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Date: Wed, 21 Dec 2011 14:43:03 -0500
Message-ID: <CALSZ9gqMVb1P06M0xoPU3R3sQLdM4aMwQPB9d+S-LHEyFvB1fw@mail.gmail.com>
Subject: Re: [geda-user] Design suggestions...
From: Rob Butts <r DOT butts2 AT gmail DOT com>
To: geda-user AT delorie DOT com
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When you say potentiometers, are they carrying the full DC motor current?
or is there some kind of motor control there already?

I actually just tied each potentiometer to an adc input pin and was going
to adjust the pwm to what the adc read so the pot is not associated with
the power to the motors.

On Wed, Dec 21, 2011 at 2:32 PM, John Griessen <john AT ecosensory DOT com> wrote:

> On 12/21/2011 01:09 PM, DJ Delorie wrote:
>
>> I'd assumed rotor inertia would be sufficient, but I suppose "that
>> depends" as usual.
>>
>
> I'm sure the inertia is plenty to keep it going.  The smoothing can be
> good if the motor
> inductive response of the bare motor makes big volt spikes between
> times it is driven by transistors.  A series wound brush DC motor with
> some added
> capacitance at the motor, downstream of the FET switches will be less
> noisy,
> less likely to have a big volt spike if run hard then cut off.
>
>
> On 12/21/2011 01:16 PM, John Doty wrote:
> > The problem with added capacitance is that it increases the dissipation
> of the driver, since it then has to charge/discharge the capacitors when it
> switches. With enough capacitance to smooth the waveform, this will be a
> big effect.
>
> I'm suggesting capacitance enough to make the PWM square wave disappear,
> not round its edges.
> and it's at the motor.  When the FETs cut on they just have to top up the
> voltage the cap stored since
> last time.
>
> This probably needs a sketch to help more.
>
> John
>

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<div>When you say potentiometers, are they carrying the full DC motor curre=
nt?<br>or is there some kind of motor control there already?</div><div>=A0<=
/div><div>I actually just tied each potentiometer to an adc input pin and w=
as going to adjust the pwm to what the adc read so the pot is not associate=
d with the power to the motors.=A0<br>
<br></div><div class=3D"gmail_quote">On Wed, Dec 21, 2011 at 2:32 PM, John =
Griessen <span dir=3D"ltr">&lt;<a href=3D"mailto:john AT ecosensory DOT com">john@=
ecosensory.com</a>&gt;</span> wrote:<br><blockquote style=3D"margin:0px 0px=
 0px 0.8ex;padding-left:1ex;border-left-color:rgb(204,204,204);border-left-=
width:1px;border-left-style:solid" class=3D"gmail_quote">
<div class=3D"im">On 12/21/2011 01:09 PM, DJ Delorie wrote:<br>
<blockquote style=3D"margin:0px 0px 0px 0.8ex;padding-left:1ex;border-left-=
color:rgb(204,204,204);border-left-width:1px;border-left-style:solid" class=
=3D"gmail_quote">
I&#39;d assumed rotor inertia would be sufficient, but I suppose &quot;that=
<br>
depends&quot; as usual.<br>
</blockquote>
<br></div>
I&#39;m sure the inertia is plenty to keep it going. =A0The smoothing can b=
e good if the motor<br>
inductive response of the bare motor makes big volt spikes between<br>
times it is driven by transistors. =A0A series wound brush DC motor with so=
me added<br>
capacitance at the motor, downstream of the FET switches will be less noisy=
,<br>
less likely to have a big volt spike if run hard then cut off.<div class=3D=
"im"><br>
<br>
On 12/21/2011 01:16 PM, John Doty wrote:<br>
&gt; The problem with added capacitance is that it increases the dissipatio=
n of the driver, since it then has to charge/discharge the capacitors when =
it switches. With enough capacitance to smooth the waveform, this will be a=
 big effect.<br>

<br></div>
I&#39;m suggesting capacitance enough to make the PWM square wave disappear=
, not round its edges.<br>
and it&#39;s at the motor. =A0When the FETs cut on they just have to top up=
 the voltage the cap stored since<br>
last time.<br>
<br>
This probably needs a sketch to help more.<br><font color=3D"#888888">
<br>
John<br>
</font></blockquote></div><br>

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