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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: Ralph Tenny rften at swbell.net
Date: Mon Dec 29 12:51:00 CST 2003

Kip:
Your example is flawless; the problem is that ANY battery system that
drops from 24v nominal to 20v is close to a rapid voltage drop.
Lead-acid battery packs will have the slowest decay and NiMH the most
precipitous and the most sensitive to full discharge conditions.
Ralph

-----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
12V.

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
need 
(This is what I call head room).  If you are running PWM at 95% duty
cycle 
at 24V if the voltage drops to 23V the PWM needs a duty cycle of about
99.13%.

If the voltage drops to 22V you do not have the voltage at a PWM duty
cycle 
at 100% to make it work.

The alternative was to use a switching power supply.  A cheap buck
switcher 
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
longer 
works? That is the voltage you tune it for, and limit it to that maximum
speed.

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
ticks 
of the encoder with that voltage) With a fresh 24V battery, that makes
the 
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
of 
90% to still provide the 18V.

I hope I am making myself clear.

Kip

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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