Hi,
I’m currently upgrading the servos for my prosthetic thumb design and I’ve found I nice big motor upgrade - but its running slower!
The specs say that it should be running almost double the speed on the same voltage:
But it seems to be slightly slower… Any thoughts?
Thanks!
Dani
From your video looks like takes 0.4 second to the small servo to turn and 0.6 to the big one.
There are different things you can have a loot at:
Is it a power issue ? (not enough current)
Is it a control issue? (the new servo need wider steps)
Sounds like it could be a at least a power issue if you are powering the circuit from your Arduino .
From specs:
https://www.servocity.com/hs-645mg-servo
|Current Drain - no-load (4.8V)|350mA|
|Current Drain - no-load (6V)|450mA|
From: https://www.servocity.com/hs-225mg
|Current Drain - no-load (4.8V)|300mA|
|Current Drain - no-load (6V)|340mA|
The total at 5V, with both servos running, would be give or take 650mA - not sure the Arduino could supply that.
Thanks @dsikar & @emuboy it definitely seems to be a current issue. I’ve been playing around with some bigger power banks, and wiring the battery to bypass the micro usb and that seems to be helping. I’ve just ordered a USB-c cable with 3.0 to try with a new battery pack into the VIN on my NodeMCU board and hopefully that will help? Not sure how else to give it the power it needs.
Dani
As long as you power your servos from your battery pack, not from NodeMCU 3V3 pins, you should be ok.
The boring maths (NodeMCU schematic and NodeMCU voltage regulator datasheet below):
Power you get from NodeMCU 3V3 pins = 500mA x 3.3V = 1.65W
Power required by servos = 650mA x 4.8V = 3.12W
Power supplied by typical USB power pack 2.1A x 5V = 10.5W
Good luck!
USB power packs shouldn’t do this (without negotiation they should current limit to 500mA) so this probably wont work. Bigger power banks also probably wont work for this reason.
For a quick workaround/test, connect 2 USB power packs in parallel. Or just connect it to 4 AA batteries. Or any of the power supplies in the space set to 5V.
That’s a type C , it uses USB PD and I do belive delivers 18W only to USB 3.1 devies.
In terms of powering two servos and a NodeMCU, the standard is backwards compatible so the example shown should deliver 5V 2A ~ 10W. This is the power pack I use, rated 5V/2.1A, attached devices drawing 1.5A ok:
It may be worth checking servos with brushless or coreless motors. There seem to be loads these days. Definitely with metal gears 
and also check some DC/DC converters that can deliver the required power. There are tons on ebay/amazon, also hobbyking may be a good source.
I don’t think you understand the standard. Usb shouldn’t provide that current without negotiation. If you have a powerbank which doesn’t comply with standard then yes it might work but wouldn’t be compliant.
That’s right, provided that all the chain in your power delivery solution are correct, you should be able to have 2A out of an usb connector. Which is extremely rare, in all my tests many many USB cables fails to deliver 2A, 1.5? yes, 2A? nope, at least for Micro USB.
The connector it’s important too, micro usb leads are very small, you can’t really fit 10W through it, at least, not for cheap.
So, it’s mostly marketing material, USB it’s not the right choice for power delivery (at least USB 2.0)
