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The following email was condensed and contributed by Mike Jones. It is a discussion that occurred recently on our email server. The information presented here is quite valuable!

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Subject:     PWM chip    
Date:     Tue, 31 Aug 1999 11:51:05 -0400      Status:     Normal    
From:     Chris & Cory         

Has anybody tried the DRV101 or DRV102 PWM chips from Burr-Brown? They are
available from DigiKey for around $5 and I was wondering if anybody had
experience with this chip. It does PWM at a fixed 24kHz and the duty cycle
is adjustable from 10%-100%. I could be wrong about this, but that's what
a quick scan of the datasheet seems to indicate.

Cory
________________________________________________________________________________
From: "Dan Creagan" on 08/31/99 09:32:48 AM
To: srs@seattlerobotics.org
cc: (bcc: )
Subject: PWM Freqs

I'm using a PIC to generate PWM for some motors and I noticed that the PWM
frequency (not duty cycle) greatly impacts performance and duty cycle
resolution. A recent message on this list mentioned a PWM chip that had 24KHz
frequency. That would be unusable for my motor setup - I'm getting best
results in the 240-580 Hz range.

So the question is this - what are typical frequency ranges used for motor PWM?
Is there a general rule to follow for these things, or is it pretty much a test
and see situation?

Dan

________________________________________________________________________________
Subject:     Re: PWM Freqs    
Date:     Tue, 31 Aug 1999 09:41:49 -0700      Status:     Normal    
From:     jbronk            

I have always wondered what the fastest PWM freq that a hc11 can generate. Any
body know?

It has been my experience with motors that the faster the PWM frequency, the
greater the efficiency of the motors. Also if the PWM freq is to low, you get
very annoying audio sounds from it.
________________________________________________________________________________
From: Eric Olsen
To: seattle robotics society "<srs@seattlerobotics.org>
Sent: Tuesday, August 31, 1999 12:45 PM
Subject: Re: PWM Freqs

PWM readers,

Keep in mind that a motor is a low pass device. The reason is that a motor
is mainly a large inductor. It is not capable of passing high frequency
energy, and hence will not perform well using high frequencies. Low
frequencies are required, and then PWM techniques will work.

Lower frequencies are better then higher frequencies, but PWM stops being
effective at too low a frequency. Knowing the low pass roll-off frequency
of the motor helps to determine an optimum frequency for operating PWM.

Try testing your motor with a square duty cycle using a variable frequency,
and then observe the drop in torque as the frequency is increased. This
technique can help determine the roll off point as far as power efficiency
is concerned.

Regards,

Eric Olsen
________________________________________________________________________________
Subject:     Re: PWM Freqs    
Date:     Tue, 31 Aug 1999 16:41:17 -0700      Status:     Normal    
From:     "Larry Barello"             

Eric suggests that High Frequency PWM will not work well. This is not the
case

Sure, the motor is a big inductor, but that is why PWM works: it creates an
average current through the motor. The more the power is on, the higher the
current flow. The problem is with how one drives the motor with the PWM
signal:

I am using the LM293D with a 15khz PWM, locked anti-phase, for those who
know the terminology, driving some Hitec HS-605BB servos. It seems very
efficient. The motor chip is cool when driving the motors at 50% duty cycle
(zero drive with locked anti-phase) and is completely silent in operation
and the control range is very wide.

Now, the way most hobbyist run their PWM is sign-magnitude, where 0% duty
cycle is no power, and a separate output bit sets the direction. I could
never get this to work smoothly, high frequencies or not, until I bridged
the motor with a .1ufd capacitor. The capacitor provides a low impedance
shunt around the motor to allow the current that flowed from the PWM driver
to continue circulating.

The idea that a lower frequency PWM works better simply reflects that the
"on" cycle needs to be pretty wide before the motor will draw any current.
A higher PWM frequency will work fine if you hang a large capacitor across
the motor or (and I have not actually tried this, but someone else out there
in SRS land has...) short the motor out on the "off" cycle (e.g. power/brake
pwm) The reason for this is that short pulses will not allow much current
to flow before being cut off. Then the current that did flow is dissipated
as an inductive kick - probably as heat through the flyback diodes. The
capacitor integrates the pulse and provides a longer, but lower, current
flow through the motor after the driver is cut off. There is not inductive
kick either, since the current flow isn't being cut off.

Locked anti-phase, by the way, the motor is always driven either forward or
backwards, but always connected to the power. 50% duty cycle has no net
current flow and the motor doesn't move. Because the motor is always being
driven, it always has a low impedance across it's terminals. A side effect
of this is that the motor, at 50%, not only doesn't turn, but it resists
turning - it is in brake mode: a low impedance (e.g. a short) is across the
terminals. No capacitors are needed. The one drawback is intense inductive
noise at the switching frequency... Guess that L293D isn't a perfect
switch...
________________________________________________________________________________
From: Dan Creagan
To: srs "<srs@seattlerobotics.org>
Date: Tuesday, August 31, 1999 6:11 PM
Subject: Re: PWM Freqs

>>A higher PWM frequency will work fine if you hang a large capacitor across
>>the motor

How large we talking about? I have already put the .1 ufd shunt on the motors
and ran it in both modes that you talk about (you sorta stumble into them when
you are experimenting) and I still noticed a marked difference in freq response.
Lower is better - noise or no noise.

While I see what you are talking about, I am curious where one would get a
large enough bi-polar capacitor. .1 uFd doesn't seem big enough. However, I
haven't tried it at anything over 4.8 kHz. I'll play around with something
much higher and see what happens.

Dan
________________________________________________________________________________
Subject:     Re: PWM Freqs    
Date:     Tue, 31 Aug 1999 18:48:12 -0700      Status:     Normal    
From:     "Bill Bailey"             

The low versus high frequency for PWM of dc motors describes two totally
different approaches. Both are valid.

At low frequencies you get a mechanical averaging. When the drive is turned
off, there is a momentary spike of voltage that the catch diodes clamp but
after that transient dies out the motor is left to freewheel. You will
typically hear the motor buzzing -- there has been mention of the
possibility of excessive wear possible. But this is usually pretty simple
to implement with small motors and low voltages (remember the motor must
freewheel when the drive is off).

At high frequencies the inductance of the motor (armature) does the current
averaging. This is similar to a switching power supply (or a chopper
drive). The catch diodes are more critical here because they carry full
motor current a substantial amount of time (not so if you are driving the
motor locked anti-phase). High frequency PWM is more sensitive to the motor
(inductance). I didn't have much luck with the real cheap, small dc motors
but with medium size typically higher voltage (i.e. pittman) 20kHz worked
fine. But I have this servo-disc servo motor that has a real low inductance
and 20kHz is not a high enough frequency.

I've always been curious about the 'cap across the motor'
suggestion/requirement, especially with a high frequency PWM. There would
have to be a current spike when the 'switch' turns on. And with a low
resistance MOSFET h-bridge and ni-cad batteries, this current could be very
high. It may be that the catch diodes are not doing their job -- i.e. they
need to be faster or of a higher current rating. Use fast catch diodes
whose current rating exceeds the current the motor is to be operated at. I
suggest schottky diodes (1N5822 for instance for a 3 amp diode)

________________________________________________________________________________
Subject:     Re: PWM Freqs    
Date:     Tue, 31 Aug 1999 22:41:16 -0400      Status:     Normal    
From:     (Robert Nansel)             

Bill Bailey said:

>I've always been curious about the 'cap across the motor'
>suggestion/requirement, especially with a high frequency PWM. There would
>have to be a current spike when the 'switch' turns on. And with a low
>resistance MOSFET h-bridge and ni-cad batteries, this current could be very
>high. It may be that the catch diodes are not doing their job -- i.e. they
>need to be faster or of a higher current rating. Use fast catch diodes
>whose current rating exceeds the current the motor is to be operated at. I
>suggest schottky diodes (1N5822 for instance for a 3 amp diode)

I don't use a pure cap, but a series RC snubber. Values that I've used that
work fairly well are C=0.0033 uF & R=47 ohm, though I've never been able to
find a textbook explanation of how to select the values, so these may be
far from optimum.

One point Bill makes, though, is that recirculating diodes are critical.
Those diodes aren't there to protect the transistors, guys, they're there
to conduct the motor current during the OFF cycle of your PWM. Leave 'em
out and your efficiency is going down the toilet.

On motor quality: I've found that the cheaper the motor the higher the duty
cycle has to be for the motor to turn. A Barber/Colman gearmotor, for
instance, won't stay "locked in" to 2 kHz PWM for duty cycles much less
than 67% or thereabouts, depending whether it's driven with a bipolar or
MOSFET H-bridge. One bipolar PWM I built would stall out at 50%, but a
MOSFET circuit with lower ON resistance would go down to 33%. I imagine a
higher quality motor would operate at even lower PWM duty cycles.

-RLN
________________________________________________________________________________
Subject:     Re: PWM Freqs    
Date:     Tue, 31 Aug 1999 22:10:36 -0500      Status:     Normal    
From:     "Dan Creagan"             

Great discussion. I just jacked my PWM up to 19.2 kHz and I got the duty
cycle to about 50% before cutoff. That is better than the 4.8 kHz range (the
cutoff was around 70%!). However, that is running my hardware PWM at full
blast (20 mHz PIC). I can still get the duty cycle to about 30 % if I use a
very low PWM (240 Hz) and put up with the noise. Unfortunately, that means I
can only run the PIC at 4 mHz - which isn't a great loss, but still not
using all the lifties.

I tried it using both anti-phase and 'tuther. It works best (wider control
range) with 'tuther (sign magnitude?) - using either PWM frequency.

I realize this has much to do with components. I'm using L293D (heat sinked)
and those Jameco Motors that I mentioned in another post. If I didn't have
a goal I was trying to reach, this would be a fun place to experiment for a
while.

Dan
________________________________________________________________________________
Subject:     Re: PWM Freqs    
Date:     Tue, 31 Aug 1999 22:23:27 -0700      Status:     Normal    
From:     "Eric Olsen"             

PWM Readers,

OK, so I didn't take that class in electro-dynamics (it's quite a complex
subject as I understand). But I still think there's a lot of confusion in
this discussion. I offer a short treatise with my understanding of simple
impedences:

I look at it this way. An inductor becomes a larger resistance as the
frequency goes up. This is fact. Therefore, POWER TRANSFER will suffer.
Maximum power transfer occurs when your source impedance (i.e. your drivers
output resistance) matches your load impedance. Most drivers try to
minimize source impedance. Look at your data sheets for your particular
case.

For many of your applications out there, this may not be a problem (like
hobby servo's, for example). However, as the resistance (known as impedance
in the sinsusoidal domain) goes up, the current goes down. There's nothing
you can do about that. For some of you operating at 20KHz, if you keep
going higher, your motor will eventually stop (i.e. I'm talking 50%
sinusoidal duty, not high speed glitches! Square waves do have low
frequency content regardless, however, and that confuses the issue a bit).

Another rule of thumb, "the current in an inductor cannot change
instantaneously, however, the voltage across an inductor can". Those of you
saying that the inductor filters out the high frequency are right. A higher
frequency will produce a very smoothed current flow as opposed to a lower
frequency that may create motor chatter. Because an inductor has a simple
exponential "charge" cycle just like a capacitor (where "charge" is current,
and current equates to magnetic flux), and since this charge cycle is
exponential even if the input is a step function (instantaneous voltage
change), I'm not sure that rise time will offer any real advantages,
however, someone please correct me if I'm wrong.

Just keep in mind that too much frequency, and you're impedance is too high
and old Ohms law will make sure that your current (and hence your magnetic
flux) is being limited. Also, I'd be very careful about placing high
capacitance across the motor terminals. I think I even suggeted this before
(oops!). The fact is that a large capacitor will exhibit low impedence at
high frequencies ... and this is for sure. In this case, you are wasting
power through the cap. Forget about the idea that the cap is storing charge
for the motor ... treat the entire parallel circuit as a lumped impedance
..... current flowing through the cap is not creating magnetic flux.

Kick back diodes will do both things mentioned. They will definitely cycle
current back into the motor when the driver turns off. However, the diodes
are primarily needed to protect the transistor. There's this funny
transistor rating called "break down voltage", and it's the voltage level
that will destroy your driver transistor! So this is why it's the primary
reason. As a bonus, kick back diodes re-deliver current back to your motor
and kind of give it a coasting effect. The reason is as I mentioned before:
"the current in an inductor cannot change instantaneously". Because of this
property, when your transistor shuts off, the current in the inductor keeps
flowing. But your transistor now looks like a very large resistance, and a
large current times a large resistance equals a very large voltage (ohms
law). This effect is an instantaneous effect, since the magnetic field
(driving the current) collapses fairly rapidly in this case, hence a "spike"
is generated. By placing a kick back diode across the motor, the current
takes the path of least resistance, which is back through the windings in
the direction that the current is already flowing. The diode also "clamps"
the voltage spike to protect the transistor. However, diode turn-on speed
is an issue, and therefore, high speed switching diodes are preferred. (You
can see this with a scope, slow diodes still let some initial voltage by,
while faster diodes let far less voltage by).

The other law that is important to remember is "voltage across a capacitor
cannot change instantaneously, but current through the capacitor can".
That's why a small .1 uf capacitor or thereabouts is a good thing across a
motor. It's definitely there to shunt motor noise. You used to see them
all the time in old tape recorders, since if they weren't there, you'd
definitely hear cracking and popping on your tape! The motor is a
tremendously noisy device, and a small capacitor helps reduce voltage spikes
while shunting current exactly when it is needed (when the motor armature
shuts the current off momentarily). Remember what happens when you toggle
a switch to a highly inductive load ... you get a spark. Again, the current
doesn't stop instantaneously, and will actually induce a very high voltage
across the air gap to the degree that the air will ionize and a spark will
fly. This goes on all the time inside a motor (as I'm sure we all know).
The cap helps to filter these voltage spikes by shorting current exactly
when it's needed. The reason is that the cap resists instantaneous voltage
changes. The goal of choosing the correct value for the cap is that it
exhibit a low enough impedence to effectively shunt voltage spikes induced
by motor switching (rise time is the metric here), while being a high enough
impedence at the frequecy needed to control your motor using PWM so that is
does not waste current. Simply take the inverse of (2*
Pi*Frequency*Capactiace) for your effective capacitor impedence at your
given operating frequency.

All of us must remember, as was mentioned by several of you, that we are
working with different motors. Some big, and some small. Also, our
applications are different, some care about power transfer, others maybe
not. We should be careful that what works for one motor may not work for
the other given different applications. However, I'd have to think that
some general rules can be applied to all motors, and it would be a great
project if someone out there would tackle this problem and create a set of
equations and/or rules for any motor circuit as well as optimum values for
PWM frequency given a specific motor. Parameterization of the motor is
therefore important, and it would be good if this could be done in a
relatively simple manner.

Any takers? (and sorry for the previous bad suggestions!).

Anyway, that's my treatise for the subject, as I reminded my self of things
forgotten and hope to inform others that there is science behind all of
this. Electrical engineering does apply, and this being the case, I'd like
to learn more about this very subject. I've offered some rules of thumb to
help make sense of all this.

Regards,

Eric Olsen
________________________________________________________________________________
Subject:     Re: PWM Freqs    
Date:     Tue, 31 Aug 1999 22:54:29 -0700      Status:     Normal    
From:     "Eric Olsen"             

One last point (because I hate it when I think of stuff after I've hit
send!)

Much of my arguments on the last message are based upon simple sinusoidal
waves ... that is, current alternating between a negative and a positive
value. In such basic examples, we often assume a zero DC current. On the
other hand, we are talking about square waves that have a DC offset.

If you consider a 50% duty square wave voltage, swinging between 0 and 5
volts, then what we have mathematically is a 50% duty square wave swinging
between -2.5 volts and 2.5 volts with a 2.5 volt DC offset (hence, zero to 5
volts). Therefore, when your driver delivers a frequency that is too high,
the inductor will impede the AC component, and only pass the DC component.
Assuming the inductor is blocking the AC component, then at 50% duty, the DC
component amounts to half of half (25%) of full DC at 5 volts. This would
be worst case attenuation.

This is why Larry and others motor circuits work fine even at very high
frequencies (which is true). However, it should be realized that in the
limiting case, when the frequency is too high, the current delivered to your
motor approaches only the DC component of the signal, since the AC component
is effectively blocked. Again, optimum power transfer is not happening
here, but in many applications, this may not be an important factor.

There, I feel a lot better!

Eric
________________________________________________________________________________
Subject:     Re: PWM Freqs    
Date:     Tue, 31 Aug 1999 23:12:28 -0700      Status:     Normal    
From:     "Eric Olsen"             

One last time,

My last analysis was on to the right idea. High frequency components are
blocked by the inductor, and only the DC component is passed.

I have reconsidered and hence retract the one point that half of half is
transfered. It appears Larry was right, the DC gets through, and this could
be as much as full DC in the limiting case that PWM was nearly 100% on.

High frequency PWM appears to be a good thing.


Eric
________________________________________________________________________________
Subject:     Re: PWM Freqs    
Date:     Tue, 31 Aug 1999 23:36:30 -0700      Status:     Normal    
From:     "Eric Olsen"             

Yes, there are a number of different ways to look at this topic.

I was going to say that while I was on a roll sticking my foot in my mouth,
Dan Miner's use of a large capacitor is also justifiable for the same reason
I mentioned in my last e-mail.... that is, the capacitor does short out the
AC component of the signal applied, but it BLOCKS the DC component, which
means I was wrong to suggest that it wastes current. There may be some
reasons to choose fast caps or something like that, but I'll refrain on
offering any sugestions at this point!

It may be interesting to note that a lumped inductor and capacitor should
create an LC tank. An LC tank should exhibit some oscillations at it's
tuned frequency when excited (i.e. switching). I'm wondering if anybody can
see any of these effects?

Eric
________________________________________________________________________________
Subject:     RE: PWM Freqs    
CC:     "'SRS List Server'" <srs@seattlerobotics.org>     
Date:     Wed, 1 Sep 1999 00:10:50 -0700      Status:     Normal    
From:     Bryan Minugh         

Richard: it *is* that the motor can't keep up.

The more massive the spinning parts are the longer it takes them to begin
rotating. A current flow that doesn't last that long produces vibration,
but not rotation. As you'd expect, the more massive the motor, the longer
it's "Mechanical Time Constant" is (the term used in spec sheets to
describe the motor's inertia). The design articles I've read in technical
journals recommend a minimum pulse width equal to the MTC of your motor, if
your goal is to slow the motor but not let it bog down under heavy loads
(Pulse Width Modulation). Before PWM, motors were typically slowed by
cutting the voltage fed to the motor with a simple series resistor (Tyco
slot cars, your Grandma's sewing machine, etc.) but that reduced the
motor's torque, so it's speed wavered, it stalled easily and it had to be
"gunned" to start it turning.

When power is first applied to a motor the current flow ramps up gradually
as the coils reach full charge. For the same reason a switching regulator
is preferable to a linear one, (less power waste and heat output) Ramp
Pulse Drive can be used to reduce the voltage across the motor's terminals
instead of simple series resistance. For this goal, a pulse (shorter than
the MTC) is used that disconnects the battery before full current flow is
reached. Diodes and/or capacitors are often added to the motor's terminals
so the current flow will continue after the battery is disconnected and/or
recycle the energy when the coils discharge.

PWM (the first goal) is based on the motor's mechanical spin-up time and
ignores the (much briefer) current ramp-up time. RPD (the second goal)
operates only during coil current ramp-up and ignores the motor's
mechanical spin-up time

I see from the other replies to your post that some people use long pulses
for PWM (to achieve the first goal) others short pulses for RPD (the second
goal). Although the two operating modes are different, both use nearly
identical circuitry so people often confuse the two and use the term "PWM"
for either.

-Bryan Minugh

P.S. does anyone know the conventional name used to refer to the ramp pulse
drive technique?
________________________________________________________________________________
Subject:     RE: PWM Freqs    
Date:     Wed, 1 Sep 1999 09:14:16 -0500      Status:     Normal    
From:     Dan Miner        

> I was going to say that while I was on a roll sticking my
> foot in my mouth, Dan Miner's use of a large capacitor is
> also justifiable for the same reason I mentioned in my
> last e-mail.... that is, the capacitor does short out the
> AC component of the signal applied, but it BLOCKS the DC
> component, which means I was wrong to suggest that it
> wastes current. [...snip...]

I think there's some confusion here.

For the record: My large (47000uF) cap is *NOT* connected
across the motor terminals. It is a SUPPLY bypass and goes
across the supply pins to the H-bridge. Crude picture:


+-------------+-----+ +--------+
| | | | |
Battery 47000uF H-bridge Motor
| | | | |
+-------------+-----+ +--------+

The important part is to get the bypass cap close to the H-bridge.
(I also have a 1uF cap as H-bridge bypass.)

Also for the record: I have *NO* cap across the motor.
(Other than unintentional parasitic cap. :-)
The LM18200 has flyback diodes built in.

                - Dan Miner
________________________________________________________________________________
Subject:     Re: PWM Freqs    
CC:     "'Johnson, Richard A'" ,"'SRS List Server'" <srs@seattlerobotics.org>    
Date:     Wed, 01 Sep 1999 07:20:46 -0700      Status:     Normal    
From:     Randy Carter         

This where you need to use a servo loop. Placing a sensor on the
drive train to check the current speed. Then you have the
electronics this with the desired speed. Compute the error and
use that to increase or decrease the power output. I use servo
chips like the LM629 (National Semiconductor
http://www.national.com), the MC1401 (Performance Motion Devices
http://www.pmdcorp.com) or the PIC-Servo (JR Kerr
http://www.jrkerr.com). With the MC1401 driving PWM sign and
magnitude to a LMD18201 I was able to get a motor shaft to spin
at 1 RPM. The output frequency of the MC1401 is fixed at 25 kHz
which makes any whine inaudible.
________________________________________________________________________________
Subject:     RE: PWM Freqs    
Date:     Wed, 1 Sep 1999 07:26:19 -0700      Status:     Normal    
From:     jbronk            

Wow, I just asked a simple question about PWM. I have decide that PWM and H
bridges are way to complicated to use. I will stick to relays and DC from now on
;>)

It is beginning to sound like different motors respond to different PWM freq in
different ways. I assume it has to do with the number of windings a motor has,
the inductance of a winding, the DC resistance of a winding and any stray
capacitance in the motor.

On my first robot, the motors had very low torque with a PWM freq less then
20k. At 30k it would burn rubber. I just assumed that higher was better in all
cases. silly me
________________________________________________________________________________
Subject:     RE: PWM Freqs    
Date:     Wed, 1 Sep 1999 11:13:26 -0400      Status:     Normal    
From:     (Robert Nansel)             

I would like to revise my comments to clarify the recirculating diodes vs.
protection diodes debate--they serve both functions, but I was trying to make
a point about the nature of PWM motor control.

--RLN
________________________________________________________________________________
Subject:     Protection diodes vs. Recirculation diodes     
Date:     Wed, 1 Sep 1999 11:42:55 -0400      Status:     Normal    
From: (Robert Nansel)             

On Tue, 31 Aug 1999 Eric Olsen said:

>Kick back diodes will do both things mentioned. They will definitely cycle
>current back into the motor when the driver turns off. However, the diodes
>are primarily needed to protect the transistor. There's this funny
>transistor rating called "break down voltage", and it's the voltage level
>that will destroy your driver transistor! So this is why it's the primary
>reason. As a bonus, kick back diodes re-deliver current back to your motor
>and kind of give it a coasting effect.

Let me clear up some confusion. It is possible to design PWM circuits where
the protection and recirculation diode function are completely separate.
Consider the lowly relay H-bridge. Here relays do direction control only,
and you must add a single ground-referenced pass transistor (bipolar or
MOSFET) to do the PWM. In software, say, you can set things up so that the
relay contacts never close or open while motor current is flowing; this
means you would not really need diodes to protect those contacts. But you
*must* have a full set of recirculating diodes *across the motor terminals*
to make the PWM work right in this circuit.

You would need a protection diode for the pass transistor, though--and that
diode would not participate in the recirculation current since the diode
bridge would steer most of the current away from the pass element.

One of the nice things about MOSFET H-bridges is not that they have
recirculating diodes built in--you can't always trust a MOSFET intrinsic
diode to have a fast enough reverse recovery time. The real reason MOSFETs
are nice is that MOSFETs conduct in either direction when ON. The
recirculating diodes may take some of the current before the MOSFET is
fully switched ON, but with a good low ON resistance MOSFET the transistor
itself will conduct the majority of the recirculating current. Here I'm
assuming that you are driving one half of the H-bridge with your PWM signal
so that the "upper" and "lower" MOSFETs of that half are alternatly ON and
OFF (i.e. one of the transistors is always ON, except for a short
"deadzone" between transitions to eliminate shoot-through current spikes).

On another point, the purpose of snubber networks is to reduce the hash
from motor commutation, and thus they don't really deal with the
recirculation current.

-RLN
________________________________________________________________________________
Subject:     RE: PWM Freqs    
Date:     Wed, 1 Sep 1999 07:26:19 -0700      Status:     Normal    
From:     jbronk            

Wow, I just asked a simple question about PWM. I have decide that PWM and H
bridges are way to complicated to use. I will stick to relays and DC from now on
;>)

It is being to sound like different motors respond to different PWM freq in
different ways. I assume it has to do with the number of windings a motor has,
the inductance of a winding, the DC resistance of a winding and any stray
capacitance in the motor.

On my first robot, the motors had very low torque with a PWM freq less then
20k. At 30k it would burn rubber. I just assumed that higher was better in all
cases. silly me
________________________________________________________________________________
Subject:     Re: Protection diodes vs. Recirculation diodes     
Date:     Wed, 1 Sep 1999 09:36:03 -0700      Status:     Normal    
From:     "Eric Olsen"             

----- Original Message -----
From: Robert Nansel
To: seattle robotics society "<srs@seattlerobotics.org>
Sent: Wednesday, September 01, 1999 8:42 AM
Subject: Protection diodes vs. Recirculation diodes


>
> Let me clear up some confusion. It is possible to design PWM circuits
where
> the protection and recirculation diode function are completely separate.
> Consider the lowly relay H-bridge. Here relays do direction control only,
> and you must add a single ground-referenced pass transistor (bipolar or
> MOSFET) to do the PWM. In software, say, you can set things up so that the
> relay contacts never close or open while motor current is flowing; this
> means you would not really need diodes to protect those contacts. But you
> *must* have a full set of recirculating diodes *across the motor
terminals*
> to make the PWM work right in this circuit.
>

Robert,

I'm not clear why one needs a full set of re-circulating diodes accross the
motor to make the PWM work right. Can you explain this.

I beleive I understand your point of MOSFETs acting to pass motor current in
an H-bridge configuration, but what happens when all MOSFETs in the bridge
are shut off? Seems like that would happen at least in a failure mode. Are
they all turned off at any other time?

Eric
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Subject:     RE: PWM Freqs - Anti-pahse vs. Signed Magnitude     
Date:     Wed, 1 Sep 1999 12:47:36 -0500      Status:     Normal    
From:     Dan Miner        

National's app note 694 has a really good description of this:

http://www.national.com/an/AN/AN-694.pdf

> -----Original Message-----
> From: Mike Jones
> Sent: Wednesday, September 01, 1999 7:03 AM
> To: srs@seattlerobotics.org
> Subject: RE: PWM Freqs
>
>
> In a previous message, Larry talked about "anti-phase".
> Exactly what is does
> this term mean?
>
> Mike
________________________________________________________________________________
Subject:     Re: PWM Freqs    
Date:     Wed, 01 Sep 1999 11:20:41 -0700      Status:     Normal    
From:     bill harrison         

Hi,

I've been using PWM to control motors for some time now, and am pretty happy
with using it. I might well have been intimidated by this thread back when I was
starting, but luckily I just built my robots not knowing how it all was supposed to
work :-)
I've found that if you aren't pushing limits on anything, PWM is pretty
forgiving and most of the time we aren't too concerned about max torque or efficient
power usage. I'd leave the fancy design work for those that have a serious
application or a product design (for what little I know about electronics, I get the
feeling that most on this thread don't have a full understanding either).
Just try your motor driver setup out with a test setup. See how it works,
play around with it a bit. If it doesn't work right, try a different setup. There
are lots to choose from and many SRS members have running robots (so it must be
possible). Just be careful with expensive components (you might do the test with
cheap stuff).
Of course the best thing to do is use a great design that some expert
designed. The best way to do that is buy a product that does the difficult stuff
for you (such as chips, boards, or plans). One of the reason that Randy Carters
robots work so well, I feel, is that he got chips that did a wonderful job of
controlling his motors. The chips seem expensive, but then you aren't buying the
chip, you are buying the software and the designing that went into it (that's in the
chip).

Opps, I'll get off the soap box now, Bill Harrison
________________________________________________________________________________
Subject:     Re: PWM Freqs    
Date:     Wed, 1 Sep 1999 11:31:46 -0700      Status:     Normal    
From:     jbronk            

Bill, I was just kidding when I made that statement.

Actually, this discussion has sparked my interest in seeing if it is possible to
determine the PWM freq that gives the max torque for a give motor by measuring
the inductance of the motor windings. I will have to blow off the dust on my dip
meter to
try it.
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