TOOK (Customer) asked a question.
I work in a product test lab where we apply pressure to products with air cylinders millions of times to test product durability.
Right now I am using a Siemens 1211C AC/DC/RLY PLC to control Norgren V61 5/2 solenoid valves which operate SMC air cylinders to push and pull on the product at a rate of 60 cycles a minute (1 Hz).
Overall this works fine, but the relay outputs on the PLC tend to go bad every 5-10 million cycles. They start to not work, or work erratically. The manual shows the PLC relays are only rated to 1 million cycle life so that is a red flag. The solenoid valves pull 0.036 A at 110 VAC.
I am looking to revise our setup to improve reliability over 10 to 50 million cycles. What would be the ideal control scenario for this? Are there PLCs with higher life relays? Change to external relays and use PLC with MOSFET digital outputs?
Most applications for solenoid valves and air cylinders do not operate this many cycles or at this fast of a rate. I'm curious if others have used/designed systems for higher speeds and long life.
Thank you.
You're on the right track.
1) Change from relay outputs on the PLC to digital (transistor) outputs.
2) Use an interposing (SSR) relay similar to: 38.31.7.024.8240, SSR Interface Module, SPST-NO 3A, 24 VDC Input, 12 -275 VAC Output, screw terminal socket | FactoryMation
3) Ensure you have flyback diodes installed.
I agree that transistor PLC outputs and a separate interposing relay is a good way to go
But since this is Automation Direct's forum, I might suggest something like https://www.automationdirect.com/adc/shopping/catalog/relays_-z-_timers/solid_state_relays/din_mount_relays_(ad-ssr8_-z-_ad-ssr2_-z-_ad-ssr6_-z-_ad-hssr8_series)/general_purpose_10a_-_65a,_(ad-ssr8_-z-_ad-ssr2_-z-_ad-ssr6_series)/ad-ssr810-dc-28rn
Or one might just use a socketed electromechanical relay that costs maybe $5 and is easily replaced.
While coil suppression is essential for solid state outputs and always desirable (though not entirely essential with small coils and electromechanical relays) but one does not want diodes with AC, an RC suppressor, "Transorb" or MOV might be used.
You're correct concerning the diodes with AC coils. I was using the term flyback diode as a general term for coil surge suppression but only a varistor or RC pair or similar should be used on AC coils.
What I don't like about the AD SSR offerings is that they are packaged units and in order to replace the relay, it has to be rewired. Separate bases/relay modules are the only way to go in my book.
I agree about the replaceable relay module. I do not mind if failures occur within this system as long as they are quick/easy/cheap to replace on occasion.
Right now replacing the whole PLC is my only option which of course checks none of those boxes.
The coil surge suppression issue is something we have never been aware of. It sounds like it is not an issue with electromechanical relays like we have always used? Only solid state relays?
Would this not also be required between the relay and the PLC? Or perhaps not since your suggested relays are solid state rather than using a coil.
"It sounds like it is not an issue with electromechanical relays like we have always used?"
No, I wouldn't say "not an issue" I said " always desirable (though not entirely essential with small coils and electromechanical relays)" And I was thinking of easily replaceable relays when I wrote that, if replacing a relay means replacing a $500 PLC then suppression is absolutely an issue, even if it would work for a while without.
"Would this not also be required between the relay and the PLC? Or perhaps not since your suggested relays are solid state rather than using a coil."
Yes, it should be used with transistor PLC outputs and electromechanical relays, however, with a DC output a simple diode will work. Many PLC outputs will have some suppression built in, and with small relays one may often get away without external suppression but an additional diode at the relay socket is easy and cheap insurance.
And yes, it is not needed with solid-state relays as they don't have inductors.
Note I'm talking about small relays, with large contactors it is more complicated and a simple diode may not be so good..
Personally I would not use a TRIAC PLC output card for anything but legacy indicator lamps, well, not again anyway. I actually do have one of the CLICK modules driving a solenoid valve, but we turn the air off before starting the machine because we've had random actuations at startup. TRIACs are delicate flowers that might be ok dimming the lights in your dinning room, but back to back SCRs are probably better for anything industrial.
What RBPLC said is definitely a great way tyo do it with minimal cost and improved reliability.
Another option would be to change out all of your hardware and go with voice coil actuators instead of pneumatic air cylinders. That should get another order of magnitude longer cycle life, but the cost to implement would be drastically higher.
If the amp draw on the Norgren solenoids is really that low, switch to a PLC with a modular TRIAC output card. Many Triac outputs can handle up to 0.5 amp load, but the whole card might be limited to a couple of amps (so you can turn on 16 0.5 loads at once if they're all wired to the same card).
Even the Click 8 point AC output card supports up to 0.3A per point:
https://www.automationdirect.com/adc/shopping/catalog/programmable_controllers/click_series_plcs/click_plus_plcs_(stackable_micro_modular)/stackable_i-z-o_modules/c0-08ta
Skip the relays altogether.
I am surprised the Norgren valves hold up to that duty cycle. I replaced many a Norgren valve stack with Mac Valves when I worked in mfg due to reliability issues.
Thanks everyone for the input! As a mechanical engineer, it is extremely helpful to have insight from others with more experience and from other backgrounds on things like this.
I hope to switch to a Siemens PLC with transistor DC outs along with SSR relays to my solenoids.
Most of the equipment we have fails often and we just keep replacing bits and pieces because no one is knowledgeable/willing to investigate whether or not the components are well matched to the task.
I'm surprised you don't have (or at least didn't express) more issues with the solenoids and cylinders than the PLC outputs. At my day job we have machines that get all the cylinders replaced every 6-18 months because after many millions of cycles they are just plain worn out. Older models of these machines favored reciprocating mechanical linkages over cylinders and the maintenance burden on those is much lighter. It might be better in the big picture to ditch cylinders and solenoids altogether, go with an eccentric drive like an old power hacksaw.
But, if that's not in the cards, like they said... DC PLC outputs to interposing relays. I use these but they're out of stock currently. You can get pop-in replacement relays by the bushel, but not from AD.