Pneumatic Cylinder Controller

Hi all,
I am doing this project in which ill be using a microcontroller to control the air valves.
Ill be taking inputs from reed switches placed on the cylinder and according to the condition switching the valves on or off.

I have come up with the following circuit, researching through the internet.
Kindly let me know if you guys find any problems or maybe some suggestion to better the circuit.

Thanks!

**broken link removed**

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If you use a ULN2803a you can eliminate the resistor network since it is designed for 5 volt logic. Beyond that the circuit should work. What is the stroke (travel) of the cylinder?

The ULN2803a data sheet.

Ron

Thanks! Ill be using ULN2803a now. I hope its available in the local store.
There are 4 cylinders and the strokes are the following: 120mm,100mm,30mm,30mm.

Can you tell me the process which one should follow developing a circuit.
Should i first make the circuit on the breadboard and then go on to the PCB. I think there is less things to go wrong in this circuit and i should move on to soldering and coding the Uc.

I would absolutely breadboard the circuit first. You will need to program the uC of your choice. Remember that obviously the cylinder shaft will need a magnet to activate the reed switches.

Ron

Your schematic shows the relays with high-side switches. Be aware the ULN2803 consists of low-side switches.

That's what I was going to say. Something looks wrong with the wiring if you use Ron's relay driver. Look at the test circuit in the datasheet. There are 2 commons, but one is ground and the other is the relay power supply. Not for power, per say, but to enable the diode suppression. The resistor/inductor is the model for the relay coil. If you were just driving LED's with the driver, a connection to +power is not needed. PCB layout is important here, to make the COM power pin close to the relay.

The schematic is drawn with multiple 5 V supplies. You might draw it with only one. Use power symbols.

I might consider putting a Schmidt trigger on the power on reset pin. I've had processors not reset when a linear supply was used, but would when a switching supply was used. Turn on time was too slow for the linear supply.

AC power/power supply is missing, but remember that there would be a power switch, RFI filter and fuse protection.

Next question is would you require an ON/OFF switch that the processor would use and not be dependent on just the power switch. Would an E-stop (Emergency Stop) be appropriate?

If this thing was put in an industrial environment, 24 VDC might be more appropriate for the relay coils unless this was a stand-alone product.

I know it's a project and not a product, but you need to be aware of what might have to change if it were a product.

Reloadron said:

I would absolutely breadboard the circuit first. You will need to program the uC of your choice. Remember that obviously the cylinder shaft will need a magnet to activate the reed switches.
Ron

Click to expand...

Yes,The cylinder shaft will be having a magnet.

alec_t said:

Your schematic shows the relays with high-side switches. Be aware the ULN2803 consists of low-side switches.

Click to expand...

I do not understand what you mean by low side switches of ULN2803.
Maybe the confusion is due to the free software i used to make the circuit.
I want to use both the contacts of the relay, NO aswell as NC.

KeepItSimpleStupid said:

The schematic is drawn with multiple 5 V supplies. You might draw it with only one. Use power symbols.

Click to expand...

I couldn't find the correct symbol in the software. Please consider all 5V supplies to be one.
That too will be through either a SMPS or a 220V supply stepped down to 12V and rectified and regulated to 5V.
I didn't know that would make a difference if the end use will be of 5V DC.

KeepItSimpleStupid said:

AC power/power supply is missing, but remember that there would be a power switch, RFI filter and fuse protection.

Click to expand...

Where would i require an AC power supply,RFI filter and fuse protection?

KeepItSimpleStupid said:

Next question is would you require an ON/OFF switch that the processor would use and not be dependent on just the power switch. Would an E-stop (Emergency Stop) be appropriate?

Click to expand...

I wont be requiring an ON/OFF switch, I plan on using the reset switch. I operation would be performed once and then the controller will go into an endless loop. Whenever the reset button is pressed the operation will be performed again.

Ill also be using a switch to put the cylinder in the initial position in case of power failure.

KeepItSimpleStupid said:

If this thing was put in an industrial environment, 24 VDC might be more appropriate for the relay coils unless this was a stand-alone product.
I know it's a project and not a product, but you need to be aware of what might have to change if it were a product.

Click to expand...

Why wouldn't a 5 VDC work on the relay coils? This will indeed be used in an industrial environment but since I'm new, I'm not aware of the industrial standards.

Low side switch
They switch the ground side of the load. The ULN drivers typically do this through a NPN transistor.

5V Supplies
OK

AC power etc.
It's a tough call depending on where it's going. But say it's a rack mounted unit. You could get a power entry module which effectively has a IEC male connector for a universal power cord, a switch, and RFI filter and even a fuse. Your power supply may or may not have one. You need to keep RFI from the power supply and the processor from radiating into the power line. Search for Corcom filters. What you use depends on a lot of things. To me it was simpler using that approach. Some things didn't get a power switch, but were fused on the front. Some controls didn't get one either. The fuse would be the power switch. It was on 24/7. In 15 years, it was turned off about 3 times.

Reset
Then you may wish to consider a separate switch for OPERATE with a separate power ON reset unless it's acceptable for the unit to start when power is first applied.

How can your switch perform two functions?
Reset and Go?

OP said:

Ill also be using a switch to put the cylinder in the initial position in case of power failure.

Click to expand...

Without power, it will be interesting. Note if your using air, you will have air. I did a project once in which I had dual acting air cylinders, but we didn't want them to change state if power failed. That was fun.

Relay power
It may be just fine when it's self-contained. A lot of plants use 24 VDC for controls. I can't really make a suggestion here. I did a project where I had to drive solenoids, so all the solenoids were mounted externally because of up to 24 air lines. The control was generic, so some of the 24 lines were used for other functions. A lot of times it's more convienient to put air near the cylinder. I know you didn't mention air, but it's typical.

I do not understand what you mean by low side switches of ULN2803

Click to expand...

I don't have the details for the ULN28031 you show in your schematic, but I assume the pin-out is the same as for the ULN2803A which is suggested in post #2. That IC contains Darlingtons with their emitters all grounded and their collectors connected to respective output pins. The output pins are intended to be connected to the bottom pins (low-side) of the relay coils; the top pins (high-side) of the coils should go to +V. The way you've presently got them connected means that the Darlingtons will merely short out the relay coils instead of energising them.

Maybe the confusion is due to the free software i used to make the circuit.

Click to expand...

No; it's down to not connecting the IC according to its intended function, as per the datasheet.

I want to use both the contacts of the relay, NO as well as NC.

Click to expand...

That's fine. The relay coil drive arrangement is independent of the use of the switched contacts.

The attached should help. It is a partial schematic with an inset of the chip. The reference to low side switching means we are switching the common of the relay coils as drawn.

Ron

KeepItSimpleStupid said:

Without power, it will be interesting.

Click to expand...

Why shouldn't a valve work without power?

Please take a look at the sketch. Made for you only!

Yea, I know.

We had these valves as you have shown, but they were attached to some Hi-vacuum gate valves and at times like when loading or unloading there would be this $3500 arm in it's path if power failed. The air would still be stored (BIG TANK).

So, a power fail in the middle of that operation would kiss $3500 goodbye. There was also a manual valve that shut off the vent side when it was open too. That was put in place before the electrical one.

This system was shut down by a fire alarm signal as well, so leaving it in it's current position upon a power fail or fire alarm would do no harm and would actually help us. The nasty stuff happened when the gate valves were closed.

Yep, the valves work without power. TRUE. But they may work unintensionally.

There were other reasons too. The chambers were typically turbo-pumped overnight and a power fail would make us waste a day if it was brief and the Turbo didn't shut down. Spins at like 40,000 RPM or so.

KeepItSimpleStupid said:

Without power, it will be interesting. Note if your using air, you will have air. I did a project once in which I had dual acting air cylinders, but we didn't want them to change state if power failed. That was fun.

Click to expand...

If the controller loses power, wouldnt the cylinder stay in the position they were in?

alec_t said:

I don't have the details for the ULN28031 you show in your schematic, but I assume the pin-out is the same as for the ULN2803A which is suggested in post #2. That IC contains Darlingtons with their emitters all grounded and their collectors connected to respective output pins. The output pins are intended to be connected to the bottom pins (low-side) of the relay coils; the top pins (high-side) of the coils should go to +V. The way you've presently got them connected means that the Darlingtons will merely short out the relay coils instead of energising them.
No; it's down to not connecting the IC according to its intended function, as per the datasheet.
That's fine. The relay coil drive arrangement is independent of the use of the switched contacts.

Click to expand...

Ohkay. I got what you meant. Thanks for pointing it out!


Also, I guess there will be a problem of the relays getting energized when there is a power on reset. What can i do to eliminate the problem. I tried to find on if there were any pnp transistor array ic available but couldn't find any.

I guess there will be a problem of the relays getting energized when there is a power on reset

Click to expand...

There shouldn't be. Hopefully the power-up time will be only 1 or 2 mS. Relays of the mechanical type need longer than that to pull in, so will 'ignore' brief spurious pulses.

Your ULN drivers probably work like the ULN2003 and I'm pretty certain about it. Because they are nothing but darlingtons they have a unique characteristic:

1. Off when the inputs are open
2. Off when the input is below VLow
3. On when the input is above Vhigh

This has a profound meaning. A lot of processors power up with their pins as inputs, so the device thinks it's off.

If the controller looses power, you may have to deal with that. I can't even begin to analyze that for you, no can I know what is supposed to happen when power is restored.

Power off usually turns off all of the relays. It turns back on. Now what?

When the power comes back on, i want all the cylinders to go back to their initial position. I have till now planned on using only 4 relays for 4 double action cylinders. For resesting to the initial condition i think ill have to use 8 relays.

I have still to go to my supplier and take a demo for the valves. All i know is it will have 2 wire. Any idea what combination does what. Does it work like a DC motor ?
T1 + , T2 - => One Direction
T1 - , T2+ => Other Direction

or just energized and DE-energized ?

Your mentioning doing something depending on the position of the cylinder. I can't interpret that very well.

If the cylinder moves slowly, then you could conceiveably do things during it's travel.

If your trying to position the cylinders, then that's another issue. I don't even know if your using air. I'm just playing "devils advocate" and asking questions. I was thinking that the cylinders were controlled by something else. I have no idea. Now, it could be a DC linear actuator. I have no idea. I won't speculate.

I will say that for DC linear actuators, you can have potentiometer feedback, pulse+org, slip clutches or nothing at all and you can't combine stuff.

Since you said you have two wires, let's assume DC linear actuator for now. DC linear actuator? Position feedback? How? Speed control required? Voltage? Current? Stroke? Force?

What about finding the origin? Power failure? Limit switches?

See why I don't want to go there yet? The problem isn't specified enough.

The entire idea could be scrapped.

Please define the problem a bit better. Your option list seems to be a closed set, because you don't know what's required and what's available.

I was initially thinking that you activate 4 relays depending on position of the cylinder and the cylinder position is controlled by something else. e.g. open legs, open arms, nod head up, knod head down are the 4 relay outputs and it's for a Halloween decoration.

Yep, I'm being silly.

Now i feel silly and stupid! Let me try and be specific.

In the attached image, the blue ones are pneumatic cylinders (air).
The cylinders will be controller using a 3/4 Port Solenoid Valve.

There are reed switches(S) on the pneumatic cylinder which i will use as position feedback to my electronic circuit.

First, Cylinder 1 in position S1 will be actuated to S2.
When S2 goes high cylinder 2 actuates towards S5.
When S4 goes high cylinder 3 actuates towards S6.
When S5 goes high cylinder 4 actuates towards S8.

Initial position S1,S3,S7,S9.
All cylinder are dual acting.
Solenoid valves control which way the air goes.

KeepItSimpleStupid said:

Your option list seems to be a closed set, because you don't know what's required and what's available.

Click to expand...

You got that one right. I am new and i don't know how things work in practice and what are some of the standards of the industry.

OK, I got it for the most part.

What kind of air is being used? (clean or lubercated)?
How will you set push/retract speeds?
Dual acting air cylinders will likely require lubercated air? (actually lubercated, dry air)
Where does the dirty/clean air exhaust go?
You can have air and/or electric pilot valves. What's your plans?
What are the valves and the cylinders rated for: (lubercated, dry) air
Where does the exhaust go? (is it dirty or lubercated)?
Been there. Done that.

I hate to make you think. You could say: "That's not my problem." Just don't take the fall.

Haha! That's not my problem!
But again, let me try.

1.I have a FRL unit. So thats clean and lubricated air.
2.For speeds, there are regulators available with the solenoid valve.
5.Ill be something like this:**broken link removed**