Servo does not move

[ruijc]

Hi Alain,

thanks for your help.

i'm currently using a futaba's S148 servo.

My idea is to test any futaba and hitec servos ( digital and analog ).


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[Acetronics2]

Hi, Ruijc

héhé ...

Futaba S148 !!! the "mechanical neutral" ( half of the two geometric max positions ) ... corresponds to ~ 1300 µS ( an heritage of the old Robbe/Futaba join venture ! )

Now ... YOU must place your servo arm to have your "application neutral" with a trustable 1500µS pulse !!! ( remember Xtals are NOT generating exactly their nominal frequency ... especially for "common" ones )

Moreover consider Futaba "analogic" servos have a quasi-linear positionning in the 900-2100 µS range .

Outside those limits, you can send from 200 to 2500 µS pulses ... but with no more linearity at all !!! Specially at low values ... ( I did those tests to try to get 180° travel without any mod to the servos ...)

to fill the "bottle" ... your servos have a deadband of ~ 4/5 µS ... around their position, so, don't ask them to show you 1µS steps !!!

I did not test any Hitec servos ... so don't ask me for them !!!

Alain

[ruijc]

Got it Alain,

just changed the xtal from 4Mhz to 8Mhz and is was easy to tune !

thanks

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[ruijc]

One last question:

i found my 7805 getting very hot when my circuit is looping through auto motion.

The code is safe in terms of not letting the servo go to extremes and overloads the circuit.

It's just moving arround ( left and right ) on it's own.

Is there a way to control it's temperature ?

I'm trying to avoid using a heatsink because it will be in a small box.

any ideas ?

[HenrikOlsson]

The 7805 is getting hot because your supplying current to the servo thru it. Even though the motor is "just turning" it consumes much, much more current than the PIC. When you start "loading" the servo it will draw even more current and will probably overheat the regulator. Basicly you have four options.

1) Mount a heatsink.
2) Reduce the powersupply voltage. That will reduce power dissipation in the regulator.
3) Use a switching type regulator instead of a linear (78xx type)
4) Only run the PIC on the regulated voltage. And the servo on the raw, unregulated voltage.

Number 4 may not be an option depending on your actual supply voltage and max voltage that the servo can take.

/Henrik Olsson.

[Acetronics2]

Hi,Ruijc

Why not run it with 4 or 5 old NiCd cells ??? if 12v batt used, only a switching regulator will do it ( LM 2575 or 2576 i.e )

Alain

[ruijc]

thanks Alain and HenrikOlsson,

all good tips

at my workbench i have a 12V/3A power supply but i designed to have a normal 9v battery.

#1 Since the 9v battery does not have that much voltage/current the regulator will not over heat ( or will it ? )

#2 The nicad pack is a good idea running the servo directly from it and keeping the regulator for the pic only . But since it will be used at the field and there are many different packs available, i'm affraid that a lipo pack would burn up the regulator :s


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