Stegmann - Incremental Encoders - Glossary of Encoder Terms

In the past, a wide variety of feedback sensors were offered for commutation, speed regulation and position determination including: hall affect sensors, analog and digital tachs, resolvers, incremental and absolute encoders.

The introduction of digital speed regulation has had a significant impact on feedback systems. This new market shift has made it necessary for more and more functions to be integrated into a single feedback device. The resolver, at first glance, seems to be the functionally ideal sensor for these new digital drives, since it combines practically all the necessary functions in a single device. The only exception is when absolute position over multiple revolutions is required, which is a low percentage of the total available applications.

However, the resolverís limited accuracy and itís systematic current behavior in the control circuit exclude this technology from serious consideration because these limitations donít allow digital drives to reach their full potential. For example the important characteristics attainable with digital speed regulation, such as high synchronization and high dynamics and/or load stiffness are exactly those which are held back by the resolverís limitations. For this reason, a wide variety of feedback systems have been developed in the past few years leading to numerous mechanical and electrical interfaces almost to great to contemplate.

The pyramid above shows the quality level and variety of interfaces in use; at the right, the standardized HIPERFACE® interface, which reaches all the way from the high end to the low end of the performance spectrum.

The basic Principle of Operation for Conventional Optical Encoders

The most common, optical shaft encoder systems in use today are conventional incremental and absolute encoders. The latter, in particular, are relatively expensive because of the great cost of producing them in the conventional way.

Functionally an absolute encoders works like this: Binary information encoded on a sheet of glass is read by an optical scanning unit. For every binary track ( one track per bit of resolution ), a corresponding optical scanning unit is required. The alignment of each scanning unit to another is very critical and must be related to the others in such a way under all conditions that no reading error can occur. The information from each of these separate scanning units is put into a digital word and then must be transmitted continuously to the control system.

The Principle of Operation for the HIPERFACE® Servo Motor Feedback Systems:

HIPERFACE® stands for High-Performance Interface, and describes the physical interface between the motor feedback system and the digital drive system.

HIPERFACE® motor feedback systems are a cross between Incremental and Absolute encoders. The absolute value in these systems is calculated only when the device is initially turned on. This absolute value is transmitted via the RS 485 interface through the parameter channel to an external counter in the drive. The drive, using the initial absolute value as a base, then increases the resolution using sine and cosine signals from the process data channel.

It has been argued that transmission of the absolute data through synchronous serial data lines is faster and safer than using an RS-485 interface. This argument, however, is found wanting if one focuses on the actual use of the data that is transmitted via this link. The purpose of this data, which is not time critical, is to communicate an initial position value at initialization or power-up. Therefore the speed of the transmission of this one-time data is largely unimportant. In fact, the use of a synchronous serial interface actually has negative consequences in that it requires four wires instead of two adding additional costs and taking up precious space.

To make it possible to reach high resolutions even at very fast shaft speeds, the incremental signal is not transmitted in the common digital data format of A quad B, but rather as analog sine-cosine voltage over the process data channel. This sin/cos signal is then interpolated inside the drive unitís input circuitry (for instance with a 10 bit analog/digital ( A/D ) converter). The result is a very low frequency signal even at extremely high resolutions. For example, at 512 cycles per revolution, a frequency of just 102.4 kHz is reached at a shaft speed of 12,000 rpm. This low frequency signal can be transmitted without difficulty over considerable distances.

Using sinusoidal signals has the advantage that the transmission channel does not have to possess an extremely broad-band range, as is true with digital signals. Rather the drive circuitry must be designed to handle only as much bandwidth as is required ñ this range in actual practice depends the shaft speed the feedback device will operate at. For instance, a servomotor using digital signals, at speeds of up to 6,000 rpm and a desired resolution of 20,000 pulses per revolution, needs a bandwidth of 2 MHz. If the SinCos® is used, a bandwidth of only 51.2 kHz is required!

A supply voltage of 8 V is recommended for these new feedback systems ( the full operating voltage range is actually from 7 to 12 V). The conventional 5vdc or 10-30vdc supply ranges were avoided intentionally! This is because, with a 5vdc supply, two additional sensing lines are needed to measure the decrease in voltage due to line loss or alternately the 5 volt supply must be well regulated, which adds to the cost and complexity of the power supply. With 10-30V supplies , the high heat generated by the regulator has negative affects on the control unit located inside the feedback system decreasing the overall temperature rating of the device on the high end.

Instead of many (up to 19 transmission ) lines, with HIPERFACE® only eight are needed!

Motor Feed back Systems with HIPERFACE®:

The SinCos® and SinCoder® are the names of the two products which comprise this complete family of servo motor feedback systems. Their decisive advantages are not limited to the standardized electrical interface they both share, in fact their common electrical interface is only one of many.

The SinCoder® was designed mainly for the low-end range of applications. It can be used wherever the resolver is at the limit of its effectiveness. Usually, the precision of resolvers is 10-20 angular minutes. The SinCoder® comes in at ±40 arc seconds.

In regards to resolution, the SinCoder® is also greatly superior to the resolver. With a resolution 1024 times greater, it offers an order of magnitude increase in resolution that is more and more in demand, due to end customerís increased requirements. To achieve the new levels of speed regulation required in todayís applications, a very high flow of information in real time is needed, especially at low speeds. For instance, if the drive uses a 10 bit analog/digital converter a resolution of 20 bits is the end result ñ a resolver would only have a resolution of 10 bits using the same interpolation factor.

In another contrast to the resolver, the SinCoder® does not have the limitation imposed by a large P-factor; that is, the feedback information is available immediately at the controller without any delay. This means the performance limiting time lag common with demodulation circuits in R/D converters is avoided.

Motor Feedback System	Number of Lines	Accuracy	Typical Resolution ( 360deg. )	Dynamic Behavior	Ease of Mounting	Integrated EE-PROM
Resolver (12 Bit A-D)	6 special type, with double insulation	±600 arc sec	4096	poor	difficult	no
Incremental Encoder	14	±120 arc sec.	20,000	average	average	no
SinCos® with Hiperface	8	±18 arc sec.	4,000,000	good	easy	yes
SinCoder® with Hiperface	8	±60 arc sec.	1,048,576	good	easy	yes

The other product that completes this new family of feedback systems is the SinCos®. The SinCos® was developed for use in the high-end drive market. It has optional mechanical gearing that allows it to keep track of multiple shaft revolutions ñ and in spite of the extra mechanics itís small physical size has been maintained. In contrast to battery backed-up systems, which accumulate errors created due to EMI/RFI noise ( a big problem in drive systems ) signal integrity can be taken for granted here. And unlike battery backed-up systems it doesnít matter how long this feedback device sits before the end machine makes it to itís final destination; no energy meter is running.

Functionally, the multi-turn information is created by four successive gears arranged in series which turn 4 permanent magnets. Each permanent magnet has two Hall elements which are used to generate one sine/cosine pair. The single-turn information is created when interpolation of the 512 sine-cosine periods
(through the process data channel) occurs in the drive system; the result is up to 4,000,000 steps per revolution. The extremely high density of information makes possible a quality of speed regulation that surpasses anything previously available, even at the lowest speeds.

The Sincos® is particularly valuable in applications that have a C-axis requirement since a high level of accuracy is necessary. For these type of applications the SinCos® dominates all other feedback devices with accuracies to ± 18 arc seconds or less!

The real sensation: the electronic name plate.

By integrating an EE-PROM into both feedback system types (SinCos® and SinCoder®), expensive and time consuming input work can be eliminated. In this electronic label, motor parameter data such as voltage, current and torque ratings are stored, which makes it possible for the drive to automatically adjust and calibrate itself when placed into operation in the field. It does this by reading the data out of the feedback device during machine initialization or routine power-ups and using the data to automatically ìset upî the drive system. Thus, time-consuming, error-prone entry through the keyboard is eliminated. This is not only a blessing for harried engineers; those who are responsible for improving cost performance and speeding up installation times will benefit from the savings that result from this feature.

A further, significant advantage of having the electronic name plate is that the fact that mechanical commutation adjustment (always necessary with resolvers and conventional encoders) is eliminated. Until now, the offset value for commutation was calculated manually and then stored in the control unit or in other cases mechanical alignments and adjustments of the encoder to motor Back EMF were made. This is taken care more easily and efficiently in the SinCoder®. Just put the motor in the proper setting, write the offset value in the EE-PROM, and youíre done!

Additionally, data that relates to individual jobs such as the model of the machine, customer name, delivery date, project names, etc. may be stored in the EE-Promm . It is important to note that the use of the electronic label is not limited to the manufacturers of the motors and drive systems but can be accessed by the machine tool builder or end user as well! Information regarding the most recent maintenance date and problem, for example can be stored and retrieved from datafields that have a total overall capacity of 192 bytes.

Relax! Expensive and time consuming data entry work is eliminated. The electronic label used for storing and reading motor parameters makes it ìautomaticî.

Revolutionary mounting system!

Since both the Sincos® and Sincoder® utilize the same physical mounting system, a manufacturer of motors can design the back side of the motor and the motor shaft in a standard way, independent of the number of performance classes and size ranges the product series may cover. As a result only one mechanical interface is required for the entire range of performance and application requirements that exist. The endless array of mounting schemes, limited in the past only by peoples imagination, can be put out to pasture.

The new mounting system makes it possible! Insert the feedback system, push the cover plate on the motor housing, and youíre done!

The most important advantages of the HIPERFACE® motor feedback systems, in summary:

Innovative product design that dramatically increases the drive systems overall performance Drastically reduced wiring effort, since there are only 8 lines; thus savings of time and costs; Smaller motor construction is possible which in turn translates to smaller machine sizes Both machine manufacturers and users of machines profit from lower system costs Just one mechanical design for the motor back end Just one electrical interface to the drive system for all conceivable applications Manual setting of motor parameters is eliminated, saving time and money while improving the quality of the installation process Electrical compatibility is guaranteed by the introduction of HIPERFACE® as the interface mandated for all physical parameters.