost below $25 in volume production?

To self-calibrate, Heilenhain sensors require the movement of the rotor. Cambridge Encoders provides complete accuracy right at power-up before the rotor starts moving. We offer subcontracted R&D services to develop your own sensors using Renesas’ new IPS2200 ASIC. The object code for either STM32G4 or RA6T2 microcontroller is given to customers, who can then manufacture their own sensors without further input from us. If you need to change the coil geometry, Cambridge Encoders is here to assist you.

Cambridge-based companies have perfected resonant target technology; German companies use metal targets, which are better suited to space-constrained applications.

A new Renesas inductive metal target ASIC, IPS2200, is designed for automotive applications.

Metal target encoders require rotor movement for self-calibration, making motor encoders an ideal application for this technology. Cambridge Encoders has developed a self-calibration solution that allows absolute metal target encoders to be used after reduction gearboxes, even in applications without much angular movement at this point.

Metal target sensor technology is well known in the industry and is used by Heidenhain and Hella. Renesas manufactures automotive ASICs for metal target absolute inductive encoders.

With the IPS2200 ASIC, Cambridge Encoders can build more compact sensors that can be easily redesigned for specific applications. Our electronics offer a 125 kHz update rate with a group delay of 20 µs.

Based on our proprietary approach to the sensor and target design, we can develop absolute metal target sensors that can be used with larger air gaps. We have designed our electronics to achieve full sensor accuracy immediately after power-up. Further self-calibration algorithm sets in after the first full turn of the rotor. For applications with an electromagnetic brake, we offer a multi-turn counter.

Case Study

Direct-drive motors controlled by metal target encoders

A direct drive motor is a popular choice among mechanical engineers who need to design highly efficient, small and lightweight systems. Various low-profile form factors with a large through hole are convenient for routing cables and other system components. A frameless direct drive kit allows easy integration into machines, robotic joints and rotary tables. Different windings and form factors are possible. However, this flexibility comes at a price, as it is difficult to match the geometry of the motor with the appropriate through-hole optical or magnetic encoder.

By using 2D resolver technology – metal target motor encoders with rotor/stator built using printed circuit boards – the NRE cost of developing custom encoder geometries is minimised. The new IPS2200 automotive analogue front-end ASIC allows a much smaller footprint for electronics and facilitates the integration of electronics on stator PCBs. Stator coils can be designed by Cambridge Encoders for specific applications without much difficulty. The task of respin of electronics, although more complex, is simplified by using IPS2200 front-end ASIC.

Inductive metal target encoders offer the following advantages:

Kit encoders have a low profile
Easy integration into customised systems, support for outer-rotor (in-wheel motors)
The freedom to choose a maximum practical diameter for higher torque values
Smooth and accurate motion control
Improved encoder accuracy reduces speed ripple
The UVW commutation signals can be produced without additional Hall effect sensors
A BISS interface with scanning at up to 40 kHz provides for rapid control loops

Subcontracted services

You don't need to spend $250+ on Heidenhain encoders and then pay even more to read their proprietary EnDat output! The cost of the BOM for your own sensors starts at under $15, and you have the option of using the industry standard SSI, BISS or SPI outputs.

Cambridge Encoders does not manufacture metal target encoders for commercial sale. Most applications require custom geometries for kit encoders, and profit margins are low. Therefore, we offer metal target encoders as a technology consulting service for OEMs. In addition, we offer our customers non-exclusive licensing rights to use our smart algorithms implemented in the object code of the STM32G4 or RA6T2 microcontrollers.

Subcontracted R&D services are available to optimise the sensor’s geometry for your application. The IPS2200 ASIC, $2.5 in 10,000 units, is used for the analogue front-end design. The smart algorithms are implemented in either STM32G4 or RA6T2 microcontroller, and Cambridge Encoders can supply the object code as part of the final project delivery to high production volume customers.

We offer our customers a range of development boards to test our solution. Our customers have the option of using existing geometries or having us develop custom geometries for a fee. Cambridge Encoders offers pre-programmed microcontrollers for small batch orders. For a one-off payment, we can transfer the object code for the microcontroller to customers with production volumes of more than 50,000 units per year.

Cambridge Encoders is committed to supporting its existing customers by providing redesign services so that similar sensors can be used in your future mechatronic product lines. In any case, the process of manufacturing your own sensors is completely independent of Cambridge Encoders, as you own all the production documentation for your specific sensor project.

It is possible to extend the temperature range of the electronics to +125°C. Above +95°C, Cambridge Encoders recommend the use of ceramic loaded PCB materials instead of high-temperature FR4. In addition to noise-free 16-bit resolution, Cambridge Encoderds also offer 14-bit (80″) accuracy, equivalent to that of Heidenhain encoders. We support shaft speeds up to 24,000 rpm, air gap deviations up to ±0.5 mm and radial misalignment up to ±150 µm.

Contact Cambridge Encoders to discuss how we can help you develop your own bespoke motor encoders based on our design and more than 20 years of experience in developing such sensors.

Case Study

Using a metal target technology for encoders in robotic joints.

The design of robotic joints is constantly evolving to enable smaller, more compact assemblies with high precision and reliability. To take full advantage of the robot’s capabilities, the motion control that defines joint movements must be as precise as possible. The joints of most collaborative robots (cobots) today have two encoders that allow separate control of input and output rotation.

Due to the relatively slow speed of robotic joints, usually below 100 rpm, a direct drive motor (large diameter and short length) combined with a high ratio, a low profile gearbox is the most efficient solution in terms of torque and size. Harmonic gears are the most popular choice in smaller robots due to their lightweight, low profile and zero backlash. A harmonic gear uses a flexure to transmit motion, reducing rotational stiffness. Measuring the input and output joints simultaneously with high-precision encoders provides enough information to create a closed-loop algorithm that measures torque and eliminates the negative effects of reduced stiffness.

For robot joints with two encoders, the best options are:

Using a faster version of the metal target sensor for motor input control. During startup, cross-channel gain is calibrated. The metal target must be rotated to update the cross-channel gain calibration.
Output control using the four times slower metal target sensor. Cross-channel gain calibration does not require rotation of the metal target.
Resonant target technology is a more expensive but ultimately more accurate solution that provides a higher resolution. With this technology you can quadruple the resolution while maintaining a fast update rate.

Since each encoder is mounted inside the robot joint without the need for precise alignment, they offer the advantage of being the cheapest, most accurate and fastest control solution.

Technical data

Electrical

Voltage supply

4.75…30 VDC

Reverse polarity protection

Yes, including any wiring faults

Short-circuit proof

Yes

Consumption w/o load

≤60 mA (5 VDC)

Power consumption

typ. 0.3 W

Function

Multiturn (with Power off brake)
Singleturn

Interface

SSI 32 bit (multiturn)
SSI 24, 25, or 26 bit (singleturn)
BISS
SPI
UART 921,600/8-N-1
UVW commutation (optional)

Steps per revolution

16 bit (20″)
17 bit (10″) through hole > Ø 40 mm
18 bit (5″) through hole > Ø 80 mm

Number of revolutions

up to 14 bit (SSI)
16 bit (BISS, SPI)

Absolute accuracy

±0.02 ° (80″)
±0.01 ° (40″) through hole > Ø 40 mm
±0.005 ° (20″) through hole> Ø 80mm

Repeat accuracy

±0.01 ° (40″)
±0.005 ° (20″) through hole> Ø 30mm
±0.0025 ° (10″) through hole > Ø 80 mm

Sensing method

Inductive, metal target

Input signals

SSI/BISS/SPI clock
Zero setting input
Counting direction

Output stage

RS422

Output signals

SSI/BISS/SPI/UART data

Clock frequency

2 MHz (SSI)
10 MHz (BISS, SPI)

SSI configuration

Parity bit
Error bit
Output code: Gray or Binary

SSI Multipath transmission

Yes

Internal cycle time

8 µs, fast option
16 µs, slow option

Fixed latency

20 µs, synced to SSI/BISS requests
30 µs, slow option

Group delay

20 µs, fast option
40 µs, slow option

Full transient

50 µs, fast option
100 µs, slow option

Cyclical scanning

up to 40kHz, fast option
up to 25kHz, slow option

Gain mismatch in receive channels

The slow option: continuous update
The fast option: only at start-up and during rotation

Breakthrough in receive channels

The last valid value at startup.
Update of breakthrough values requires rotation of the metal target.

Interference immunity

EN 61000-6-2

Mechanical

Dimension

up to Ø 125 mm through hole
minimum size NEMA 17

Working temperature

-40 °C … +90 °C
-40 °C … +125 °C

Max. rotational speed

24,000 rpm

Installation height

from 7mm

Form factor

It is possible to mount electronics on top of a thick PCB (~3mm) with sensor coils (Heidenhain style)

Typical through hole size

Increased by ~12 mm compared to the resonant target technology. The typical relationships between internal and outer diameters are:
20 (ID) – 54 (OD)
25 (ID) – 59 (OD)
30 (ID) – 64 (OD)
40 (ID) – 75 (OD)
55 (ID) – 90 (OD)
68 (ID) – 105(OD)

Why CamEncoders?

Cambridge has become the leading centre for developing 2D resolvers - absolute encoders that use PCB coils. Over the years, IP developed in this cluster has been licenced to automotive and industrial companies. Cambridge Encoders developed sensor solutions for metal target sensors, licenced to Balluff GmbH and TT Electronics.

Stator PCBs with excitation and receive coils

Kollmorgen’s inventors first proposed a design for absolute rotary sensor using 2D variable transformer technology about 35 years ago. They proposed integrating the sensor coils on two planar pieces of an ordinary printed circuit board (PCB). One target PCB is attached to the rotating shaft and another PCB is stationary with additional ASIC electronics to measure the mutual angle of rotation between the two PCBs.

It took another 15 years for independent development teams in Cambridge, UK and Germany to perfect this technology. A crucial automotive rotary sensor – the combined torque and angle sensor on the steering wheel – is now manufactured using metal target sensor technology. A PCB-based rotation sensor was the only solution that could achieve the required level of robustness and accuracy, forming the basis for modern drive-by-wire vehicles.

Cambridge Encoders has over twenty years of experience in the development of resonant target sensors and metal target sensors. Our IP for metal target sensors are part of a successful range of linear sensors used in automotive and industrial applications by TT Electronics and Balluff.

The development of more powerful microcontrollers has allowed greater flexibility in the development of electronics for such PCB-based rotary sensors. It has therefore lowered the hurdle for the development of new customised PCB-based rotary sensors. The commercial availability of Renesas’ analogue ASIC simplified the layout of the electronics.

At Cambridge Encoders, we have experience in developing PCB-based rotation sensor technology that easily exceeds competitors’ specifications. Over the years we have carefully researched and optimised all aspects of the sensor:

Developing tools for designing more accurate coils on the PCB.
Improving the measurement algorithms to support faster rotation speeds.
Introducing important self-calibration algorithms that increase accuracy

Until now, the two main manufacturers of metal target sensors have been Hedidenhain for industrial use and Hella for automotive use. However, current advances in electronics make it possible for anyone to manufacture such sensors. With our help, it’s as easy as ordering PCBAs from the factory. Contact Cambridge Encoders to find out how we can help you make your own kit encoders comprising just two PCBs.