Absolute Encoder Servo Control with BiSS Technology

INDUSTRIAL ENCODER CORPORATION

Member of the GESgroup of Companies
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BiSS Technology Encoders

From Industrial Encoder Corporation

March 7, 2008
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Industrial Encoder Corporation (IEC) has extended the features of its Absolute Encoder line to include precision servo motor absolute position and speed feedback in industrial control and hazardous duty applications. These encoders include encoder disks that cannot be broken, modern Opto-ASIC design, and rugged machined housings and covers.

For Single Turn Absolute Encoders, the standard resolution is 12 bits with an option to increase resolution to 16-21 bits. In addition, an analog sine/cosine signal is available with 2048 counts per turn resolution.

For Multi-Turn Absolute Encoders, the standard resolution is 24 bits (12 bits single turn with 4096 revolutions) and an option to increase resolution to 25-28 bits. The Multi-turn is also available with analog sine/cosine signals of 2048 counts per turn resolution.

The interface for servo motor control is derived with the use of BiSS technology. IEC selected BiSS, an open interface protocol, to provide the flexibility to satisfy the requirements of multiple customers and to provide a highly reliable solution to the industry.

This servomotor control solution is available in Solid Shaft and Hollow Shaft absolute encoders, and with IP66 and Explosion Proof ratings. List prices start at under $300 and depend upon options selected and quantity used.

IEC’s products have been proven and used extensively in Robots, Cranes, Forestry, Oil and Gas, Machining, Elevator, Factory Automation and Food Processing, and Energy Production applications. IEC is certified to certain UL standards and for Explosion Proof requirements.

For more information regarding a specific application or the IEC competitive advantages, please contact either Tony Petrecca at 888-277-6205, or tony.petrecca@globalencoder.com or Jon Gum, Motion Control Research, at
435-627-1086, or jon.gum@globalencoder.com.

www.biss-interface.com

Review www.biss-interface for several interesting downloads.

Dynapar on Interface Protocols

Open vs. Closed Encoder Communication Protocols:
How to Choose the Right Protocol For Your Application
Mr. Cory Mahn
Applications Engineer
Danaher Industrial Controls Group
Gurnee, IL

I. Introduction
Imagine that you are specifying an encoder to provide motion control feedback. You have
already decided whether the encoder needs to be incremental or absolute, and you now have to
decide on the communication protocol. The application has too many devices to use parallel
communication, and too few to require a bus such as DeviceNet or Profibus. The best choice
appears to be a serial communication link. Unfortunately, the numerous choices in serial
communication links can make the choice a confusing task. On what basis should you choose
one protocol over another? This application example highlights the key questions typical for
most applications. This paper is intended to answer and demystify some of these questions and
help you choose the right serial communications protocol for your encoder application.

II. Open vs. Closed Communications Protocols:
What does it mean to me and my application?

You might have noticed that some communication protocols are available from a limited
number of manufacturers. Chances are that these protocols are closed-type communications
protocols. Closed communication protocols are proprietary, meaning that they require a user
license and license fee for users to design their own interface with a closed protocol slave.
Detailed specifications on a proprietary protocol are restricted; they are available to license
holders and not the general public. Closed-protocol devices are essentially ‘black-box’ devices
because not much is known about the specifics of their operation. The most common closed
serial communication protocols for encoders include EnDat® and HIPERFACE®, which were
developed by individual encoder manufacturers.
Open communication protocols, on the other hand, are non-proprietary and non-restrictive.
The developers of communication protocols freely share specifications regarding data format
and electrical design. Designers of products that use serial communications, such as sensors
and encoders, can create their own interfaces without paying a licensing fee. This means that –
when you are shopping for a replacement serial communication device – you’ll find many more
encoders available on the market. The most popular open communications protocol is
Synchronous Serial Interface (SSI). SSI is a well accepted and time-tested protocol. More
recently, the Bi-directional Synchronous Serial Interface (BiSS) open serial protocol has come
into the market. Using SSI as a basis, it represents an alternative to bi-directional
communication protocols such as EnDat® and HIPERFACE®.

III. Why use a closed communications protocol?

There is one good reason to use a closed proprietary protocol; simplicity. For instance, the
communication protocols slave configuration software is usually pre-designed, saving time and
development costs. Another reason may be that a user prefers a particular vendor’s product
and agrees to pay the licensing fee in exchange for the ability to completely rely on that
manufacturer for all levels of required support for the encoder or sensor. Of course the
drawback to this approach is that the user limits his/her options in the future, when a
replacement device is needed.

IV. Why use an open communications protocol?

The key benefits to using an open communication protocol include:
• Availability – Encoder buyers have more selection options and alternative
manufacturers to choose from.
• Cost –Manufacturers of products using closed communication protocols can charge
any fee that the market will bear for a replacement encoder, since there is virtually no
competition from other manufacturers. On the other hand, competition among suppliers
of open-communication-based products promotes price and product alternatives.
• Information - Closed protocol developers limit the amount of information that is
published and available to buyers. However, if the need exists, an educated user can
check the compliance of an open protocol from a specific manufacturer. Information
regarding that protocol is freely posted on the internet.
V. How do the various communication protocols operate?
In order to select the right protocol, buyers must fully understand the operating capabilities and
functional limitations of each of the available feedback control protocols.
SSI
SSI is the foundation on which all of the aforementioned protocols are based. SSI
communication requires two twisted pair wires plus two wires for power. Encoders may
also be supplied with additional incremental outputs for speed feedback. For serial
communications, one pair of wires is for a differential clock signal and the other pair is
for data feedback from the sensor. Clock frequencies can be as high as 1.5MHz.
However, as clock frequency increases, the maximum cable run decreases, which is a
limitation common to all protocols.

Cable Length Data Rate
50 m 400 KHz
100 m 300 KHz
200 m 200 KHz
400 m 100 KHz
Table #1, Cable Length vs. Data Rate
The data frame length depends on the device and its resolution. In an SSI interface,
there is one slave per master, referred to as a point-to-point connection. The clock
remains high until the master needs information from the sensor. It then sends a stream
of clock pulses equivalent to the number of bits of information from the sensor.
Samples of each bit are usually taken on the falling edge of the clock pulse. This
insures that the propagation and process delays are accounted for. Sensor
manufacturers are free to insert alarm bits if required, but the requirement must be
provided before the product is shipped, and a single alarm bit may have several
meanings. See Figure 1 for an example of what an SSI trace may look like.
Figure #1, SSI Communication Format

VI. EnDat®

EnDat is a proprietary protocol created by Heidenhain of Germany. EnDat is similar to
SSI in that it is a point-to-point connection. EnDat is also a synchronous protocol like
SSI, meaning that the data, with a speed of 4 MHz, is received as clock pulses are sent
simultaneously. However, much more data can be read from an encoder and stored
due to the provided internal memory. This can include diagnostics, identification,
alarms, and warnings. It can also contain information about the motor in which it was
mounted to such as model and serial numbers. Varying clock pulse stream lengths are
also required from the varying data that may be received or sent to the device. Also,
using a function called a Datum Shift, an encoder can easily be reset to a new zero or
reference point. The Datum Shift is a value that is added to the physical position of an
encoder. SSI provides a direct reading of the physical position, so it requires the end
user to rotate the shaft to the zero position.
The hardware level minimally requires six wires for communications. Depending on the
version of EnDat, there may be an additional wire that carries an analog incremental
output for faster speed sampling. This is useful for speed-controlling drives that can be
more demanding than the rate at which serial feedback is normally provided.
For more information on the EnDat protocol refer to the Heidenhain website at
www.heidenhain.com.

VII. HIPERFACE®

HIPERFACE is a proprietary serial protocol created by Max Stegmann GmbH.
HIPERFACE can be point-to-point like SSI and EnDat, but it can also use bus
connections in which several encoders can be wired together and addressed by a
single master. HIPERFACE is the only one of the four major serial protocols that is
asynchronous. It uses bidirectional RS-485 communications to send and receive data.
The data comes at a relatively slower rate of 38.4Kbps, but contains much more
information than provided in SSI communications.
Like EnDat, it has internal memory that can hold motor information such as voltage,
current, and other parameters. Using a licensed master, you can also define an
absolute zero position. Because HIPERFACE is proprietary, the user doesn’t know how
the communications work and how parameters are addressed. Setting and storing
parameters are essentially “plug and chug” with the manufacturer provided software.
Generally, the data sent and received from the master is at a fixed length and is not
controllable by the end user or OEM.
At the hardware level, eight wires are required. The proprietary SinCos® output requires
four wires for each differential pair, with two wires required for data communication and
two for power.
For more information on HIPERFACE, refer to the Stegmann website at www.sickstegmann.
de.

VIII. BiSS

BiSS is the latest protocol designed and developed by IC-Haus, Germany. It was
created to be an open non-proprietary protocol alternative to EnDat® and HIPERFACE®
but designed with the capabilities of the proprietary protocols in mind. Combined
features such as alarms, warnings, and the ability to store motor information to the
encoder exist in the BiSS protocol.
BiSS has two modes; sensor mode and register mode. In sensor mode, the sensor or
encoder communicates in a manner similar to SSI. The master begins to send a stream
of clock pulses. Eventually the data line level will drop low and data sampling will begin.
The data can be received and clocked at a blazing 10MHz. Due to the speed of
transmission, many drives may not require additional analog incremental outputs to
control motor speed. In ACURO encoders available from Danaher Industrial Controls
Group, the propagation and calculation delay is so brief that communication is
backwards compatible with the SSI protocol. Figure 2 shows the communication format
and a data frame map.

Figure #2, BiSS Sensor Mode
In register mode, the protocol modulates the clock pulse width to address specific
slaves and parameters. This mode is unlike any other protocol. If in the sensor
mode a warning or alarm bit is set high by the sensor or encoder, the end-user may
access the register mode and find specifics on the alarm or warning. This might be
an over-temperature warning or, in the case of an encoder, a disk pollution alarm.
Danaher Industrial Controls Group is currently developing the ability to provide
single step alarms in case improper position is being provided in sequence. Other
information such as acceleration, temperature, and identification can be retrieved
from an encoder with BiSS. Also, as mentioned before, register mode allows for
sending and storing data to the encoder. Figure 3 and 4 illustrate the
communication format and data map of a BiSS device in register mode.
Figure #2, BiSS Register Read Mode
Figure #3 BiSS Register Write Mode
On a hardware level, the same cable used in SSI and EnDat can be used with BiSS
applications. For more information on IC-Haus and the BiSS protocol , refer to the
BiSS Website at www.biss-interface.com.

IX. Summary of the Four Dedicated Serial protocols

Table 2 shows some of the key differentiators for each protocol.
Table #2, Serial Interface comparison

X. Conclusion

Choosing the right communication protocol for an encoder application can be difficult. This
primer gives OEM project engineers and end-users enough knowledge of the basic
operating principles to make an educated buying decision. Encoders that offer open
communications protocols offer the most flexibility, and new non-proprietary protocols such
as BiSS will accelerate the trend away from using closed communication protocols.

The Bliss of BISS - Encoder Interface

The Bliss of BiSS By: Kristin Lewotsky, Contributing Editor (posted 10/02/2007)
Open-source protocol for absolute encoders offers networking benefits. Find out what it can do for you!

Motion control is all about precise positioning and movement. Even if you’ve got the best servos in the world, however, you can’t maintain accurate positioning without a feedback loop. Enter the encoder. An encoder monitors the motion of a system and provides feedback to the controller and driver, allowing them to position with an error defined, in large part, by the resolution of the encoder. Encoders can be classed as incremental and absolute. An incremental encoder keeps track of distance moved. Systems that use incremental encoders must be mechanically rehomed each time at start up. An absolute encoder tracks movement from a physical zero point on the device, so it always knows where home is. Providing the encoder with a virtual home different from the zero point only requires transmission of an electrical signal, not a physical re-homing operation. That does not, perhaps, seem like a great advantage for a single-axis system, but in a motion-control world in which systems routinely feature dozens of axes, the time saved can be significant. “We had a recent application that involved variable package sizes, and the packaging film needed to be tensioned in order to achieve proper film thickness,” says Mark Langille, BiSS product manager for North America, Danaher Inc. (Gurnee, IL). “You can imagine the amount of time it could take to go through a complex machine and reset cams and reset gears as opposed to being able to broadcast a new home point or a new set point to the encoder.” More meaningful are the advantages of speed and resolution offered by absolute designs. In terms of cost-efficiency, the cut-off for resolution in incremental encoders is around 8192 counts, says Langille. “Upward of 8192, traditional incremental encoders become more like lab instruments than industry-grade sensors.” As the MEMS and nanotechnology markets stand poised on the edge of explosive growth, manufacturers need high resolution, industrial-grade sensors. Absolute encoders provide that capability. The Protocol of Communications Of course, a feedback device is only as good as its communication protocol. In the case of encoders, there are several: serial synchronous interface (SSI), the 20-year-old de facto standard; EnDat 2.2, the latest version of the Heidenhain (Traunreut, Germany) proprietary interface; Hiperface, the proprietary interface from SICK/Stegmann GmbH (Donaueschingen, Germany); and the bi-directional serial synchronous interface (BiSS), an open-source protocol working to position itself as the new standard. SSI is a unidirectional digital protocol only, and thus unable to perform smart sensor communication. Hiperface requires the drives to interpret analog signals from the encoder in order to maintain knowledge of location. “BiSS eliminates all that computation in regard to position and keeps track of it on the encoder,” says Langille. “The other thing that really wasn’t available at the time [SSI was developed] was the ability to do all of that on one piece of silicon, which is how BiSS is designed. All of the computation for interpolation up into 22 bits or even 26 bits occurs on the ASIC in the encoder, whereas before that it was shoved back to the drive. That’s one of the reasons why a BiSS system is more robust than an SSI system.” The unidirectional nature of SSI means that data can only flow from the position sensor (or any other slave chip) to the master. Bidirectionality also allows the transmission of calibration data and diagnostic information through the network. The fact that BiSS is bidirectional opens up the prospect of “plug and play” systems in which components plugged into a system can send out identifying information and receive back the required operating parameters. A technician could fine-tune a system initially and save the motor parameters to the encoder. When the servo is swapped out, the new drive could query the new encoder and motor to receive all of the information it needs. “All that info can be stored directly by the encoder, thereby making the motor a smart device, as opposed to sort of a big dumb animal that really doesn’t know what to do and how to do it,” Langille says. Getting On the Bus Unlike SSI, EnDat 2.2 and Hiperface, which are point-to-point protocols, BiSS is a bus solution. That allows a single master chip to run multiple slave chips with less cabling and a smaller footprint. Of course, we live in the real world, which means there is a tradeoff: the higher the number of devices on the same two-wire bus, the lower the speed. “If you put ten slaves in a daisy chain then of course you will drop the max speed for each single one,” points out Heiner Flocke, co-founder of ASIC-manufacturer iC-Haus GmbH (Bodenheim, Germany), developer of BiSS. “If you want to maintain the maximum speed then you also have to do it in several peer-to-peer connections.” For the right application, though, say a three-axis system, a single BiSS master could do the job quite nicely. “Maybe you have just one encoder and you need the temperature or anything else from another sensor within the system,” Flocke says. “Then you can easily put it onto one BiSS bus line.” Which brings up an important point -- BiSS is not just for encoders. The protocol can accommodate any appropriate device, such as a thermister or vibration sensor. And users are already thinking about how to leverage the capability. “People are coming back to us and saying, ‘Can we use BiSS to do additional transmissions?’” notes Langille. “A lot of times, motor manufacturers have a thermister in their motor that requires them to run a separate set of cables back to the drive. Now we’re talking about running that temperature sensor into the encoder and then transmitting that temperature signal back to the motor drive, which reduces the overall installation cost for a motor vendor.” Opening Up Ah, cost, the perennial issue. Cost brings us to one of the primary benefits of BiSS -- the fact that it’s an open source protocol. “To create a standard there are in my opinion two choices: one is that you are the monopoly player. The other is that you are like Linux, you give it for free,” says Flocke. “We decided to have an open source interface. The strategy is to have a bigger cake and get a chance to get a bigger piece out of it.” Accordingly, iC-Haus just sells BiSS chips. BiSS is a low-overhead protocol, in that on the slave side, it requires about 1600 gates, says Flocke, which is only a few percent of a typical 10 mm2 die. Companies wishing to build master chips integrating BiSS can license the technology for free. That means they have free support and free, complete access to the code. “When they build a master chip like an FPGA or a controller with the BiSS microcode on there, then it’s seen as a BiSS device, which helps to establish the standard,” he explains. Vendors developing devices integrating BiSS devices are likewise unaffected, as are users. For that matter, even IC manufacturers wanting to build BiSS slave chips can license the protocol for free. “There’s a whole list of encoder manufacturers that make BiSS compatible encoders -- not only those who buy their base chip from iC-Haus but other companies who produce their own,” says Langille. Like, for example, Danaher. “You really can’t consider BiSS proprietary because the base code is freely given out to people to research and develop, whereas with EnDat and Hiperface, all of that is done behind closed doors. Other companies are not allowed to adopt those protocols into their encoder.” The benefit comes out of the numbers. “BiSS gives you a much broader range of product and also price and lead time that can’t be matched by just two encoder companies.” What About the Future? So what’s waiting up ahead for BiSS? Sophisticated safety applications, for one. A new cost basis, for another, Langille predicts. “The status quo had been for so long that if you wanted to reach this upper echelon of performance, you were going to pay a premium. In that regard, BiSS is really a game changer. I think there’s going to be a major shift in that price point over the next year or two,” he says. That price reduction will put absolute encoders within reach for more applications. Flocke notes that BiSS continuous mode (c-mode) also incorporates actuators, which significantly broadens its capabilities. “It’s not only a sensor interface with which you can collect data from your sensors, you can also, on the same bus line, give signals to actuators, maybe control data to motors, relays, brakes, or pneumatics.” As prices come down, he has visions of smart sensor technology moving beyond motion control, even. “One idea was to have a lamp system that illuminates streets. That is a system that contains several sensors like daylight sensors and vibration sensors. It is a little sensor system that could also be done in a bus protocol.” Given its open-source, bidirectional, digital nature and given that BiSS already has 130 licensees, it seems likely that we’re going to hear a lot more about protocol going forward.

Interface Protocol

Motion Control Systems will post information on Interface Protocols used in Motion Control Systems.

Jon Gum