Possible actuator problem

Waggly tails
Posts: 280
Joined: 16 Feb 2018, 06:35

Re: Possible actuator problem

Post by Mike_K » 18 Nov 2019, 19:51

Where do I start?

Tobe is correct to say that most Tobe Rand actuators don't suffer from the problem. So if you aren't suffering from the "glitch/freeze" problem don't try to fix it! And how brief the "glitch/freeze" is, you'll probably not notice it when flying. I noticed it on the bench, but still went flying and didn't notice it in the air.

One of my two Tobe Rand actuators stopped misbehaving once I'd freed the linkages/hinges up, the problem definitely shows up more when the actuator is working against a load. The other one still occasionally "glitched/froze" every 5 to 10 minutes, but a lot less than before. But for the tests, I reverted to the stiffer linkages to see if the diodes/resistor were helping.

It was Phil's idea to try a TVS diode and I received mine today and they seem to fix the problem. I bought mine from Farnell, but RS has them too:

https://uk.farnell.com/stmicroelectroni ... EN-SKU-MDC
https://uk.rs-online.com/web/p/products ... lsrc=aw.ds

As well as the TVS diode there's a 200R 0.6W resistor across the motor terminals.

Before I got the TVS diodes, I used two zener 5V6 1.3W, back-to-back with the 120R across the pair as it was all I had. It/they work well too. You could use higher-rated zeners, but these ones survived over an hour of running without failing.

https://www.rapidonline.com/vishay-bzx8 ... de-47-3058

I also tried separate batteries for the actuator motor and Rx (linking the GND but splitting the Vcc and Vin) and it worked OK without any diodes/resistors. But again I'd still fit the diodes/resistor as well.

If you are programming your own ATtiny85 to drive the Pololu DRV8838, I've modified Phils program to separate the Enable and Phase pins, and turn the enable off to let the motor "coast" fractionally before changing direction. I shamelessly copied this from what Martin (ceptimus on this forum) has done with his pulse recoder/driver. It solves the "glitch/freeze" problem and the motor runs cooler too. And you can still fit the TVS or zener diodes for "belt and braces". Rudder and Elevator are still on P0 and P2 of the ATtiny85, the enable is P1 and the Phase on P3. But you can change the order to suit.

And finally, I have tested all the above on the bench for hours, but I haven't had a chance to test fly any of them yet. And the above worked for my actuators, but as we know, they are all slightly different, most work well without any modification, but we might find that the above doesn't solve the problem for all.

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Posts: 280
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Re: Possible actuator problem

Post by Mike_K » 19 Nov 2019, 07:00

Phil_G wrote:
18 Nov 2019, 17:30

Incidentally, has anyone tried a simple decoupling capacitor across the receiver power rails (like the famous Spektrum "capacitor-on-a-plug"), say at least a few hundred μF ? (I think the Spekky ones are 4700μF)

Hi Phil

I tried plugging in an old Spektrum capacitor, but it makes no difference. But the component that is causing the problem is the DRV8838, not the PIC/Arduino or the Rx. I tried connecting a 220uF Low ESR Electrolytic capacitors across the GND and Vcc/Vin on the DRV8838 and it worked well, but unless you can get one that is physically smaller, then it's not so practical.

In fact I'm still not certain what is causing the DRV8838 problems. Is it resetting, or is it one of its many self "protection" functions kicking in. If you read the TI data sheet, there is over current, over temperature, low voltage etc, etc. My storage scope is not quite quick enough to look at the transients, I really should buy my self a new one.


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