Servomotor drive manufacturers agree that the performance of today's digital drive is largely determined by the performance of its feedback device. Clearly, previous feedback solutions, such as resolver technology and conventional encoders, no longer meet the increasing market demands for higher accuracy and resolution, lower P factors, and easily adjustable commutation information - all of which must be available in a single small, easily mountable package. An innovative approach by Stegmann Inc. for its Hiperface™compatible feedback systems meets and exceeds these demands.
The Sin/Cos by Stegmann is a Hiperface compatible feedback device developed for the drive market, offering better resolution and improved performance over traditional encoder technology. Benefits over an incremental encoder include a resolution of 4-8 million counts, well above the 20,000 count of most encoders. Accuracy is ±2-3 arcsec, compared to ±2 arcmin for incremental encoders. At 50 mm in diameter, the Sin/Cos is the smallest multiturn absolute encoder in the world when compared with devices that have similar performance ratings. It has an optional mechanical gearing system that lets it keep track of up to 4096 shaft revolutions.
Hiperface is an eight-wire interface standard that makes it possible to get commutation, speed regulation, and position information from a single device. The physical interface includes two wires for an RS-485 link, four wires for sine and cosine signals, and two wires for the power supply, a reduction from as many as 19 leads in traditional systems.
These innovative systems combine the functions of both incremental and absolute encoders. The absolute value in these systems is calculated only when the device is initially turned on, and is then transmitted via the RS-485 interface to a counter in the drive. The drive in turn uses this initial absolute value for the commutation of the motor and also as a pointer to identify the exact period of the sine/cosine signals that it is currently evaluating. The drive then increases the base resolution of the sin/cos signal by interpolating these signals in the drive with an A/D converter. The final result after 12-bit interpolation, when using a feedback system with 1024 periods per turn, is more than 4 million counts.
An advantage of using sine and cosine signals to transmit the position information is that a very low-frequency signal can be used to provide extremely high resolutions. This is because the incremental signal is not transmitted in the common digital data format of A-quad-B, as with conventional encoders, but rather as analog sine-cosine voltage over the process data channel. This signal is then interpolated inside the drive unit's input circuitry to get the increased resolution. This low-frequency signal can be transmitted without difficulty over considerable distances.
The resulting benefit is that the drive circuitry can be designed to handle only as much bandwidth as is required; this range in actual practice depends on the shaft speed at which the feedback device will operate. For instance, a servomotor using digital signals at speeds of up to 6000 rpm, and a desired resolution of 20,000 pulses per revolution, needs a bandwidth of 2 MHz. If a Sin/Cos motor feedback system from Stegmann is used, a bandwidth of only 102.4 kHz is required.
Hiperface compatible motor feedback systems also outperform existing feedback devices in accuracy. Motor feedback systems provide signal linearity of the sine/cosine signals down to ±5 arcsec, resulting in total system inaccuracies (including mechanical mounting of the encoder and A/D conversion in the drive) of less than ±30 arcsec. This is at minimum a tenfold increase in accuracy over today's conventional incremental encoders and resolvers.
By integrating a nonvolatile memory device into the system, important data can be stored in the feedback device and accessed over the RS-485 link by the drive when needed. As an example, the electronic label of the motor can be stored during manufacture, then accessed by the drive during system initialization and used to automatically configure the drive, eliminating the need for keyboard entry of this data. Other information, such as routine diagnostic information and internal device temperature, can also be accessed. These features add another dimension to drive systems that incremental A-quad-B encoders or resolvers cannot hope to match.
Other Hiperface products are available for differing applications and mounting arrangements.
For more information contact Andrew Monnin, the author of this brief and sales and marketing manager for Stegmann, Inc. , 7496 Webster St., PO Box 13596, Dayton, OH 45413-13596; 800-811-9110; fax 937-454-1955; E-mail: