Mechanical: Latching? Solenoid Principles? Not opening? - Page 1
Forum section: electronics / repair

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 21, 2017, 02:32:45 pm
content: I have an automatic door that has a pair of solenoids which control direction in and out... basically each solenoid drives a tab which either allows or denies travel through the flap. (OK there are felines involved... don't tell Dave)...Anyway... it has stopped operating reliably in the inward GREEN direction.The solenoid that drives this (appears to be connected to an H-Bridge L293NE and supply is 12V) is pictured.It seems that when 12V is supplied in one direction the plunger is pulled in and then locks in the pulled in position such that it remains there when power is no longer applied. (The spring is compressed in this state)I think the idea is that if you apply 12V the other way around the device unlocks and the spring pulls open the plunger to the extended state... and then when power is no longer applied it stays extended.However... the pulling in and locking operation works fine.... but when 12V is applied the other way I get only a twitch and the plunger doesn't extend and hence the door remains locked. (I have to apply quite a lot of additional force to encourage it to open)Am I correct in my interpretation of this latching arrangement?Is there some maintenance I could perform to help the 'unlock' part of the solenoid extension?Thanks in advance

author: retrolefty
author_rank: Super Contributor
timestamp: July 21, 2017, 03:09:39 pm
content: QuoteYour interpretation describes no solenoid I've come across. Most have pull inward with applied power,and the spring weakly extends plunger when power is removed. Any 'latching' action would be in circuit control logic or mechanically with additional parts. At least that is my best guess at what you have.

author: Armadillo
author_rank: Super Contributor
timestamp: July 21, 2017, 03:19:37 pm
content: WD40 lubricating spray on the mechanism first I would say.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 21, 2017, 03:40:15 pm
content: I think I have worked it out... at least partially....There is a steel plunger, and the coil in a steel frame but also a permanent magnet and a spring. I've taken it out and cleaned and added a bit of WD40It seems that the spring isn't enough to extend the plunger when the power is applied in the extend direction.I have stretched the spring out a bit (assuming it had become a bit compressed) and am getting a few extends as well as contracts now.I guess either the coil lacks ooompf to counteract the permanent magnet on the extend motion (do solenoids lose oompf over time?) or the spring isn't as springy (do springs loose springy-ness over time?)

author: Armadillo
author_rank: Super Contributor
timestamp: July 21, 2017, 03:58:00 pm
content: Quote from: NivagSwerdna on July 21, 2017, 03:40:15 pmThe WD40 did the magic!.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 21, 2017, 04:02:18 pm
content: Quote from: Armadillo on July 21, 2017, 03:58:00 pmIt's better but it isn't reliable.... maybe some more spring stretching....

author: Armadillo
author_rank: Super Contributor
timestamp: July 21, 2017, 04:14:15 pm
content: Quote from: NivagSwerdna on July 21, 2017, 04:02:18 pmLubricate liberally [not a bit] the "locking mechanical mechanism - rod, shaft, hole, cylinder, plunger" not the solenoid mechanism. Got to *dissolve* the "stiction" of the rust on the moving mechanical surfaces. That means they already pitted not smooth surfaces.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 22, 2017, 01:02:04 pm
content: I think this is working now.... although I think I did the wrong thing in stretching the spring out a bit... A good clean and some WD40(!) and it now works but I'm starting to suspect actually that is only half the problem... the device is powered by a 12V wall-wart and I've noticed that it is quite hot... perhaps I have voltage droop and that's partially responsible for the solenoids not activating fully. The wall-wart is continuously on but I'm surprised how hot it is... might replace it anyway.

author: SeanB
author_rank: Super Contributor
timestamp: July 22, 2017, 01:35:29 pm
content: Put a capacitor on the incoming supply, 4700uF 35V, which will help a lot with an old wall wart that likely is using a simple transformer rectifier and the capacitor inside ( probably wishes it was a 125C CapXon electrolytic rather than a measly 85C WhongFungKungLiChleep special) and the extra current capability on energising the coil will help a lot with the initial pull in energy.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 22, 2017, 02:06:30 pm
content: Quote from: SeanB on July 22, 2017, 01:35:29 pmIndeed.There is quite a large capacitor on the board near the input.. not sure of the spec (it's back in the box currently). I measured the wall-wart temperature with an IR thermometer... around 45 to 50C. The solenoids are latching types so there shouldn't be solenoid load during quiescent times... the major consumption presumably being the field for the RFID chip coils which I guess is small. I think I need to go shopping for 12V 250mA wall wart.

author: SeanB
author_rank: Super Contributor
timestamp: July 22, 2017, 03:30:35 pm
content: Well, it is a 2200uF 16V electrolytic 85C rated Elite branded capacitor, so likely it has both high ESR and low capacitance by now just from ageing. replace it with 4700uF 25V or 35V and it will operate the driver with a lot more kick.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: July 22, 2017, 04:33:01 pm
content: If the power supply runs hotter than originally, there might be a problem with the coils or the driver. If that was always the case, I would suspect the spring. Springs can deteriorate over life, but the exact mechanics of failure involved are a bit mysterious even to experts in part reliability. I had a longer correspondence on this topic with someone from RAC and we decided that more data is needed.On the other side, coil insulation becomes worse at well. When this happens (shorting one ore another winding) the coil runs hotter AND a higher current is necessary to produce the same force, making it run even more hot and accellerating the insulation failure.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 23, 2017, 11:08:49 am
content: I took the 2200uF out and measured it with my swanky LCR meter... see attached photo. My interpretation is that the C is OK; In any case I don't have anything equivalent or larger so that mod will have to wait.I did un-solder both the left and the right solenoid and then swap them. In this configuration the working one has also swapped so I think that proves it is solenoid trouble.I also measured the supply and it gives 10v when disconnected and a stable 12v when connected to the door. With about 60mA quiescent current. When a solenoid is operating it's a bit over 250mA which seems about right. I think the power supply probably needs individual investigation but I'll probably just swap it out for now (or more precisely in a few days when my EBAY cheapo replacement arrives).The solenoid that is misbehaving has operated probably 10 times a day for the last ten years and it's tolerance to it's bi-stable operation appears pretty slim... by tweaking the spring I can get it to fail in always-in or always-out mode but I've pretty much failed to get a configuration that operates reliably.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 23, 2017, 11:11:57 am
content: And some photos of left & right solenoid and a close-up of the one which retains some lettering. The solenoid are identical (one is located further into the surround)I imagine sourcing a replacement would be pretty impossible.

author: Armadillo
author_rank: Super Contributor
timestamp: July 23, 2017, 12:24:56 pm
content: Rewinding solenoid is the easiest of transformer windings.I would rewind the solenoids and give them a new 10 years.Since you have the Der Ee, measure the inductance of the good and the bad solenoids and do a comparison. It would be interesting to see the difference.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 23, 2017, 12:44:52 pm
content: I should have said... I measured both solenoids at 50(Forgot to measure inductance and physical dimensions before I put it back together)

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: July 23, 2017, 12:48:38 pm
content: Do you have access to a collection of mechanical parts? Try exchanging the spring instead of extending it.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 23, 2017, 01:37:39 pm
content: Quote from: Neomys Sapiens on July 23, 2017, 12:48:38 pmI don't think I have anything suitable. The spring sort of attaches to the plunger in a strange way.It's reassembled now and working on one side should be good enough to keep things going in and out correctly during myvacation.I have ordered a suitable 4700uF and will replace the existing cap at some point on my return. (I see D of >0.5 which feels a bit high; despite having used 1kHz range).(I also note that the current model of this is designed a bit differently so unlikely that current spares are inter-changeable... I think ours is very early production.. might have to reverse engineer it one day).Thanks everybody.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: July 24, 2017, 12:34:31 am
content: Well, good luck with the repair - especially as there are felines involved!

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 24, 2017, 11:30:13 am
content: It's a sad story... the saga continues... After 24hrs of encouraging operation the behaviour has recurred... the green (left hand) solenoid has stopped operating.... Now I have swapped the solenoids so I therefore conclude that was just a red herring. I swapped out the 2200uF for a replacement (which is less but the biggest I have to hand in a similar form factor... 1500uF) and the behaviour is the same... I'm pretty sure the C is good enough so I put it back. As far as I can see the logic part is working fine it is just the driver... I have also powered via a bench supply and then an alternative 12V wall-wart but same behaviour... logic says... L293NE part knackered.... I don't have a spare of that.(I also don't see any diodes.... wouldn't I expect snubber diodes?)I might have to order a L293NE but looks like due to time concerns I might have to buy a replacement now to tide me over.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 26, 2017, 07:53:44 am
content: Well I found that I had a motor shield in the parts box that had a L293D so I swapped that in (despite its max current rating being smaller than the L293NE) but same behaviour. I'm struggling to get my head around why this doesn't work... when I watched the inputs to the L293D they look good and there isn't much beyond the L293D than the solenoids. Maybe it is still a mechanical issue or the driving has intermittent issues.It's a slightly odd arrangement. Rather than using the four half bridges in pairs to each solenoid it is set up as three bridges to the solenoids with a common to one half-bridge. The final half-bridge is used to drive the coil for the RFID.400ms pulse to drivers. Looks healthy

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 26, 2017, 07:59:19 am
content: Scan of PCB. It's one layer. On the other side are only the L293NE as DIP package and a couple of capacitors and a single jump wire. (there is an earlier picture).Is that ST chip a quad op-amp?

author: Armadillo
author_rank: Super Contributor
timestamp: July 26, 2017, 08:22:26 am
content: Go to youtube see rewind solenoid. You have not checked the inductance still.Ohm measurement is not same as inductance.Few turns shorted likely, weaken the pull-in torque. It happens to solenoid.Edit; Alternatively apply voltage directly to the solenoid terminal and compare the pull-in torque of both the solenoids.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 26, 2017, 10:35:28 am
content: Quote from: Armadillo on July 26, 2017, 08:22:26 amOK. I will do that the next time I have a go.It's a b****r to get the door assembly off and on because it is located in a tunnel under a kitchen cabinet.I had assumed that a short would reduce the resistance measureable.In any case, I did swap solenoids and the problem remained on the same side.Running out of time before my trip to the US of A.

author: Armadillo
author_rank: Super Contributor
timestamp: July 26, 2017, 11:05:09 am
content: Is that a lighting effect?The solder look pretty dull if you ask me.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 26, 2017, 01:07:57 pm
content: The solenoids are across P2&P4 and P1&P3. I did re-solder them when I did the swap. I think the joints are OK. Power is -ve to P6 and the +ve arrives above R1. To the left of the +ve is a wire link that goes down to near Pin 16 on the L293NE, this seems to carry the +5V across the board (which is single sided). So Pins 1,9 & 16 are always at 5V... i.e. bridges always enabled.The solenoids are therefore common P1&P2 connect to 1Y (pin3), P3 to 3Y (Pin6) and P4 to 4Y (pin14).It is possible I am misleading myself since I cannot test the unit with the RFID coils (JP1) connected since these are currently glued into the wall where the door fits. It is theorectically possible that the behaviour is different when driving those coils. I bought a 4700uF electrolytic today and will try that (as long as I solder it in the correct way around!)It's quite a fun circuit. The two buttons control operation (always open, always closed, allow matching RFIDs in/out etc) with current mode shown on the green (DS2) and red (DS1) LED. The coils (I imagine two seperate coils) attach at JP1 with a bit of analogue front end at U3 et al. PIC16F88 for the logic supplied with 5V via U4.If only I could get it to work!

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: July 26, 2017, 10:03:38 pm
content: Yes, LM324 is a quad OPA (albeit a lousy one).U4 is voltage regulator.