Latching circuit help please

Hello all,

I am a novice trying to build a circuit that will extend and retract a double action pneumatic cylinder. I have a fob operated 12V DC 10A 433MHz relay switch module that has four separate buttons / relays on board, of which I would use two for the extend and retract cycles of the cylinder. The module would deliver a momentary voltage to the relay to latch it and then turn off. The latching relays are Bosch style 40A automotive relays.

The valve being used is a single inlet double acting, double solenoid valve that feeds one side of the cylinder while venting the other. It has a center resting position that stops all airflow and holds the ram either extended or retracted when power is disconnected.

I'm using a pressure switch similar to that of an air compressor, it has two independent sets of contacts, one of which would be used for this circuit. When a button is pressed, I need the appropriate relay to latch and power the air solenoid for that stroke, the compressor to come on and everything stay on until the set amount of pressure has been reached in the cylinder, then everything turn off. The compressor has a one way valve inline and naturally bleeds it's cylinder pressure off ready for the next start-up.

My question is, do you think the circuit will work as shown, with the solenoids in line with the relay coil, or will I need secondary relays to power the solenoids? The solenoids are actually 3W, I put 4W in the drawing. Also, am I missing anything obvious here?

Apologies for the rudimentary drawing and lack of knowledge, hopefully at the least this will be a good laugh for the day.

Thank you,

I think that you are missing some logic devices to run the correct sequence of actions.

You need some device to remember that an action has been initiated. As I see it, when the button is pressed, the compressor will start, but will carry on running when the button is released, until the pressure switch operates. So for the compressor you need one on-off state to be remembered. That could be done with a simple relay.

The remote control relay may have some latching feature, but then you might need a separate action to turn thing off.

You should write down all the states, so that the order of what is required to happen is obvious. If two different actions are required for the same input, some device needs to remember that part of the sequence. For instance, if the compressor runs until the pressure is high, and then stops, and then the pressure leaks away, but you do not want the compressor to restart, something has to change state when the pressure is high enough, so that the action is stopped and will not restart.

Any device with start and stop buttons has to remember which was pressed last.

On your circuit diagram, you shouldn't put wires behind symbols. It's not clear where the power supply is, or where the other end of the compressor is. There doesn't seem to be any way for the solenoids to be connected to ground. The function of the remote switch and the pressure switch aren't shown.

Hi Diver, thank you for the reply.

The remote relay will be a momentary signal, so I would like it to just start the latch on the extend or retract relay. I want the compressor to stay running the whole time the cycle is completing as there is no accumulator tank, the compressor is filling the cylinder directly. I was hoping that having the negative of the compressor's relay coil hooked before the pressure switch would cause it to lose power until the next cycle.
My hope was that the arrangement of the pressure switch would turn the whole shebang off, including cutting the signal to the compressor relay. I omitted the compressor's relay circuit as the part I did took me way too long to draw up on my lunch here at work. The compressor's relay coil is connected to positive and grounds before the pressure switch. The whole thing will be run by a 100W solar panel and deep cycle battery.

OK. The best thing to do is to list the stages.

It seems that it would be something like:-

1 Start. Pressure may be low or high. Solenoids off. Compressor off.
2 Momentary signal from remote relay.
3 Pressure low. One solenoid on. Compressor on.
4 Pressure high. Solenoids off. Compressor off.
5. Pressure drops to low. Solenoids off, compressor off.

3 and 5 have the same inputs, in that the pressure is low and there is no signal from the remote, but in 3 the solenoid and the compressor is on, and in 5 they are off.

You will have to have two relays to remember whether retract or extend had been pressed last.

What do you want the system to do if it is in the middle of extending and you press the retract button?

Good point on partial cycle stops. I hadn't accounted for that yet. It would be ideal if everything would shut down, stopping the extend or retract at that point in the cycle IF the same button that initiated the sequence was pressed but I'm not sure of the best way to do that. If the opposite button was pressed, system would lock up as it is right now, as both sides would be latched open.
The remote module does have a latching feature, but I used momentary with a separate latching relay because using the module's relay in that way would require a second button press after the cycle to release the relay, which isn't ideal as it would likely be forgotten.
I would like to also add a timer relay between the pressure switch and neutral that would turn everything off at one minute if there was an anomaly. The timer relay would reset with each cycle. That should be easy enough. My main concern is if the primary circuit as pictured would be viable.

I failed to note that the pressure switch cuts off at 60 PSI. Once I find a pressure switch that will adjust down to 40 PSI I will change that out as adjustability of cylinder force is of importance. Cycle time to fill the 3.5" x 30" cylinder to 60 PSI is around 25 seconds.

Steps would ideally be:

1) Cylinder would be pressurized to one side or the other from the previous cycle, everything is off.
2) Button press latches one of the relays and opens that solenoid, if the cylinder is already in that position, compressor fills lines, pressure switch cuts everything off.
3) Relay and solenoid latches open and compressor fills cylinder.
4) System reaches full pressure and pressure switch releases ground, unlatching relay and ending cycle.
5) One way valve near compressor allows pressure to bleed off at the compressor only, reducing cylinder head pressure enough for it to restart when needed.

Your patience and knowledge is appreciated. Thank you.

Are you able to use limit switches? These would enable the cylinder to "remember" where it is. The limit switch(es) is wired in series with the latching relay so when the limit is reached and the switch opens the relay is no longer latched. One is needed at each end for the in and out movement of the cylinder. If power fails when it is in the middle of operating then it will simply stop until one of the buttons is pressed again.

Mike.

Good morning, Mike,

Thank you for your reply. I don't have a good way of using limit switches, the cylinder isn't set up for Hall effect type sensors and the environment is very rough around the cylinder. I was trying to limit protrusions to two copper lines for the cylinder and try to protect those, in fact the primary reason for going pneumatic over the linear actuator I had previously is to eliminate wires that can be damaged. The other reason is the actuators don't hold up well to the load and a cylinder should do a better job.
I can't remember the last time I scrapped a project but this one is becoming too complicated for me. I feel like the best way to tackle this would be via Arduino but I have zero experience with that. I really don't have the drive to learn it all either, at close to 50 years old. I may just keep having to replace the actuators 2-3 times a year at $500 a pop.

Spikes said:

I feel like the best way to tackle this would be via Arduino but I have zero experience with that.

Click to expand...

Seems what is needed is logic, timers and some i/o. Have you considered using a small PLC? PLC Programming semantics is similar to using relay logic and they are fairly inexpensive.

eTech said:

Seems what is needed is logic, timers and some i/o. Have you considered using a small PLC? PLC Programming semantics is similar to using relay logic and they are fairly inexpensive.

Click to expand...

I really don't have any experience with PLC's either but I'd be more willing to learn that so I'll do some Googling. Thank you.

You may want to consider something else regarding your 433Mhz Wireless RF Switch. I have a few laying around and they both offer the option of Latching or momentary where the relay is energized only while the key fob button is held down.

Ron

I am using the momentary option on the module as the other users aren’t going to remember to press the button again once the cycle is complete