comeng

lets look at another scenario away from your project and see how the elements match in An office building hundreds of people simple aircon they select how much air is coming to each desk but every hour the numbers in the building go up and down, as people leave their desks the air con shuts. So the system is setup with a variable speed fan the air con engineer initially assess's that the system pressure for optimum flow is 100mbar (PID setpoint) a pressure transducer feeds the actual pressure back to the input of the PID giving a plus/minus variation from the setpoint to which it responds by increasing/decreasing the speed of the fan so as the air load varies dependent on the people opening and closing vents then the system responds to keep a constant system pressure. OK lets say first motor speed set from a pot (could be a value on an HMI), bring encoder from first motor as an input and scale it to an N10 value (then using another pot or HMI value create a mulitplier number N11 say 0.9) multiply N10 by N11 to give value that can go itno SCP for creation out put for speed of second motor Both encoders would display speeds and the you could cascade speeds down to 3,4,5 drives not in office at minute will try and create something for you tomorrow

Sorry still do not see why you are trying to use a PID, this is normally used if say you have a fabric material that you want to keep at a constant tension then you would have a loop and some form of sensor whether dancing bar, photocell, ultrasonic unit with which try to adjust the speed of the 1st motor to keep a constant loop feeding it, the second motor would then be controlled by the speed of the 1st simply using the speed of the first to calculate the speed of the second from the mechanics plus any offset you feel appropriate for shrinkage/growth of product. All you are trying to do is control one motor speed with another you can do this by linking two drives together you do not actually need a plc to do this although using the encoders should a give indication of the speeds and give more accurate control. Why would the second motor run slow if you are feeding an output signal from the first motor speed unless the second motor is heavily loaded and can thus be braked What is actually causing the speed to vary this is the element that should be in your PID loop

depends on how much reliance you place on the MS and the comms system if 55kpa means plant goes burp if MS offline and gives operators time to hit big red button and run then alarm OK in MS if at 55kpa the plant goes bang then suspect alarm handling better off in PLC

Simple answer the wondeful world of sensor electronics can only deal with currents and voltages 0-20ma 4-20ma 0-10V -10+10V etc the real world deals in temperature, pressure, length, speed. Thus raw and engineering units why pass both to the MS only if someone does not believe PLC can perform the math correctly

I think we need more information the important thing is not necessarily the speed of the motor but the speed of what is being driven say that each drive is driving a roller of equal diameter then using SCP to give an input over the range 0-117 can be used to SCP an output which will go to your second drive to follow the speed if the rollers are different diameters then you will have to do the math the 2nd encoder would only be of use to prove the speed of the second roller was what you expected or generate an alarm

if you put your input into a counter for every revolution you get a count value of 360 so if you have timer set for 1 minute and then look at the acc value of the counter you will get a value that when divided by 360 = rpm slight problem of course is that as the actual rpm rises so the acc value will approach it's limit where overflow occurs (and the counter reset) you may have to use a timer set in seconds or hundreths, or if going slower than might have to use hours or days obviously when not using minutes as the time you have to correct for this to give rpm. If things are happening at high speed you might have to look at a certain number of counts triggering an event, everything of course depends on what is driving the roller and it's diameter

I think you need to pause and think about the mechanics here, firstly the fabric must be coming in roll form therefore will have to sit in some sort of let off device which may or may not be driven if it is not driven (or braked) then somewhere along the route something must be pulling either a nip between a drive and idler roller or the windup of the material. If you have neither a nip nor a windup then you are only driving the fabric via the friction of the rollers to the fabric when I have tried this before I have found the drives have to run faster than you would calculate the line speed simply to produce the frictional effect Unless the first drive roller you are talking about is a nip roller how do you know the fabric is travelling at the same speed as the roller You need to establish what is ultimately controlling the line speed and use this as your master

Am I missing something here normally I would be pulling a fabric through a chamber supported on freewheeling rollers and the tension being controlled from a braked let off why would you need 4 drives also if you do need 4 drives I would not assume that the shrinkage would be the same at all speeds of the master because the heat history of the fabric would be different. Don't know about AB drives but Telemecanique can have remote displays connected easily to the drives through standard ethernet patch cable would then go with previous suggestion of pot below each display

I agree with Steve your biggest problem will probably be the control side and may I suggest you are not the person ensuring it complies with the relevant European safety legislation. Not that I am suggesting you would not be fully competent to do it but it can be a minefield

hi paul have used these a lot http://www.scansmart.com/catalog/datamax_allegro.asp even when covered with carbon black they somehow survive Phil

Thanks to Bob and Paul for two interesting suggestions I am going to look into both when I can get back onto the plant and play in safety Problem stemmed from long period of running drive in manual where the long acc/dec times in the drive wee essential to protect the fan, the argument arises that we need PID control but occasionally will need manualand will therefore need to protect the drive, we might have to have switched values within the drive Incidentally I have thought of incrementing the SP in the PID loop but don't know if this has any advantages/disadvantages in comparison to the other suggestions Will get back to you when I have the answer Phil

Need some help with setting up a large fan on PID control, supplier of control system SLC 5/03 has configured PID loop in his way with a large acceleration/deceleration setting (approx 60secs) within the AC inverter (255kW) which seems reasonable given the size of the fan however the drive manufacturer says that normally when using PID control they set these figures around 3 seconds and control speed up from start via the PID loop, control supplier says does not think this can be done with AB PID statement as effectively using two different ramp figures one for startup and another for normal running conditions. Fan currently very unstable when setup this way All suggestions gratefully received Phil

You can buy a standard 25 pin to 9 pin converter no chance of USB with a PIC Phil

not enough information to give an answer but if the program change is urgent why not use online editing and test each line in case something is inherently wrong in what has been written, daft question he has turned key to program position