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Model Forum / Radio Controlled / Helicopters / June 2004Strange Servo Failure Mode, near end of travel (one direction only)
Can you please tell me if you have ever seen any servo exhibit this failure mode (described below)? What is happening within a servo to cause this? I discovered a subtle but serious problem with the extreme left response from my aileron servo. When I give a full left cyclic demand the servo goes steady with no observable problems. However, when I add a little torque resistance with my fingers, while slowly returning the cyclic stick just off the extreme, the servo loses a significant amount of torque and starts to shake the horn. This very strange behavior occurs only at left extreme and not right extreme. Have you seen this failure mode in a servo before? I swapped the original 9202 with another 9202 servo and the problem goes away (strong steady torque at both extremes). I tried the original servo again and problem returns (validated consistency by swapping 3 times each). I disassembled the original servo and verified gears are in good shape and no obvious signs of internal damage. However, after reassembling the servo, the problem is gone! Strong and steady torque through both extremes! Now I feel a little uneasy about whether the problem will return someday (and cause another crash), because I still don't know what was happening within the servo. This problem results in locked roll and loss of aileron control. However, the servo seems to be performing consistent for now, after several minutes of testing with torque resistance. Dan > Can you please tell me if you have ever seen any servo exhibit this failure > mode (described below)? [quoted text clipped - 20 lines] > (and cause another crash), because I still don't know what was happening > within the servo. If in doubt, bin the servo. A 60 quid servo isn't worth twatting the heli for. Or some poor spectator. This problem results in locked roll and loss of aileron > control. However, the servo seems to be performing consistent for now, after > several minutes of testing with torque resistance. And it MAY peform faultlessy for the next 20 years, but on the other hand...... Servos do fail occasionally even if the failure doesn't manifest itself as a "dead in the water" deal. Don't put it back in the heli.  SignatureBeav Please note my E-mail address is "beavis dot original at ntlworld dot com" (with the obvious changes) Beavisland now lives at www.beavisoriginal.co.uk > > Dan Thanks Beav, I always value your advice. If I cannot narrow the scope of possible causes to an acceptable risk, I will be left with no choice but to "bin the servo". Thanks again, Dan > > Can you please tell me if you have ever seen any servo exhibit this > failure [quoted text clipped - 41 lines] > > > Dan > Thanks Beav, > > I always value your advice. Thanks for that Dan. > If I cannot narrow the scope of possible causes to an acceptable risk, I > will be left with no choice but to "bin the servo". The problem with servos is they REALLY should be considered as consumables. They don't last forever and if you get a couple of seasons flying from a set, you're ahead of the game. After that, I don't consider them worth even looking at if they show any signs of "offness" SignatureBeav Please note my E-mail address is "beavis dot original at ntlworld dot com" (with the obvious changes) Beavisland now lives at www.beavisoriginal.co.uk > The problem with servos is they REALLY should be considered as consumables. > They don't last forever and if you get a couple of seasons flying from a > set, you're ahead of the game. After that, I don't consider them worth even > looking at if they show any signs of "offness" Just *what* on a heli isn't a consumable?? I think I've replace all the parts on mine... no wait.. I haven't smashed the receiver yet... ;-) .S. > > The problem with servos is they REALLY should be considered as consumables. > > They don't last forever and if you get a couple of seasons flying from a [quoted text clipped - 5 lines] > > .S. Uh-Oh, you just jinxed yourself on that one! :-o ;-) Fly Safe, Steve R. >>Just *what* on a heli isn't a consumable?? I think I've replace all the >>parts on mine... no wait.. I haven't smashed the receiver yet... ;-) [quoted text clipped - 5 lines] > Fly Safe, > Steve R. Damn! When servos were much more expensive in real terms in the 80's, I regularly replaced feedback potentiometers. However, Ripmax in the UK a couple of years ago started not to want to sell `electronic' spares for servos - citing Health & Safety issues. (and no doubt product liability creeps in as well) - unless of course they repaired the servo. Not sure what their current position is. Comment from Ripmax - if they read this - would be useful. But I really don't like the `consumable' culture - and I hate binnining an expensive servo for the sake of a feedback potentiometer. And the `'neutral' position of the servo is always where the potentiometer track wears most ..... = the jitters around neutral. CW >> The problem with servos is they REALLY should be considered as consumables. >> They don't last forever and if you get a couple of seasons flying from a [quoted text clipped - 5 lines] > >.S. > When servos were much more expensive in real terms in the 80's, I > regularly replaced feedback potentiometers. Same here. Changing pots was almost a weekly affair, particularly when the servos began appearing with paper tracks. > However, Ripmax in the UK a couple of years ago started not to > want to sell `electronic' spares for servos - citing Health & > Safety issues. (and no doubt product liability creeps in as well) > - unless of course they repaired the servo. Not sure what their > current position is. Comment from Ripmax - if they read this - > would be useful. I wouldn't hold me breath:) > But I really don't like the `consumable' culture - and I hate > binnining an expensive servo for the sake of a feedback > potentiometer. I don't think any of us LIKE the consumable idea, but most people wouldn't tear a servo apart and it can cost as much to get someone else to do it as it would to replace the whole servo. > And the `'neutral' position of the servo is always where the > potentiometer track wears most ..... = the jitters around neutral. That can usually be treated with a good dollop of vaseline too, not that many would try it these days. Back to the "CYA" world we live in nowadays. SignatureBeav Please note my E-mail address is "beavis dot original at ntlworld dot com" (with the obvious changes) Beavisland now lives at www.beavisoriginal.co.uk > > The problem with servos is they REALLY should be considered as consumables. > > They don't last forever and if you get a couple of seasons flying from a [quoted text clipped - 3 lines] > Just *what* on a heli isn't a consumable?? I think I've replace all the > parts on mine... no wait.. I haven't smashed the receiver yet... ;-) Just the flyer and some of them are too:-) SignatureBeav Please note my E-mail address is "beavis dot original at ntlworld dot com" (with the obvious changes) Beavisland now lives at www.beavisoriginal.co.uk Now that I'm paying more attention to these consumables, I noticed my elevator servo starting to shake today just around center position. I won't be flying the rebuild until both cyclic servos are renewed. However, I will take the old ones to work and look at them thoroughly with a microscope to see if I can learn anything or if I can find a cracked solder joint that might allow me to restore servo performance. If not, they're trash. > > Thanks Beav, > > [quoted text clipped - 9 lines] > set, you're ahead of the game. After that, I don't consider them worth even > looking at if they show any signs of "offness" > Now that I'm paying more attention to these consumables, I noticed my > elevator servo starting to shake today just around center position. I won't > be flying the rebuild until both cyclic servos are renewed. However, I will > take the old ones to work and look at them thoroughly with a microscope to > see if I can learn anything or if I can find a cracked solder joint that > might allow me to restore servo performance. If not, they're trash. You'll probably find the feedback pot track that the wipers run on to give/take the signal is in need of cleaning and re-greasing (vaselining). If that's all it takes to sort the servo, it's not a binnable item and this sort of thing is WELL within the abilities of most modellers (or it WAS). SignatureBeav Please note my E-mail address is "beavis dot original at ntlworld dot com" (with the obvious changes) Beavisland now lives at www.beavisoriginal.co.uk I thought that was oil from exhaust fumes. That was vaseline? I placed several of my servos under a microscope today and learned a little. I was about to post the results, but I'll edit the part about the oil and then post it as a separate thread. Thanks again. Dan > > Now that I'm paying more attention to these consumables, I noticed my > > elevator servo starting to shake today just around center position. I [quoted text clipped - 9 lines] > that's all it takes to sort the servo, it's not a binnable item and this > sort of thing is WELL within the abilities of most modellers (or it WAS). > I thought that was oil from exhaust fumes. That was vaseline? It's VERY unlikely to be oil from the flying. More likely to be whatever the manufacturer put in their when the servos were made. The tracks need SOMETHING to lubricate them and vaseline has worked for years. > I placed several of my servos under a microscope today and learned a little. > I was about to post the results, but I'll edit the part about the oil and > then post it as a separate thread. I got it about a minute ago:-) SignatureBeav Please note my E-mail address is "beavis dot original at ntlworld dot com" (with the obvious changes) Beavisland now lives at www.beavisoriginal.co.uk Maybe one of the servo motor drive transistors/FETS is iffy? I think they are normally in a bridge to the servo motor - maybe you have intermittently lost half of the bridge? This has happened to me - no appreciable torque in one direction of servo travel. C W >Can you please tell me if you have ever seen any servo exhibit this failure >mode (described below)? [quoted text clipped - 24 lines] > >Dan C W, Thanks for the reply. I believe I am observing slightly different behavior than you describe. Rather than less torque through an entire direction of movement, I am observing strong torque in both directions. However, at one extreme (and only if some torque resistance is applied), the servo starts to lose it only during the first small portion of the path back to center. After crossing this shakey small area, the servo regains its strong control through the remainder of its movement back to center. Thanks again, Dan > Maybe one of the servo motor drive transistors/FETS is iffy? I > think they are normally in a bridge to the servo motor - maybe you [quoted text clipped - 31 lines] > > > >Dan `shaky' areas and jitter are often associated with feedback potentiometer track problems in a certain part of the travel - but that does not explain why the problem is only there when torque is applied. I used to build - and wreck - my own servos from kits many years ago - but these digitals are a whole new game for me ! I have been playing with a JR8700G super tail servo - and that causes all sorts of jitter on other servos when used with Futaba PPM - it needs an opto isolator and separte battery to work with PPM. (PCM filters the interference out from the 8700G - or masks it !! - which is why I am not keen on PCM having had one lock out crash) So try the servo with a different Rx all on its own - and with PPM and PCM. CW >C W, > [quoted text clipped - 53 lines] >> > >> >Dan sounds like it could be a dodgy feedback pot to me. Rgds Alan > C W, > [quoted text clipped - 53 lines] > > > > > >Dan |
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