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'RC Servos [OT]'
1997\10\25@123026 by Alec Myers

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I'm having a bit of difficulty with some cheap RC servo units (being driven
from a PIC).

Every now and then, the servo unit goes into a funny oscillating mode
(frequency the same as the driving pulses). It draws lots of current
(>500mA), and the stiffness of the unit drops off to about zero. If I
change the length of the pulses, the servo moves off to the right place,
and (usually) stops oscillating. I've tried adjusting the amount of
friction in the system, but that doesn't help. I don't think it's a
'hunting' problem, because you can move the servo a long way off it's
correct position without any effect.

The driving pulses are quite stable in length - it's not a problem at the
PIC end, I don't think.

PIC and servo run from different power supplies, Control pulses are between
~1mSec and ~2mSec applied at about 50Hz.

If anyone's got a clue what's happening, I'd be grateful for their suggestions.


Alec

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1997\10\25@145931 by Eric Naus

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Hi Alec,

Are you sure that all the servos have been accessed within a 20 ms window.
I don't know how many servos you have connected or what type of program
is running in your PIC.

    -----                   -----
   |     |                 |     |
----|     |-----------------|     |------

---> 1-2ms <----

    <------ <20ms  ------->

This might help

Eric Naus

At 05:09 PM 10/25/97 +0100, you wrote:
{Quote hidden}

suggestions.
{Quote hidden}

1997\10\25@170420 by Andy Kunz

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>PIC and servo run from different power supplies, Control pulses are between
>~1mSec and ~2mSec applied at about 50Hz.

This isn't unusual.  Most modern servos are intended for 35% or so of
supply voltage as the input signal level.  The PIC is driving it
rail-to-rail, and this messes up the servo amp chip.

I simply add a  270 ohm resistor in series in the signal lead.  Works well.

You should also check your pulse output frequency to make sure it really
has some time between the pulses.  50 Hz is fine, 70Hz is also.  Actually,
they should need no more than about 6mS between the falling edge and the
next leading edge, so you can really hammer the snot out of them and they
still work.

Andy

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1997\10\25@175229 by Lee Hewitt

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Hello Alec,

>PIC and servo run from different power supplies,

Have you got a GOOD low impedance common GROUND connection between both
supplies ?.

Is the voltage interface between the PIC, ?Driver? and servo compatible
(level shifting devices etc.)?


Lee Hewitt (Manchester ENGLAND)



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Manchester ENGLAND

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1997\10\27@131948 by Mike Keitz

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On Sat, 25 Oct 1997 17:09:13 +0100 Alec Myers <EraseMEAlecspam_OUTspamTakeThisOuTW5.CO.UK> writes:
>I'm having a bit of difficulty with some cheap RC servo units (being
>driven
>from a PIC).
>
>Every now and then, the servo unit goes into a funny oscillating mode
>(frequency the same as the driving pulses). It draws lots of current
>(>500mA),

Drawing 500mA is normal.  It takes power to turn the motor.  When the
servo reaches the set position (without having to hold against external
torque), the motor will cut off and the power consumption drops to just a
few mA.  The motor is driven with PWM at the drive-pulse frequency, so
even at less than maximum effort, the servo power rail will have to
supply relatively large pulses of current.  Budget about 1A peak for each
servo.  If the servo power supply voltage is dropping, the servos may
malfunction like you describe.

1997\10\27@165142 by Alec Myers

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>On Sat, 25 Oct 1997 17:09:13 +0100 Alec Myers <Alecspamspam_OUTW5.CO.UK> writes:
>>I'm having a bit of difficulty with some cheap RC servo units (being
>>driven
>>from a PIC).
>>
>>Every now and then, the servo unit goes into a funny oscillating mode
>>(frequency the same as the driving pulses). It draws lots of current
>>(>500mA),
>
>Drawing 500mA is normal.  It takes power to turn the motor.  When the
>servo reaches the set position (without having to hold against external
>torque), the motor will cut off and the power consumption drops to just a
>few mA.  The motor is driven with PWM at the drive-pulse frequency, so
>even at less than maximum effort, the servo power rail will have to
>supply relatively large pulses of current.  Budget about 1A peak for each
>servo.  If the servo power supply voltage is dropping, the servos may
>malfunction like you describe.

Mike, I'm afraid it's not that simple. Most of the time, the unit draws
about 500mA while moving, then reaches the set position and stops. Then the
current drops to about 10mA. Only sometimes, it gets to about the right
place, then oscillates, and the current doesn't fall off.

It _feels_ like the pulses are random lengths - the motor is turning full
torque one way then the other - but on the 'scope, the pulse lengths  are
steady as a rock. If I push against the servo, or stall it with excess
friction, then I'd expect heavy current draw. But not at the low load it's
under.

Actually, running the servos from a current limited source (say max 200mA)
helps, because the oscillations aren't as severe. But it doesn't seem like
a good solution to me. I have tried different signal voltages (including
Andy's suggestion of a 270 ohm resistor from the PIC output) but it doesn't
make much diference. I will report more when I get a bit more time to play
with the system.


Alec

1997\10\27@175810 by Walter Banks

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You might want to check the frame rate to the servo's
some are 20ms some are faster. I at least one of the
systems I had had a frame rate of 16ms. We used
model A/C servo's on a keyboard tester several; years ago
and reduced the hunting by changing the frame rate.

Walter Banks
----------
> From: Alec Myers <@spam@AlecKILLspamspamW5.CO.UK>

> Mike, I'm afraid it's not that simple. Most of the time, the unit draws
> about 500mA while moving, then reaches the set position and stops. Then
the
> current drops to about 10mA. Only sometimes, it gets to about the right
> place, then oscillates, and the current doesn't fall off.
>
> It _feels_ like the pulses are random lengths - the motor is turning full
> torque one way then the other - but on the 'scope, the pulse lengths  are
> steady as a rock. If I push against the servo, or stall it with excess
> friction, then I'd expect heavy current draw. But not at the low load
it's
> under.

1997\10\27@183453 by Clyde Smith-Stubbs

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On Mon, Oct 27, 1997 at 10:23:01PM +0100, Alec Myers wrote:

> Mike, I'm afraid it's not that simple. Most of the time, the unit draws
> about 500mA while moving, then reaches the set position and stops. Then the
> current drops to about 10mA. Only sometimes, it gets to about the right
> place, then oscillates, and the current doesn't fall off.

Try bypassing the signal leads with a small cap - there may be RF noise
that you're not seeing on the CRO. The motor in the servo generates noise,
apart from anything else. I'd go with the largest cap that you can
put on without distorting the pulses too much (and the 270 ohm resistor).

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1997\10\27@193513 by Anil K. Patel

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On Monday, October 27, 1997 1:23 PM, Alec Myers [SMTP:spamBeGoneAlecspamBeGonespamW5.CO.UK] wrote:
[ ...snip...]
{Quote hidden}

You probably have already tried this, but have you tried other servos?
Including possibly servos from another brand (you did mention you were using
cheap servos).

I have had servos fail on me after use in a R/C helicopter that have exhibited
strange positioning behavior like you described above.

--Anil

1997\10\28@133133 by Martin Nilsson

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Alec,

> >>I'm having a bit of difficulty with some cheap RC servo units (being

<snip>

> >>>Every now and then, the servo unit goes into a funny oscillating mode

<snip>

{Quote hidden}

This is a very common problem. It stems from RC-servos using only
simple P-regulation.  This is prone to oscillation. For this type of
regulation, the motor is given an (average) feedback voltage that is
proportional to the offset from the desired (set) position.
However,the motor might overshoot this position, and is then given a
reverse voltage. It might overshoot again, and so on. The effect is
usually especially pronounced if you have large inertial loading on
the shaft.

Unfortunately, there is not that much that you can do about it. The
origin of this effect is theoretical (control theory), and it occurs
even if you use capacitors, change voltage, etc. A better control
algorithm - such as PID - can help, but isn't perfect, because the
optimal PID coefficients depend on the load, damping, time lag, etc.
Some more expensive servos have (larger) "dead zones" around the set
position, where the feedback is zero. In this way, there are no
oscillations for small overshoots, but instead, some precision is
lost.

Servo manufacturers have a difficult problem, because they must guess
a suitable gain for the P-regulation. Invariably, there will be cases
where this doesn't work well (for very light loads and very
heavy loads). Also, the Mabuchi motors used in most cheap servos are
highly non-linear, and this will also affect the regulation.

The only thing I can think of right now is that you add ballast
inertia and introduce external damping by adding friction to the shaft
in suitable amounts, i.e. change the "plant" to fit the controller,
instead of the other way around.  Otherwise, buying a more expensive
servo is the option. Or write your own 16C84 adaptive RC servo
PID-controller with built-in system identification, incidentally which
would be quite a commercializable product...

Cheers,

-- Martin

Martin Nilsson                           http://www.sics.se/~mn/
Swedish Institute of Computer Science    E-mail: TakeThisOuTmnEraseMEspamspam_OUTsics.se
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1997\10\28@135847 by Jon Bertrand

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    I've got a servo with a dead spot in it's pot near the neutral
    position.  The dead spot isn't very wide but at that spot the pot
    wiper opens up.  I'm guessing vibration caused the spot to ware
    through - the servo is in my RC car with zillions of hours of
    operation.

    It does exactly what you describe - provided it tries to set to that
    point 'exactly.'

    Jon Bertrand
    RemoveMEjonbspamTakeThisOuTcirris.com

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