Encoders and associated signal conditioning influence motion control system parameters such as positioning accuracy, speed stability, drive bandwidth (command-signal response and disturbance rejection), and power loss. For standard drives, manufacturers primarily use encoders with TTL-comparative output signals and 60,000 measuring steps per revolution are sufficient. Resolution of an encoder with TTL signals is four times the counts per revolution. The most advanced digital systems require much higher resolution.
Digital speed control on machines with high dynamic requirements typically requires more than 500,000 measuring steps per revolution. Texas Instruments’ Miroslav Olijaca from Motor Control Strategic Marketing, Data Acquisition Products notes that encoder sinusoidal incremental signals with signal levels of 1 VP-P can be highly interpolated in the subsequent electronics. For example, Olijaca cites a rotary encoder with 2048 signal periods per revolution and a 1024-fold subdivision in the subsequent electronics produces approximately 2 million measuring steps per revolution. This corresponds to a resolution of 20-bits. Even at shaft speeds of 12,000 rpm, the signal provided to the control system has a frequency of about 400 kHz.
To obtain greater resolution, suppliers offer different approaches providing engineers options. Four examples of the pursuit for higher resolution are interface boards, higher resolution encoders, higher resolution A-D converters, and a complete digital industrial drive.
Sinusoidal Encoder Interpolation Interface Board
Galil’s DB-28104 board (in the upper left hand
corner) requires no cables to mount directly to their DMC-21x3 Ethernet
motion controller. The unit interpolates the output from up to four
Galil Motion Control’s DB-28104 board interpolates the output of up to four sinusoidal encoders delivering higher position feedback resolution without using an ultra high-resolution encoder. Used in conjunction with any of the Galil amplifier boards, once an axis is controlled using a sinusoidal encoder, the maximum allowable speed and acceleration settings are increased to 50 million counts per second, and 109 counts/s2 respectively.
With the DB-28104, position tracking occurs at two levels. Similar to a quadrature encoder, the unit counts coarse cycles. For higher resolution, bit-wise interpolation of the two sinusoidal signals provides the precise position inside the cycle. The user establishes the interpolation rate, which ranges from 25 through 212 points per sinusoidal cycle. The unit accepts both single-ended and differential encoder inputs, as well as differential index signal inputs.
True Absolute Encoders
Gurley A58 absolute encoders provide 12, 14, 15 and
16-bit resolution in a relatively small package with a 58-mm diameter and
depth from 42 to 55 mm, depending on the
Gurley Precision Instruments A58 Absolute Encoder is a single-turn absolute rotary encoder available with 12, 14, 15 and 16-bit resolution. The optical sensor’s code disk is etched chrome on glass. The unit has a non-operating slew of 10,000 RPM. Output formats include serial and multiplexed versions of Gray code or natural binary for higher resolution models. The accuracy of these units is ±1 LSB. The 12-bit model also has a parallel output format and its accuracy is ±0.5 LSB. The maximum rotational speed for valid code is 30 RPM for all units. The 58-mm diameter unit comes with a 3 mm to 0.25-inch diameter stainless steel shaft.
12-Bit Motor Control AD Converter
Texas Instruments ADS7869 is a 12-bit, 12-Channel
motor control specific analog to digital converter in a 100-pin TQFP
With a maximum sample rate of three one million samples per second (MSPS), Texas Instruments ADS7869 provides 12-Bit resolution. Designed specifically for demanding motor control applications, the ADS7869 integrates three analog-to-digital converters (ADCs) with a total of seven sample-and-hold capacitors and 12 differential input channels. The unit has four sign comparators connected to four input channels and three additional fully differential inputs.
The ADS7869 has a parallel port that can be configured to different standards, a serial peripheral interface (SPI), and a specialized serial interface with three data lines for VECANA01 mode. This allows the ADS7869 to interface with most digital signal processors or microcontrollers. To ensure that the analog input of the encoder is held at the same point of time as the counter value, the chip integrates two up—down counters to analyze the position sensor data.
All Digital Industrial Drive
Danaher Motion’s Kollmorgen S200 series brushless servo drives are all digital industrial units with a velocity loop bandwidth up to 800 Hz providing increased system throughput and simplified tuning. To achive smooth motion and rapid start and stop action for optimized machine performance, the units have 24-bit resolution and 3-5 kHz current loop bandwidth. Using smart feedback and high bandwidth facilitates "plug and play" commissioning by eliminating the need for servo loop tuning in most applications. The units feature optically isolated inputs/outputs, positive locking connectors, full fault protection, and separate keep-alive power input.
The drives come standard with torque or velocity control and optionally support Indexing, CANopen, or DeviceNet. The servo control units target applications in semiconductor fabrication, electronic assembly, packaging, medical, and woodworking equipment.
Galil Motion Control DB-28104
Gurley Precision Instruments A58
Texas Instruments ADS7869
Danaher Motion Kollmorgen S200 series