what is the difference ? 3

[syab]

hi everyone ,i often see these topics in some sites or other forums these topics are : motion control, open control and process control .could anyone please tell me what is the difference ? thanks

 

[Skogsgurra]

Interesting question. Most people dwell in one of these areas and do not bother about the others.

Motion control is almost always about servos with encoders or resolvers as a feed-back element. They are usually electric, even if some hydraulic systems still exist. The technology has shifted from DC motors to AC permanent magnet synchronous motors. Some linear motors are also used, notably from Anorad. Typical applications are in machine-tools, robotics, guidance, tracking radar etc.

Process control normally is about keeping temperature, pressure, flow, salinity/acidity/concentration etcetera on a set value (the setpoint). All sorts of technologies are used, from simple level and pressure switches and start/stop pumps and open/close valves to analogue sensors for pressure/flow/temperature/concentration etc with variable speed pumps, proportional valves and other countinously variable actuators. Typical applications are in steel and paper production, food processing, chemical, refinery, heating and HVAC. It is way too wide a field to be summarised here. Do a google search for "process control".

Both types of systems (motion and process control) have sensors that tell the controllers what the actual value is so that the controller can act to increase or decrease its output to keep the actual value at its correct level.


Open loop control, on the contrary, means that you just set an operating point and hope for your device to stay there. There is no feed-back to tell your setting device what the result actually is. A typical open loop system is a dimmer for lighting. You set the light to a suitable level and leave it there. If the sun shines brighter, the dimmer doesn't care. You get the same brightness from your lamp. And when it gets dark outside, your lamp doesn't increase to compensate for missing daylight. It has no feed-back and does not know what the total light level is.

The "loop" is the feed-back loop in the first two system types. There is no loop in the third type of system. Or, rather, there could be a loop, but it has not been closed since there is no sensor to produce a feed-back signal.

 

[syab]

thank you for your help.

 

[jsummerfield]

Skogsgurra answered the question well. However, the question includes the phrase "open control" instead of open loop control. In a communication protocol context, "open" means published public domain as opposed to proprietary. The control context of this communications topic applies to assorted computer control interfaces, etc.

John

 

[jmw]

Open loop control is not necessarily totally without feedback.
For some processes this term is interpreted to mean a system where operator intervention is required and possible i.e. there are instruments indicating control parameters, or the process is sampled and lab evaluations performed, that allow the operator to make periodic adjustments to the set point.

In some processes there are problems with finding a suitable control parameter i.e. one that can be measured in process. For successful open loop control the system should exhibit some stability/predictability.
Failing this, or even with stability, the product may require collecting in an intermediate tank where it can be sampled and final corrections made.

A good example is ploymerisation end-point spotting in methyl methacrylate production. A typical batch reaction is exothermic, batch duration may only be around 2-2.5hrs and the end-point window only 20 seconds wide.
The duration of the process doesn't allow for sophisticated lab measurements.

Instead, the operator must take frequent samples and make a rough-and-ready viscosity measurement using a simple cup type device (viscosity is a function of the molecular weight).

The reaction curve is then plotted and the end point time predicted. The success rate is not high though many batches can be recovered by mixing back in with fresh feedstock for the next batch. 10% losses are non atypical. In those plants where process viscometers are suitable the success-rate is near 100%.

The point is while closed loop control is desireable in many processes it is not always possible.


JMW

http://www.viscoanalyser.com

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[CARF]

Dear JMW, all,

In Control Engineering terms the process you describe is MANUAL (feedback) CONTROL by operators, the term OPEN LOOP is often refered to when an automatic PID controller is set to MANUAL. CLOSED LOOP will be the mode where the controller is on AUTOMATIC.

In OPEN LOOP or MANUAL mode sometimes (step) changes are applied on the controller output in order to find the correct PID parameters. From the resulting "OPEN LOOP step response curve" the correct PID parameters can be derived, giving a good controller tuning / performance.

see thread698-98418

(well you know this thread ; )

CARF