Anselmo
Posts: 3
Joined: Thu May 02, 2013 2:56 am

Servo drag & motor

Mon May 06, 2013 6:41 am

Hello,

I'm building a raspberry controlled turret and I'm having a problem:

The dart is shot using two motors and a small servo for the trigger. The problem I'm having right now is: when the motors are running and I move the servo, both motors slow down. (Motors and servo are connected to the same power supply).

But that doesn't make sense because I'm using a 2A power supply (Well, at least that's what the wall adaptor says) and the drag of motors + servo is way less than that.

I'm pretty much new in electronics, but I read that a capacitor could stabilize the current drag. Is that true?.

Thank you

richardp
Posts: 117
Joined: Thu Jan 12, 2012 11:46 am

Re: Servo drag & motor

Mon May 06, 2013 7:38 am

Depending on the size of the motors, a capacitor should help, but depending on what energy you need and for how long will determine the size; Every motor has a large startup current to overcome the stiction. Although your power supply is 2A and the motor is perhaps 300mA, it will consume multiples of 300mA in its initial loading. The final current usage depends on the torque required.

Put an ammeter(multimeter) in series to see what the average current usage is Directly from the PSU would be ideal, but on the motors would be fine. Alternatively use a voltmeter on the PSU before the device starts running, then start the motors; if the voltage drops, then the PSU is not beefy enough.

Richard
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Anselmo
Posts: 3
Joined: Thu May 02, 2013 2:56 am

Re: Servo drag & motor

Mon May 06, 2013 7:34 pm

The motors I'm using are those toy motors (I think the stall current is 500mA max). And the servo is a HS-81 (I don't know the stall current of that one, and I think the servo is applying the max torque).

The reason I mention the stall current is because the moment the dart goes through the motor, I think they apply the max torque.

Do you think that I should use a capacitor on the motors or on the servo? (Or both?).

EDIT: Also, how do I calculate the uF I need for those capacitors?

Thank you!
M

richardp
Posts: 117
Joined: Thu Jan 12, 2012 11:46 am

Re: Servo drag & motor

Mon May 06, 2013 7:42 pm

No idea on a calculation... but the bigger the capacitor the more energy is available. 4700uF and then just see what happens.

I still think its the PSU that cant handle the inrush current loads.
Richard
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richardp
Posts: 117
Joined: Thu Jan 12, 2012 11:46 am

Re: Servo drag & motor

Tue May 07, 2013 6:39 am

Addition - The HS-81 Servos are reported to hit a peak of about 700mA (according to a website that tests these things)
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mikerr
Posts: 2774
Joined: Thu Jan 12, 2012 12:46 pm
Location: UK
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Re: Servo drag & motor

Tue May 07, 2013 9:12 am

Try running it from 4-6 rechargeable AAs at least as a test - they're less likely to drop voltage under load,
and are good for several amps.

You're pulling 1.7A which is close to the max of your 2A supply, so its heavily loaded,
a higher power PSU would be better.
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