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Opinions sought on User Interfaces for Machine Control Applications.

Regardless of what machinery is being controlled and what the user's view of the system looks like on the desk (whether it is a web-based system or a control mimic) I am seeking some opinions from the rest about what the most appropriate response time maxima should be. I have found, followed and read a few links and so far the best advice gives three zones (0.1 seconds, 0.2 to 1 second and 1.1 seconds to 10 seconds) with advice on the style expected for each of these zones.

What I am looking at would be some quotable figures that would apply to an embedded (perhaps PLC based) system being conrtrolled through a centralised server based farm of terminals.

Some parts of this conglomorate system gives responses in the two seconds region (by being given a little more priority for the sub-system traffic). Other parts are operating with a latency, between commanding a plant action and seeing confirmation of the action having occurred, of four to 8 seconds. This does not feel so good to operate.

In operating with the programmers interface to a PLC I was able to confirm that toggling the bits that caused the actions to occur and seeing the response status from the plant was seen within 3 scans (28ms per scan). This was for the operation of a small, fast, solenoid valve. With the central user terminal system this same action was at least 4 seconds and almost took 8 seconds on occasions (first scan sets the output, one scan while the device changes state physically, the third scan returns the new state of plant - approximately 84ms end to end of the change of state).

This indicates to me that the central system is much too loaded and could do with a boost in capability or an off-loading of some of the work that it is carrying out.

So, how swift should I specify the plant change of state response be displayed on the users terminal? This figure would be more interesting if it was also backed by some good document links. In the mean-time I am going to see if I can write a justification to support demanding less than 0.25 second response to any change of state of the plant.

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Paul E. Bennett ....................
Forth based HIDECS Consultancy .....
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-811095
Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
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Reply to
Paul E. Bennett
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It's often the case that the actual time taken matters less than the amount of variability. A countdown indicator can be useful for operations that take a lot of time.

Leon

Reply to
Leon

It depends on the value of the swift feedback. An interactive control, such as adjusting the volume on the stereo, needs very quick feedback because it is important in achieving the desired goal. The house thermostat can be be a lot slower because, with experience, we make a reasoned adjustment and expect the results within several minutes to meet our expectation.

A response time with significant variability causes doubt about the robustness and reliability of the system. If there are high costs associated with failure, I would certainly want to maintain robustness and the _appearance_ of robustness. What would it take to make the customer (managers and operators) happy, satisfied, and confident with the system?

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Thad
Reply to
Thad Smith

As I operate as well as design and maintain the lower strata of this system (the PLC and Embedded Controls end) it is a great annoyance to me that it is taking a significant number of seconds to display a change in plant state. The upper strat of the control system is out of my hands and is maintained by the IT team. The overall system has evolved over a twenty years and I feel that the upper strata has not really kept pace with the needs of all of the users.

I know how long my part of the system takes (change of state seen on the programmers terminal within the 28ms scan time) and, even given the time for the communications link to the upper strata of the control network, one gets to wonder why there is a delay of this change of state at least four seconds later.

I am just seeking papers that will support my proposition to the IT team that they improve the response times of their part of the system.

The equipment I am dealing with, by the way, involves keeping control of Cryogenics and potentially explosive gas mixtures. The PLC and embedded controllers look after the safety but the upper strat makes anything you do with the system feel very very sluggish.

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********************************************************************
Paul E. Bennett ....................
Forth based HIDECS Consultancy .....
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-811095
Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
********************************************************************
Reply to
Paul E. Bennett

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