Controlling 128 heaters with PWM in FX2N

[resitec 0]

Greetings Can i use 128 PWM cycles for heaters control on a FX2N plc? This system has 2 plcs (FX2N), one is controlling the heaters and the other is controlling all the movements. I was thinking use PID cycles instead of PWM, is that possible? Regards Amilcar Antunes Edited 31 May 2007 by resitec

[Paul Ked 0]

Not exactly sure what you are trying to do, but (from the programming manual) "The PWM instruction may only be used once in a users program" PID seems to be the way to go. Incidently, there is a pre-tuning feature for ease of PID set-up in the FX2N, see manual (download at http://forums.mrplc.com/index.php?autocom=downloads&showfile=693) Regards Paul Ked

[resitec 0]

Thanks for your reply. I will try to explain, I'm trying to make a percentual control to 128 independent heaters. The only way i know to make a percentual control is with a pwm cycle. Although i'm not interested in a PID control function. Regerds

[Sergei Troizky 26]

Just create PWMs programatically.

[Paul Ked 0]

I'm sorry, but I still don't understand what you are trying to do. If they are independant heaters and you can only have one instance of PWM in the PLC, how are you going to do this? Regards Paul Ked

[panic mode 246]

How often you think heater would have to be turned on and off (PWM frequency)? There are many ways to do this and you really need to explain a bit better what exactly you want to achieve. FX series has only one PWM channel (hardware) so only one PWM instruction can be used to control it. Normally PWM is using freq in order of 100Hz or higher. This is why there is dedicated function for it otherwise one could do it easily in software. If done in software, it can be as many channels as number of outputs. but this can't be as fast as built in PWM anymore (you need to count time or scans or whatever for on and off periods). PWM works great for DC but AC is a bit different (unless AC frequency is significantly higher than PWM frequency, otherwise it's pointless). Controlling on/off duty cycle in PLC should not be a problem as temperature is slow changing variable anyway, but it may be a problem depending on how exactly or precisely heaters are to be controlled (is that the only goal?). Are you just using contactors to turn the heaters on and off? They are too slow to respond to normal PWM anyway so software for control of duty cycle is more than adequate. Maybe you use Solid State Relays or something else? Are you aware that turning off solid state device in AC circuit will happen only on zero cross? So how about zero crossing? It would be really nice to both start at stop at zero cross and just vary number of on and off half-periods but for this PLC is not going to cut it even if you use PWM (requires syncing with line frequency and fast I/O and PLC scan...).

[resitec 0]

First of all, thanks for your reply. I will try to explain better what i want. I have to make an individual temperature control for 80 heaters. This is for a thermoforming machine. This type of machines doesn't have a temperature closed loop controller. They use a percentual control, e.g. the user just have to put the percentual value for the respective heater. In my case the machine has 80 heaters, the plc will controll them through SSR's I was thinking controll them through PWM, but the plc has only one pwm instance. As the pwm period cycle is 1 second can i use 2 timers for each pwm ? I need at least 160 timers, plus the timers for the machine movements. Is it possible that with so much timers the plc will slow down and go "crazy" ? I already have the folowing products: 80 SSR (zero crossing) 1 UNIT FX2N 32MR ES 5 UNITS FX2N 16EYT Can i add the system 1 unit FX2N-8AD, so i can read the temperature in 8 points? Or i will have problems because there are too many modules conected ? Best regards

[Crossbow 295]

Adding that number of output modules is too much for the PLC's internal power supply. You cannot use that many unpowered extension blocks. You need to look at the FX2N-32ER and FX2N-48ER powered extension units. Are these heaters spread over a large distance? if so, you can potentially use networked I/O and a CC-Link master module.

[resitec 0]

What you mean with "powered extension units"? Is it an extension that can provide more power to the system, helping the plc with the extension modules? The heaters are close to the plc. Regards

[Crossbow 295]

Just what I said... The powered extension units do NOT require power from the CPU's extension bus, they are externally powered. Have you looked at a catalog for these units yet? The power supply in the base unit is not capable of so many output modules without the additional power of the extension unit. Look for the FX2N Hardware Manual for more details. It's in the downloads section here.

[resitec 0]

Thanks Crossbow, i've searched the manual for powered extension units and found one that will help the system, but i'm not sure that is enough. Can you say if this system will work, or i'll have to make some more changes? 1 UNIT FX2N 32MR ES (i already have it) 4 UNITS FX2N 16EYT 1 UNIT FX2N-32ET - Pow. extens. unit 1 UNIT FX2N-8AD (i already have it) What i need: 80 transistor outputs 8 analog outputs 16 inputs 16 relay outputs Regards

[Crossbow 295]

You need to look at the hardware manual. The power supply capacity for extensions is limited. By putting the powered extension unit last, you're not making use of it's power supply. I would put a couple of the extensions on befire it, and a couple after it. Everything after the powered extension unit is calculated from it's supply not from the CPU supply. It's all clearly detailed in the FX2N Hardware Manual.

[tofty 0]

Hi resitec, I am a plastic thermoforming software engeneer in the uk. we use a simmilar setup to you apart from we have our own heater cards made. the way we do it is have one PLC doing all the controll side and one transistor out PLC doing all the heater controling. We use fx2n 48MR, Fx2n 128MT, E1151 touchscreens and 485 BD comms cards on both PLC's. The heater controler is simple, permantley run a 200ms timer, when it times our reset it and start again so you have a loop 0-200ms then 0-200ms asn so on. Then use [ <= above timer Tnumber - compared to element %] out to (y0) for example [<=t250 d100]--------------------(y0) repeat this 80 times changing the dataregister d100 and out coul y0 and tou have a heater controler which runs fast. in your E pannel you will have to do some messing with the d100 number to double it ( gain 0.5) will do. then repeat on the e pannel 79 more blocks all with different data registers. as i mentiond above we use 485BD cards as when we tried combining the sequence controler with MT extensions the scan time of the PLC went very slow. we need a fast scan time so quartz elements dont flicker to much, if this isnt a problem for you just increase the Timer value up high so if you have a long scan time it will not affect the SSR's to much.

[Paul Ked 0]

Where have you been all my life? Nice solution, but 3 months too late! Just jesting, welcome to the forum, all/any help always appreciated Best Regards Paul Ked

[panic mode 246]

while this is hands down simplest and cleanest programming solution, it is the least favourable one for the power grid. what is the power rating of those heaters?

[Sergei Troizky 26]

Can you explain why? I see no reason for such judgement.

[panic mode 246]

i know nothing about that industry and proposed solution may be very common. i just can't help trying to build "better mousetrap". you have heaters (probably one of most significant loads among electrical devices) and more -> you have hundreds of them... proposed mechanism would cycle each output according to the preset (which can be anything 0-100%) and this can't be avoided. the downside is huge inrush when all loads kick in, and -> all at the exact same time (and... this is after all of them ware off...). the only exception would be outputs running either 0% and 100%. in my opinion it would be much better to have program trying to equalize load and reduce "pumping" effect (surges which are repeating like clockwork).

[Govind Lathkar 0]

I have done 400 ( Digital Output ) heaterwith PWM in Q02 PLC for PID buit in pid present Q64TCTT ( total 10 Points i.e. 3 cards used) is ued. It is working .

[panic mode 246]

trust me, i know very well that it does work but that was not the point and so far nobody cares to explain what size loads we are talking about....

[tofty 0]

panic mode Posted Oct 11 2007, 01:11 AM trust me, i know very well that it does work but that was not the point and so far nobody cares to explain what size loads we are talking about.... We use 500W heating elements in pairs per zone = 1000w / zone. 80 zones = 80Kw = 111A / Phase. Quite a lot to be pulsing on and off. You can use a number of timers and compare them just stagger the start times. if you use 10 timers 8/zone then you will only have 11A / Phase staggerd every 100ms.

[larry818 0]

Most resistive loads don't have an "inrush" current, they just draw what they draw. I say most 'cos some have some inductive loading and some reduce current as they get hot. For inductive loads I always stagger the starts, but for resistive I mostly don't bother.

[artetavn 0]

Hi every one , I'm using the PWM functions of FX2N to control the speed of a DC Motor . But I don't know the frequency of pulses of this PWM , can I change it ? Help pls

[vaibhav 0]

hi this is vaibhav from india first tell me why you are using PWM instruction? you can directly use PLSY instruction by keeping pulse field constant to 0 use as [PLSY D0 K0 Y0] in D0 you can vary the freq. from 0 - 100000 hz fro FX1S & FX1N plcs & fro FX2n plc you can use 0 - 20000 hz freq. hope this will help you finding the solution for your problrm vaibhav

[panic mode 246]

hi vaibhav, welcome to forum. your suggestion sounds interesting so i have some questions: - how many PLSY instructions can you have in same PLC? (and running simultaneously! requirement was 128 independant channels running at the same time...) as far as i recall, one can only use one or two instances of PLSY at a time. - pulse output of PLSY has 50% duty cycle. what kind of circuit is suitable and inexpensive enough to drive 128 heaters when driven by frequency output (like from PLSY output)?

[Alexander K 0]

vaibhav's quote was regarding one DC motor :)