Problem with Servos responding to my PSCU

Britannicus

I'm suffering a bit of grief and a bit baffled why ....



I have coupled up my Propeller Server Controller USB unit (PSCU) and find that this will drive my 4.5 g servo quite happily - I have received a set of 9g Servos (SG 90 Tower) which function fine in my RC equipment, but now I want to drive them from my PSCU - no joy, the damn things just twitch. I've supplied plenty of power (7.4 V lipo to a single servo) through the PSCu and still just twitching. A similar 9g servo seems to work fine.

I've attached a picture of the offending servo and the one that works 4.3 g works - I have tried another 9g of a different make, and this seems OK Looking at the PSCU instructions, I can't work out why one make of servo would work and another not - clearly I'm missing something obvious -could I be providing an incorrect Baud rate ??

to provide a bit of background I'm using a BS2 from a BOE board at the moment as a test base - only one servo linked to a 7.4 Lipo supply at the moment as outlined in the datasheet - PSCU is revision B. Servo requires 4.8 V for both power and signal

Help !

Publison

Try putting a 1K resistor in line with the signal (white) wire.

It is based on this discussion:

http://forums.parallax.com/discussion/118121/digital-servo-s-on-usb-servocontroller/p1

Your offending servo is analog, but it's worth a shot .

Britannicus

Thanks I will give this a try - otherwise I guess my controller is going to have to be junked :-(

Britannicus

When you say in line with the signal wire - I'm assuming that's the servo signal wire out of the PSCU and not the wire in from the BOE

Publison

Perhaps overvoltage to the servo?

only one servo linked to a 7.4 Lipo supply at the moment as outlined in the datasheet - PSCU is revision B. Servo requires 4.8 V for both power and signal

You say servo requires 4.8V, but you are delivering 7.2V. If I remember, that Tower servo is sold by several people, (knockoffs), and some specs say 4.8V is max. Try lowering the voltage to the servo.

Britannicus

OK - will try that - I thought that the output of the servo controller was a regulated 5 V - but reading I see that this might be the solution - hope so as I have just bought 16 for my hexapod !

Britannicus

just thinking on this - presumably I have to reduce the voltage between the controller and the servo - hence the need for the resistor yes ? ? - this would be in line with the red wire I assume

Publison

The power for the board, (5volt), comes through the serial in headers, (center terminal) or USB temporarily. This does not supply voltage to the servos, which come through the green connector.

The reason for the 1K resistor on the WHITE servo line, (if you read the thread I posted), is because of the voltage translators on the controller board needing them to run certain servos.

I have attached the schematic if that helps.

Britannicus

OK Publison - thanks for bearing with me on this one - you're really helping - so the board is supplied from the BOE - so I can just crank down the supply to the green connector and all should be OK ? -


I'm a dummy in this respect so finding my way by trial and error - mostly error !

Publison

I would try to reduce the voltage to the green terminal first, and if that does not work, try the 1K resistor in line with the WHITE lead on the servo. I have used up to 10K resistors in order to have digital servos work.

Britannicus

OK Publison - you're a real help, my reason for going with a 7.2 V lipo had been to increase the voltage from my initial 5V fortunately just built myself a variable power supply so will try that at low V and crank up . Will try the 1K ohm as well and see what happens -

Publison

After you have made the changes, try another Tower servo, as you may have burnt out the first one.

Britannicus

cool - working my way through plenty just now :-)