Application Digest 7603

Reducing cost with motion controller/PLC integration

By Garyh Hager, Acroloop Motion Control Systems, Inc., Chanhassen, MN

Traditionally, servo and stepper motion controllers are antiquated in their handling of digital I/O. Most motion controllers handle digital I/O with high level code, such as IF-THEN statements. However, IF-THEN logic programming is often awkward and time-consuming.

One alternative to this time-consuming technique is the use of an integrated PLC and motion controller. Using servo and stepper motion control boards that take advantage of the processing speed of a floating point digital signal processor, the on-board PLC is programmed using Boolean logic commands. The Boolean commands are then translated into machine code on the fly with the floating point DSP. This programming technique allows the PLC scan rate to be on the order of a dedicated PLC scan rate.

Because the motion controller/PLC uses a pre-emptive multi-tasker, several motion control programs and PLC programs can be operated simultaneously. The ladder logic diagram shown can be programmed into the PLC program spaces of the motion controller. A further advantage is that the on-board PLC can access built-in hardware flags through the motion controller. Thus the PLC controls not only the digital I/O, but also the motion control hardware flags.

Currently, the motion controller/PLC cannot replace all PLC operations. But it can be used in applications requiring both motion control and simple PLC logic control. If the application requirements allow the motion controller PLC to be used, a separate PLC no longer is required, resulting in material and labor cost savings.

To speak with an Acroloop Motion Control Systems engineer, call 612-474-4500.

Simpler, more accurate blood pressure measurement

Peter Hutchings, Pneutronics Div., Parker Hannifin Corp., Hollis, NH

In traditional automated, non-invasive blood pressure monitoring, a pump inflates the patient cuff to 250 mm of Hg. Two digital (on-off) solenoid valves are cycled to control the deflation of the patient cuff in steps of 8 mm of Hg. This approach, although adequate, results in relatively slow blood pressure determination and patient discomfort due to initial high cuff pressure.

An alternative technique substitutes a VSO-NC normally closed proportional valve for one of the digital valves. The NC proportional valve controls the deflation rate and eliminates the need for the 8 mm of Hg decompression step, as it can achieve any inflation or deflation rate. The remaining digital valve is used merely as a safety valve.

Yet another alternate circuit substitutes a VSO-NO^TM normally open proportional valve for the NC proportional valve and the digital valve. The NO proportional valve controls the inflation or deflation rate, and since it is normally open, the need for the other digital valve is eliminated. As with the NC proportional valve circuit, it can accurately simulate almost any orifice size smaller than its maximum.

By employing this technology in non-invasive blood pressure monitoring, measurements are more accurate and patient comfort is improved.

To speak with a Pneutronics applications engineer, call 603-595-1500.