Purchase now: Allen-Bradley 1756-M02AS Servo Module
Purchase now: Atlantechdrives provides you the 1756-M02AS, a highly specialized, industrial-grade 2-Axis analog servo module with SSI (Synchronous Serial Interface) absolute feedback, engineered by Rockwell Automation for the ControlLogix 1756 platform. While modern motion control relies heavily on Ethernet-based CIP Motion, thousands of industrial facilities still operate massive legacy hydraulic servo valves, custom linear actuators, and specialized analog servo drives. The 1756-M02AS is the ultimate bridge between modern ControlLogix PACs and these analog systems. It outputs a highly precise ±10V DC analog reference signal to dictate motor speed or torque, while simultaneously closing the position loop by reading real-time absolute position data from SSI encoders. Because SSI provides absolute positioning data (rather than incremental pulses), the machine never loses its physical location during a power outage, completely eliminating the need for slow and dangerous «homing» routines when the factory restarts. It is heavily utilized in massive metal stamping presses, robotic welding positioners, and heavy hydraulic extrusion machinery.
Technical Specifications of the 1756-M02AS
The 1756-M02AS module offers 16-bit resolution on its analog outputs, providing incredibly smooth and granular control over the connected servo drives to eliminate mechanical chatter and resonance.
- Part Number: 1756-M02AS
- Platform: ControlLogix 1756 System
- Number of Axes: 2 Independent Axes
- Drive Command Output: ±10V DC Analog (16-bit resolution)
- Feedback Type: Synchronous Serial Interface (SSI) Absolute Encoders
- SSI Data Rates: 200 kHz or 1.25 MHz (software selectable)
- Loop Update Rate: Up to 500 microseconds for tight positional control
- Backplane Current Draw: 700 mA at 5.1V DC; 2.5 mA at 24V DC
Expert Installation Advice: RTB Selection and SSI Cabling
Servo modules deal with incredibly sensitive, low-voltage signals that dictate massive mechanical movements. Expert Tip: The 1756-M02AS does not include wiring terminals out of the box. You must purchase a 36-pin Removable Terminal Block (RTB), such as the 1756-TBCH or 1756-TBS6H. When wiring the SSI absolute encoders, cabling hygiene is paramount. SSI data is transmitted via high-frequency clock and data pulses. You must use high-quality, individually shielded twisted pair cables. Furthermore, the cable shield must be grounded at one end only (typically at the chassis ground lug) to prevent ground loops. If you route the SSI feedback cables in the same conduit as massive 480V AC motor power cables, the electromagnetic interference (EMI) will corrupt the data pulses, causing the axis to violently run away or trigger fatal positional errors.
Maintenance: Tuning the PID Loop inside the Processor
Unlike smart digital drives where the position loop is closed inside the drive itself, the 1756-M02AS passes the analog and SSI data directly into the ControlLogix processor. The actual PID (Proportional-Integral-Derivative) motion loop is calculated by the main L7 or L8 CPU. When replacing a worn-out mechanical actuator or a legacy analog drive connected to this module, maintenance technicians must often perform a «Motion Direct Command» tuning procedure within Studio 5000 Logix Designer. Properly adjusting the proportional gain and velocity feed-forward parameters is essential to ensure the new mechanical components respond accurately to the ±10V signal without oscillating or overshooting the target physical position.
Frequently Asked Questions (FAQs)
Q: Can I use standard incremental quadrature encoders with the 1756-M02AS?
A: No. The «AS» in the part number specifically stands for Analog/SSI. If you have standard A/B/Z incremental quadrature encoders, you must use the 1756-M02AE module instead.
Q: What happens to the axis position if the power goes out?
A: Because this module uses SSI absolute feedback, the exact physical position is maintained by the encoder’s internal mechanics or battery. When power is restored, the 1756-M02AS instantly reads the absolute digital word, knowing exactly where the machine is without needing to move.
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This dual-port analog servo module provides precise motion control when managed by a high-speed ControlLogix 5572 controller.
