Trends Towards Single-Chip Optical and Magnetic Encoder iCs

Interview conducted by Will SoutterMar 10 2014

In this interview, Joachim Quasdorf, the sales and marketing manager for Encoder ASSPs at iC-Haus, talks to AZoSensors about the development of integrated single-chip systems for optical and magnetic encoders.

First of all, how do you define a single-chip encoder iC?

In our definition, a single-chip encoder includes integrated magnetic or optical sensor functions, signal conditioning, analog-to-digital conversion, respective interpolation, and a driver to interface with the motion controller.

By this approach the complete encoder function comes available in a small package of maybe 5mm x 5mm or less, requiring only an external magnet, or in the case of an optical encoder, the code disc and a light source.

This simplifies the integration of small encoder PCB-modules into the motor housing, to cater for space-constrained applications, and leads to important cost savings as well. For magnetic or optical encoder manufacturers, single-chip devices can open the door to upgrades in performance regarding speed and resolution.

So, integration is key! What are the core technologies which have enabled this type of integration?

The sub-micron CMOS technologies we are using are capable of integrating the magnetic and optical sensor functions on the same chip, together with all the required evaluation circuits.

With this aim in mind, the focus needs to be on high-performance analog circuit design, which is essential for precision signal conditioning and fast interpolation circuits to achieve the measurement performance required by various applications.

To reduce total system cost any further, in some cases line drivers are integrated as well. Advanced optical packaging is important too, such as  our optoQFN packaging which we had to develop. Last but not least, device programmablility is often appreciated by encoder and motor manufacturers, to allow them to use multiple output options depending on their industrial application requirements.

Figure 1. 3rd generation single chip magnetic encoder iC

What is the status of single-chip magnetic encoder iCs in the market?

A few years ago the first single-chip magnetic Hall encoder offered a resolution of 6 to 8 bits for scanning a rotating magnet or a linear magnetic tape. Next chip generations featured programmable resolutions of 6 to 12 bits, supported motor speeds of up to 120,000 RPM, and offered multiple output options with line drivers on the same chip.

For instance, the output options of the iC-MH8 included serial interfaces, SSI and the open BiSS (BiSS Interface), incremental quadrature outputs (ABZ), commutation outputs for BLDC motors (UVW), as well as differential sine/cosine outputs (1 Vpp).

Now, the 3^rd generation iC-MU allows a hollow shaft and we get up to 18 bit resolution from off-axis and linear applications by scanning two Vernier tracks (see Fig. 1).

And again, all output functions are configurable: UVW adapt to motors of up to 16 polepairs, and ABZ can have any count between 1 to 65536 CPR thanks to the unique FlexCount^TM arithmetic unit. This also sets the resolution for the serial interfaces, BiSS, SSI or SPI.

Despite its complexity, this system-on-chip occupies just 25 mm² on the PCB. The magnetic target can be varied -  linear tape, rotating drum or flat disc formats are all feasible. Due to its real-time processing, the full resolution is available at 20,000 RPM or at 16m/s if linear.

What about single-chip optical encoder integration?

Early bipolar opto encoder ICs combined the separate photodiode-amplifier-comparator channels, but the outputs were simply parallel. The next generation added differential scanning and LED control, but without embedded interpolation the resolution did not exceed 14 bits for a long time.

Figure 2. 3rd generation single chip optical encoder iC

CMOS technology changed the landscape a lot, boosting 3^rd generation devices’ complexity 10-fold by incorporating the features we know today.

New reliable optical packages were the key enabler for further integration. The packages needed to have a structured light window to adapt the photodiode pattern to suit the specific code disc required for the application. And re-defining resolution, diameter and CPR can now be offered as a turn-key design at reasonable cost and leadtime.

For instance, our modern iC-LNB single-chip optical encoder offers an absolute resolution of 18 bits, using real-time vector tracking interpolation from a 1” disc with sine/cosine of 1024 CPR and another 10 digital tracks for coarse position. Although the chip contains 26 photosensors, the radial scan length is just 5.5 mm. Absolute data output can be serial or parallel, but programmable incremental outputs with index are also available (Fig. 2).

Our other single-chip optical encoder iCs, likethe  iC-LGC, can even reach 21 bits and higher, limited only by the permissible code disc diameter. Fast serial motion control interfaces are also embedded, i.e. SSI and CRC-protected BiSS for up to 10 MHz, and another serial multi-turn interface can process revolution sensors, extending the position data length to up to 45 bit.

What are the differences between the capabilities of optical and magnetic integrated sensors?

The use of magnetic or optical encoders is driven by application demands. Traditionally, optical encoders have led the pack in terms of resolution. But when they areintegrated into motor housings,  optical scanners need to be protected, and potentially sealed against dust and oil.

For these reasons, magnetic encoders are often preferred in applications like this, as long as the resolution is sufficient. And today, magnetic single-chip Hall encoders achieve nearly the same resolution as the optical solution, and this technology is only going toimprove further.

There is almost no difference in flexibility between the two technologies regarding programmability and interface options. We have compared the major aspects of the technology and summarized our results in a technical White Paper, so that customers can better understand the trade-offs between magnetic and optical solutions.

How can users benefit from these trends towards greater integration?

Single-chip integration and innovative packaging is more than a trend. Customers can already benefit from existing components, to save board space, to cut component count, to increase reliability, and to reduce total system cost simultaneously.

The flexibility of having incremental ABZ, serial SSI/BiSS interfaces, analog Sin/Cos and digital commutation by UVW outputs reduces the mechanichal variety, efforts in development, device logistics and related costs, and can make it easier to respond to market demands worldwide.

For instance, single-chip motor encoder iCs replace the Hall switches used for block commutation, and allow speed and direction control, improving the efficiency of BLDC motor applications.

What is the outlook for the next few years?

A big topic at the moment is integrated multiturn functions, based on energy-harvesting solutions or utilizing a battery to maintain the position data. Mechanical gears hinder the mechanical integration, are expensive, and limit speed. A Hall chip-set soon coming available will offer a space-saving battery-buffered solution and achieves a resolution of 46 bit (iC-PV and iC-MHM).

Then there are also a few areas of ongoing development, which are always of interest - increasing the resolution, reducing total cost, but also adding valuable functions, for instance operation monitoring capabilities for maintainance and safety.The data to monitor will include the motor’s temperature, and provides redundancy for safety-oriented applications. As there is a fast-serial data link in place anyway, closer operational and environmental monitoring functions can be imagined.

Since drive makers are primarily driven by cost, from material to installation, it is also clear that motors, feedback mechanisms and power controls will continue to be combined further. Overall, system integration trends which can be observed right now will remain a long term development, certainly for application-specific motion control chips, and likely in other fields as well.

About Joachim F. Quasdorf, Dipl.-Ing.

Joachim F. Quasdorf works for iC-Haus GmbH, Germany, as Sales and Marketing Manager for Encoder ASSP.

He obtained his Solid State Electronics masters degree at Darmstadt University in 1988, and started afterwards at iC-Haus as bipolar IC designer.

Opto ASiCs were his special design focus, before he took over applications engineering a few years later.