Follow up on Digital CR Servo Board ESCs

All things antweight

Moderators: BeligerAnt, petec, administrator

User avatar
Kyro
Posts: 374
Joined: Fri Jan 18, 2019 12:36 am

Re: Follow up on Digital CR Servo Board ESCs

Post by Kyro »

so which component is responsible for the dead band?

something that could be easily switched to make the servo read 0v at neutral? or is it the programming inside one of the chip's?

i ask as i am waiting for an order of cheap 360° servo's for use as esc's and if i have a few that give some movement at neutral which be can't fixed with trim (4wd with 2 esc's per channel) what potential fixes are there?

i have ordered 10 of which i only really need 4 good ones however i may end up using more further down the line if i have decent enough success.
Team Rocket
Esper(4wd flipper)
Mantis (4wd grab 'n' lift)
Dominus (Vert SPINNAAH)
Breakout(modular cluster)
Remembering Rex (antweight cassius)
Fennec (4wd flea) ex world champion

User avatar
MarkR
Posts: 329
Joined: Mon Dec 18, 2017 12:46 pm
Location: Reading Hackspace
Contact:

Re: Follow up on Digital CR Servo Board ESCs

Post by MarkR »

All the dead-band logic is inside the MCU, so good luck tuning it.

These tiny little MCUs have their own free-running oscillator which goes at whatever speed it feels like. So the MCU might be nominally clocked at 1 Mhz, but in practice it's 1 Mhz +- 10% or maybe worse than that.

So the firmware might be coded to give a dead band at exactly 1500us which is 100us wide, but that could vary significantly because the clock isn't accurate.

The real problem is that if you got several of these servos (or reused their controllers) and wire them in parallel on the same controller channel, there's no guarantee that those MCUs clocks are running at precisely the same speed, so their dead band might not be in the same place, which means a 4wd robot cannot be made totally still, its front and rear motor won't have the dead band in the same spot. You can't change the tx trims independently, unless you put them in different channels (maybe that's ok with a 4+ channel receiver and sufficiently clever tx).

Controllers which have a clock crystal can run at precisely the right speed. This is an improvement.

Radios need to use an accurate crystal anyway to get the correct radio frequency, so they will always output accurate pulses; all the modern types are digital over-the-air so they don't depend on any timing of the radio signal or tx.

Also, S-Bus or i-bus wire protocols are fully digital so they don't depend on exact timing to find the dead band; every device should behave identically.
Robots: Betsie - RaspberryPi controlled flipper bot with gyro stablisation - too clever for her own good?
Stacie - tidy flipper; 4wd driven by hair bands

MySolderIsOlder
Posts: 143
Joined: Mon Jul 30, 2018 1:22 pm
Location: Kent
Contact:

Re: Follow up on Digital CR Servo Board ESCs

Post by MySolderIsOlder »

Since bench tests are no substitute for the real driving experience, yesterday I knocked up a rough and ready test bot, based on a simple 4wd pusher, to see how the Feetech servo board-based ESC would perform.

Image
Image

Suffice to say, I’m delighted with the results. Response definitely feels more ‘proportional’ than either the DasMikro or generic 2S/3s 5A Dual ESCs that I have been using until now. Even with my lousy driving skills it’s possible to do slower, more precise manoeuvres, rather than just the abrupt jumps from stationary to flat out, though the acceleration is still there if you need it. Here’s a quick demo on my (rather dusty) workbench:
https://youtu.be/ZYtiQEpDRak

Also tried running continuously (wheels off the ground) at full speed for 3 minutes- no appreciable increase in temperature of the controller.

So in conclusion- Feetech FT90R board based Poor Man’s Nano is likely to be the basis for all my builds going forward (except where I need 3S). Costs about £12 to make , weighs about 5g - and drives better than anything I’ve been able to get my hands on before.
Stuart (Anthony's dad)

MySolderIsOlder
Posts: 143
Joined: Mon Jul 30, 2018 1:22 pm
Location: Kent
Contact:

Re: Follow up on Digital CR Servo Board ESCs

Post by MySolderIsOlder »

Ps.
Obviously no onboard mixing so here are the mix settings I used on the Spektrum;

Image
Image
Stuart (Anthony's dad)

Post Reply