A-B 1746-BLM Module

A-B 1746-BLM Module

Description

The module performs its servo control task independently, but is dependent on the  SLC processor for all of its configuration and run-time information. The processor  may be also be used to supply process data or timing information over the  backplane in certain situations (e.g. parison drop synchronization on continuous  extrusion machines, or accumulator position in reciprocating screw machines).

This 4-axis position-control module has these features: 

• Open-loop or closed-loop control 

• Independent and coordinated axis control 

• Position- and time-based control 

• Accumulator push-out control 

• Zero-scale/full-scale (offset & span) calibration for position inputs 

• PID with anti-windup, bumpless parameter changes, setpoint weighting, and  limited high-frequency derivative gain. 

• Profile interpolation (linear or cubic spline) between setpoints 

• Converging/diverging tooling (direct/reverse acting control) 

• Three hold values per axis: manual position, purge, or die gap 

• Independent profile scale and offset adjustments  

• Automatic parison weight adjustment 

• Setpoint marking

Module’s microprocessor 

The module processor is a 16-bit fixed-point digital signal processor (DSP). It  communicates with the analog I/O channels over a high speed (2MHz) full-duplex synchronous serial link. Serial connection between the processor and analog I/O  hardware facilitates electrical isolation. Digital I/O is performed in a similar fashion.

The module processor manages all communications between the module and the  SLC processor. It performs such functions as interpolation between profile  setpoints, loop tuning, and calculation of calibration coefficients in addition to  executing the control algorithm.

Module’s PID control algorithm 

For servo control, the module uses a Proportional + Integral + Derivative algorithm  with anti-windup, high-frequency derivative gain limiting and setpoint weighting.  Anti-windup is achieved by modeling the actuator (normally a valve amplifier) as a  nonlinear device that operates linearly over a limited range, beyond which it  saturates.  

An additional error signal is formed by taking the difference of raw controller  output, v(n), and control output, u(n), which is clamped at the actuator saturation  limits. This signal is multiplied by gain 1/Tt, where Tt is called the integrator  tracking time and summed into the integral term. High-frequency derivative gain  limiting lets you compensate for derivative term susceptibility to high frequency  noise. Setpoint weighting provides a mechanism for independent tuning of setpoint  and load response. 

Digital I/O 

There are four fully isolated digital inputs on the module. They are of the  current-sinking type. Their primary use is for start-of-parison-drop synchronization  on continuous extrusion machines. The digital inputs may be used as general  purpose inputs if the start of drop synchronization feature is not needed. 

There are four isolated digital outputs on the module. They are of the  open-collector (current-sinking) type and share a common 24VDC (nominal)  external power supply. Their primary use is as profile step-synchronization  indicators. The digital outputs may be used as general purpose outputs if the step  synchronization feature is not needed. See page 41 for complete specifications.

Analog I/O 

There are four analog I/O channels on the module. Each channel consists of a  14-bit analog-to-digital converter and a 14-bit digital to analog converter. As a  group, the four I/O channels and excitation output are optically isolated from the  remainder of the module. The high common mode input range of the input  amplifiers and the isolated nature of LVDTs and linear potentiometers make it  unnecessary to isolate the channels from one another. See page 41 for complete  specifications.