Pre modded servo boards for escs?

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MySolderIsOlder
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Re: Pre modded servo boards for escs?

Post by MySolderIsOlder » Mon Apr 15, 2019 7:30 am

Having come quite late to the Antweight world, I missed out on the necessity for sourcing DIY ESCs made from servo boards. I was always curious about giving it a try but wasn't keen on the faff of balancing those tiny potentiometers. Hence, when I saw Mike's post about using the boards from digital continuous rotation servos, I got straight on to AliExpress and ordered up a 10-pack of those DomanRC units (15 quid, with postage). They arrived last week so got down to some testing over the weekend. Results were.... interesting.

For starters I tested them as servos - plugged into a Redcon 4-CH Rx, powered by a 6v NiMH battery. Four of the ten were DOA, the other six buzzed along in a sprightly manner, though I did notice an alarming tendency for some of them to creep slowly around, even when the stick on my DX6i was centered. This was true whichever channel I used and trimming didn't have any effect - nudging the adjuster-bar along, the slow creep would remain the same until a certain point, when normal rotation in the other direction kicked in. Should mention that I've never had this problem with any pukka ESC.

Notwithstanding this unpromising start, I proceeded to dismantle all ten units and extracted the tiny speed controllers for direct testing, both with an N20 and with a multimeter. As expected the four dead ones remained dead - no voltage at all on the motor leads. As for the other six, with the stick in neutral, they all gave a steady 0.3v, which was enough to make a zero-loaded motor idle at around 15rpm, though it only took a small amount of pressure to stop it moving. Adjusting the trims, it read a constant 0.3v until reaching the tipping point when it would jump to -1.5v
I'm sure I could cure this lack of a proper dead-zone with a small resistor or something. Slightly more alarming is what happens at the other end though - with the stick at maximum deflection, I never got more than 4v going to the motor. Switched the 6v NiMH battery for a fully charged 2S LiPO but same result. Thinking it might be the Receiver, I repeated the tests with an Orange Rx614 but again same results. Out of curiosity, I also dismantled an old Feetech FS90R (analogue C/R servo) that I had lying around from an Arduino kit. Didn't manage to get the potentiometer on this one any nearer a true 0v dead-zone either, although I did find that at full stick, this board was giving a more respectable 5.2v.

So... not entirely successful but an interesting experiment nonetheless. I will certainly find a use for these cheap and tiny ESCs (maybe time for my first cluster) but it's possible I was just unlucky in the usual gamble over Chinese manufacturing quality control. Game's not over yet though - while looking at the Doman servos on AliExpress I noticed that Feetech are also now making a digital C/R servo (the FT90R) so I've got a few of those on the slow boat from China and will post test-results when they eventually arrive.

Possible I'm expecting too much from servo boards - but given how cute and tiny they are, it would be a shame not to find a use for them (especially if I can find a suitable small PWM receiver to replace the currently unavailable Rx614).
Stuart (Anthony's dad) Our ants;
"Flippin' Ugly" (flipper),
"Other way up" (invertible pusher),
"Ffythiana" (front flipper),
"Spindoza" (basic shover)

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MarkR
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Re: Pre modded servo boards for escs?

Post by MarkR » Mon Apr 15, 2019 7:53 am

Sounds more like a poor design or poor choices of parts, than a manufacturing defect.

A voltage drop of 2V at the kind of current we'd see through a N20 is extremely high, did you have access to an oscilloscope? Maybe the duty cycle is less than 100% even when the stick is at the extreme position?

--

Most of these parts we're working with (ESCs etc) don't have a clock crystal in, so they can't really measure 1500us accurately. So you're unlikely to get them dead centre, and it might vary with temperature, etc.

Radios, on the other hand, need an accurate crystal to work, so the digital radio should always send very close to 1500us when stick is centred.
Robot: Betsie - RaspberryPi controlled flipper bot with gyro stablisation - too clever for her own good?

Ant Ipodean
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Re: Pre modded servo boards for escs?

Post by Ant Ipodean » Tue Apr 16, 2019 10:47 am

Hi MySolderIsOlder
Not questioning your obviously thorough testing, but this is quite different to the results I have been getting.
I received another batch of 10 units recently for "stock", but haven't needed to dip into them yet.
You have me wondering now if the Chinese have been "tampering" with their original spec.
I am travelling at present, but will test them when I get home.
Mike.

MySolderIsOlder
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Re: Pre modded servo boards for escs?

Post by MySolderIsOlder » Tue Apr 16, 2019 12:37 pm

Hi Mike - please don't ever worry about questioning my testing - my understanding of electronics is rudimentary at best :D

I did wonder whether my el-cheapo Neoteck DMM was correctly reading RMS voltage, so I tried the same test with one of my few surviving Das-Micro ESCs. There however I was definitely seeing the full range of 0V (give or take ~20mV) up to 6V, so I think the test methodology is legit.

The real issue may well be changing spec. When I first tried putting in an order, it was subsequently cancelled due to 'stocking issues'. Then when I tried again a few days later, the Doman servos were back in stock and I was able to re-order, so maybe they were in the process of switching component suppliers? Perennial problem with ordering from China - you never really know who made the item, whatever the name badge may say, nor whether it will be the same when you next order one. I'm hoping I just got unlucky, as the ability to cobble together 2-channel ESCs for less than a fiver each is too tempting.

Do please let us know how you get on with your next batch.
Stuart (Anthony's dad) Our ants;
"Flippin' Ugly" (flipper),
"Other way up" (invertible pusher),
"Ffythiana" (front flipper),
"Spindoza" (basic shover)

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