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[DPRG] Driving an H-Bridge with a switching power supply

Subject: [DPRG] Driving an H-Bridge with a switching power supply
From: Kipton Moravec kip at kdream.com
Date: Sun Dec 28 10:42:01 CST 2003

That is what I believe.


At 07:51 PM 12/27/03, you wrote:
>OK, Granted. If the system has enough "head room", then it won't be
>necessary to worry about the maximum voltage because the PWM output will
>never reach 100%. However, lowering Vmax will lower the maximum dP/dt,
>it's just that the system should be designed such that this isn't a
>Is that the gist of it?
>-----Original Message-----
>From: dprglist-admin at dprg.org [mailto:dprglist-admin at dprg.org] On Behalf
>Of Kipton Moravec
>Sent: Friday, December 26, 2003 6:55 PM
>To: Rick J. Bickle
>Cc: DPRG List
>Subject: RE: [DPRG] Driving an H-Bridge with a switching power supply
>No. Something is wrong in the control loop.
>For example, lets say you have a 24V battery.
>When you are running PWM a 50% duty cycle gives an average voltage of
>If the battery drops to 23V (slowly over time) it now takes the PWM with
>about a 52.17% duty cycle to give the motor the same 12V.
>This is not a big deal when you have a lot of voltage above what you
>(This is what I call head room).  If you are running PWM at 95% duty
>at 24V if the voltage drops to 23V the PWM needs a duty cycle of about
>If the voltage drops to 22V you do not have the voltage at a PWM duty
>at 100% to make it work.
>The alternative was to use a switching power supply.  A cheap buck
>needs at least 2-3 volts above the desired voltage.  So if you want 24V
>out, you need 27V (or more) in.  Well if you are going to boost your
>batteries to 27V for a switcher,  You can do the same by boosting the
>voltage for the PWM with no switcher.
>You have two choices.  1. Make it work at a lower speed (or voltage). 2
>Increase the voltage in.
>Since we have already talked about raising the voltage, lets look at
>lowering the maximum speed.
>How much do you want to allow the voltage to sag before the robot no
>works? That is the voltage you tune it for, and limit it to that maximum
>Back to our 24V battery.  Lets say I want it to work if the battery is
>20V.   I still want a little head room (to deal with differences in your
>two motors), so I tune it to work at 18V. That means the maximum speed I
>can go corresponds to 18V being applied to the motors.  (Work out the
>of the encoder with that voltage) With a fresh 24V battery, that makes
>PWM have a 75% duty cycle.  That is typically the maximum PWM at that
>voltage. But if the battery drops to 20V now you have a PWM duty cycle
>90% to still provide the 18V.
>I hope I am making myself clear.
>At 06:14 PM 12/26/03, you wrote:
> >Kip,
> >
> >Even though you have a feedback control system capable of compensating
> >for the lower voltage, wouldn't the response of the control system
> >still be reduced with a lower supply voltage? Take the example of David
> >Anderson's balancing bot - if with a full battery charge the controller
> >is capable of providing more power/time to the motors, it can then more
> >quickly compensate for deviations. With a lower battery voltage
> >however, the same power transfer requires more time, making the
> >response more sluggish.
> >
> >Rick
> >
> >-----Original Message-----
> >From: dprglist-admin at dprg.org [mailto:dprglist-admin at dprg.org] On
> >Behalf Of Kipton Moravec
> >Sent: Friday, December 26, 2003 2:33 PM
> >To: dprglist at dprg.org
> >Subject: RE: [DPRG] Driving an H-Bridge with a switching power supply
> >
> >
> >I disagree with everybody.  :)
> >
> >I do not think a regulator on the input power to the robot is the
> >correct answer.
> >
> >There is something wrong with your control loop if it can not
> >compensate
> >
> >for a battery slowly dropping voltage.  Of course if the battery drops
> >enough then the system will not work with or without a regulator.
> >
> >What you need with a regulator is more head room, so you end up
> >increasing the voltage of the battery pack so you can add a regulator.
> >If that fixes
> >the problem, then just add more battery voltage without the regulator.
> >The
> >regulator is nothing more than another control loop.  They should be
> >able
> >to be combined.
> >
> >I really think the problem is that the control loop is too slow.  I
> >would recommend going from 25 Hz to 100 Hz or more.  When I talked to
> >some control people they said they do PID for motors at 1000 Hz.
> >
> >Kip
> >
> >
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