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Servo and PCA9785

 
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Jody



Joined: 08 Sep 2006
Posts: 182

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Servo and PCA9785
PostPosted: Thu Dec 21, 2023 4:59 am     Reply with quote

Hello,

I'm trying to run a servo motor without stuttering.
I see on the scope that the pulse going to the servo is exactly what I would like (500us ~2500us, 50Hz, 5V). This pulse is coming out off the PCA9685.
But my servo stutters a lot. I have already used another one but to no avail.
Does anyone have an idea where I should look for this?
Power supply is a 5V, 3A. Servo is a DS-S006L. I am only moving from 0 - 60 degrees.

Best regards,
Jody
temtronic



Joined: 01 Jul 2010
Posts: 9136
Location: Greensville,Ontario

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PostPosted: Thu Dec 21, 2023 6:34 am     Reply with quote

hmm, that's actually an RC style servo so....
is ...
fully CW would be 500us ON, 2500us OFF.
fully CCW 2500 us ON 500us off
...
I don't know what the 'frame' of a new RCservo is and it's not in the first 'hit' for that servo you posted, though it said PWM. Old servos had 'slots' for 8 servos,2ms each in a 20ms frame.PPM

You should post your code, at least the actual data going to the servo.

stuttering is usually the wrong frame rate(either too fast or too slow). if using PWM the actual frequency could be wrong ?

heck I still have 1/2 tube of the original Signetics 544 rcvrs and the xmtr here...dang gotta be 1/2 century old now....
Jody



Joined: 08 Sep 2006
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PostPosted: Thu Dec 21, 2023 8:14 am     Reply with quote

oke an update:
Removed the servo from the setup..
And stand alone it really runs very smooth....

mine conclusion is that it is not the controller but that the servo is just to weak for this what we ask off it.....

I am really disappointed in the force of this servo.....

will look for a stronger one...
Ttelmah



Joined: 11 Mar 2010
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PostPosted: Thu Dec 21, 2023 9:03 am     Reply with quote

How is the grounding routed?.
How is the decoupling done?.

The most likely thing is that you are getting RF spikes on the supply as
the motor moves, which are interfering with the signalling somewhere.
Remember the load spikes will increase with the load on the motor....

The quality of servos do vary enormously. This is why 'branded' types like
Futaba are preferred for anything at all serious.
temtronic



Joined: 01 Jul 2010
Posts: 9136
Location: Greensville,Ontario

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PostPosted: Thu Dec 21, 2023 10:16 am     Reply with quote

..and SLO-SYN for REAL serious servos !

hmm..'heavy mechanical loading' so sounds like wrong RCservo for your application. You should be able to calculate the required force the servo needs to accomplish your task. Last ones I did were in late '70s. After you do that, get the one that's X2 more powerful !
Ttelmah



Joined: 11 Mar 2010
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PostPosted: Thu Dec 21, 2023 10:28 am     Reply with quote

Dead right. Remember also that the power needed rises with the torque
a motor can deliver. Standard servos, usually draw 1A at stall. Super
high torque units in some cases can draw over 10A.
For example the D845WP has a rated stall current of 10.6A.
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