I’m a complete newbie when it comes to designing my own electronics so I need some help.

What I would like to achieve is some electronics to take a reading off say a 0-5v sensor and then use a feedback loop to maintain a set reading off the sensor by controlling the voltage of some 230v ac motors.

The actual logic would be very similar to that driving your narrowband o2 sensor.

My biggest question is what is the bit of kit with inputs and outputs that I program this logic into and how?

 

Do you want something off-the shelf?

http://uk.rs-online.com/web/search/s...duct&R=3651752

I don't quite get your reference to o2 sensor though...

 

Need more info !

What are exactly are you trying to acheive and what compoents do you have or intend to use. ?

 

 

Your requirement isn't very clear!

 

his requirement seems quite clear to me

he wants to control the speed of a couple of 240V motors with a PID loop so that they maintain a speed necessary to achieve an output on some kind of 0-5V sensor

 

Nick, you have it spot on

I want to use this sensor

http://www.freescale.com/webapp/sps/...6&fromPage=tax

With a feed back loop to auto set my fixed depression on a flow bench to 28" WC (7 ish kpa, 1 ish psi) by controlling the vacuum motors.

 

well perhaps you could translate for us lesser mortals then?

firstly, controlling the voltage of a 230 ac motor *may* or *maynot* have any impact at all upon its speed / torque. depending on the motor type that you want to control, you might be better off converting the ac to dc to start with or other types of motors rely alomost entierly on the ac frequency. need more information!

 

Are you saying that with a single phase ac motor I would be better off converting the frequency than changing the voltage?

 

And its obvious to me but its nice to know the application details.


One problem you have is controlling the speed of AC motors requires a device called a "variable speed drive" or VSD as its known.
These are not cheap !!
Motor speed of AC motors is dictated by the AC frequency and not the voltage given to it !
You cant use a variable resistor and if you could it would be BIG !

There are some cheap single AC phase drives out there but I dont use them so cant recomend any. (I use 3 phase mainly ! )
This VSD will take an input signal as a voltage or current.
This will either be a 0-10 volts input or 4-20 milliamp signal (industry standards)

Some exampes here.... http://www.abb.com/product/seitp322/...b002d1ad2.aspx




As for a controller, thats easy, there are many such PID controllers available from about Ł50 upwards.
These output standard signals as described above and take a multitude of inputs.

Some VSD's contain built in PID controllers so may be more of a compact solution.

Hope that gves you an insight

 

as much as you are a cock Simon lol.. well done with that reply. lol

 

Editted my post above

 

well depends what you mean by single phase. all 230v ac motors are single phase but some eg vacuum cleaner / electric drill are able run on dc as well if they have a comutator (brushes). on the other hand many ac motors are of an entierly different design which typically uses a capacitor to create a phase shift for staring. these motors have no brushes and wont run on dc. the type with brushes will have a very high no-load speed and this will reduce when the motor is loaded. the other common type will try and maintain the same speed irrespctive of load. traditionally, the brushed type was the easiest to control but with modern electronics many things are possible. its more common however to use a low voltage stepper motor as control is easier.

 

LOL, I will take that as a compliment

 

Single phase for those who dont know is just normal mains supply

Most of what you suggest runs at a fixed speed or 2 or 3 fixed speeds as they have diffferent switched windings.
No good for for PID control !

Drill controller will NOT be isolated from the mains so thats a safety issue if you link it to anything else.
Besides, those controllers are tailoreded to a custom motor in the drill so adapting it may be another issue !


As for stepper motors providing airflow needed for this application - No chance .... LOL

 

Thanks for the reply Simon. Knew you would be the man to ask.

If motor control is a prob I could always have to feed back loop drive some sort of bleed off valve and leave the motors running at their normal speed.

What does PID stand from. Are they off the shelf input output devices with standard logic built in?

 

PID = Proportional, Integral and Derivate. Its actually a mathmatical equation..

Look here... http://en.wikipedia.org/wiki/PID_controller

ANd yes, they are off the shelf devices with a standard set of interfaces and functions etc...

 

who mentioned air flow or the application?


did you read my post before tearing it apart??
simon

 

Got to love wikipedia! I just found that myself.

There is an off the shelf product that will do this job but the manufactures charge nearly five hundred quid for it. Just wondered if I could do it myself for a bit less??

 

PGT,

I can see that my wording may be in a "chip" style

I appologise if I seemed to "tear apart" your informative post but that was not my intention.

The thread starter mentioned airflow..

 

What for just a PID controller.. ? Rip off !!!!!

 

Here is a cheapish PID controller... http://uk.rs-online.com/web/search/s...duct&R=4822573

Ł120 ! (will be half that price from other suppliers...LOL RS components are expensive ! )

 

Single phase VSDS... Here ...

http://uk.rs-online.com/web/search/s...e=1#breadCrumb

Again RS catalog prices

 

[ASS COVERING]

FYI, My suggestions above are just that...suggestions.

Suitability will depend on total system design..

[/ASS COVERING]

 

No probs Simon. Thanks for the help. Mucho apprecated.

Looks like I get to learn something and save a few quid at the same time.

 

no probs Simon - you're better qualified to answer it than me anyway. lol at "chip style"!

 

Simon if i wanted to digitise the output from the above 0-5 v sensor that has a range of 50 kpa what sort of resolution would i get if i used this bit of kit.

It has a 10 bit processor.

http://www.dataq.com/products/startkit/di148.htm

 

I cant see why you would need digitise it ?

Normally, you would feed the output of the sensor into the PID controller and that would accept the signal and scale it to your required scale of 50 kpa.
You would set up the controller to display the range and number of decimal places (to a limit)

Depending of the resolution of the PID controller you will have the following resolutions....

8 bit 0.1953125 per bit 256 possibities

10 bit 0.048828125 per bit 1024 possibilities

16 bit 0.000762939453125 per bit 65536 possibilities

The more expensive the PID controller the higher the internal resolution.

Having said that, the controller would have a digital filter and averageing which would effectively double these ranges as measuring anything with movement will never be stable.

Even a low spec 8 bit controller will work just fine IMO.

 

I was think further.

Seeing as i have found a suitable and relativly cheap sensor could i use it to ditch my manometers and take measurement directly from it??

That way i could use the datac to take my measurements and then import into an excel spreadsheet that will do all my flow calculatiions and then of course is also an easy way of producing nice easily readable graphs for comparison.

 

Ah I see, yes you could do that quite nicely but depending on the sample rate you may need to average the result to provide a stable signal but thats easy to do on that kit.
This is the trouble with digital electronics and high speed measurements.

 

50 kpa / 1024 = 0.0488 kpa

28 inch WC = 6.9 kpa

6.9 kpa = 141 posibilities

If i use a 200 cfm calibration orifice that gives me a resolution of 1.42 cfm. Hmmm not sure that is enough.

One solution would be use their + / - 7kpa sensor rather than + / - 25kpa but 28 cfm would be nearly at the endo of its range.

That would give a resolution of:

14 kpa / 1024 = 0.01367

6.9 kpa = 504 posibilities

200 cfm orifice plate gives resolution of 0.4 cfm which would be fine.

Does that sound about right Simon?

 

Sounds about right. although I am no expert on the airflow needs you are after.!
The sensor is key wether your are monitoring and/or controlling.

Given the fine readings you are looking at, for the PID control part, controlling the speed of the motors may not give you fine enough control. !!!

This could be one of thsoe applications where all items must be specced properly first and tested virtually !