What passes as Pulse Width Modulation in DC Motor Control?

[Rich]

Can anyone please tell me what passes as PWM in Motor Control?

Is it limited to supplying the motor with a square wave, where a set and
constant level of voltage is switched on or off to the motor?

Or can other waveforms consitutute PWM control of a dc motor? Such as where
voltage does not drop to zero, but has periodic peaks having square,
triangular, sawtooth or some other waveform?

Can PWM control consist of dc pulses that rise and fall in voltage in a
sinusoidal fashion?

At the moment I am believing that PWM is just on/off of a contant
level of dc voltage. But wonder if that belief is correct or not. Thanks.

 

[John Fields]

On Tue, 27 Jan 2009 13:38:51 -0000, "Rich" <notty@emailo.com> wrote:

Can anyone please tell me what passes as PWM in Motor Control?

Is it limited to supplying the motor with a square wave, where a set and
constant level of voltage is switched on or off to the motor?

---
Yes, but not necessarily a square wave, which would represent a 50% duty
cycle PWM signal.

What's done is that the duty cycle of a constant voltage signal is
varied in order to change the speed of the motor.
---

Or can other waveforms consitutute PWM control of a dc motor? Such as where
voltage does not drop to zero, but has periodic peaks having square,
triangular, sawtooth or some other waveform?

---
No.
---

Can PWM control consist of dc pulses that rise and fall in voltage in a
sinusoidal fashion?

---
No.
---

At the moment I am believing that PWM is just on/off of a contant
level of dc voltage. But wonder if that belief is correct or not. Thanks.

---
It is.

JF

 

[Rich]

"John Fields" <jfields@austininstruments.com> wrote in message
news:n84un4pdjlplqlk0uoapb4ppdpue7803b5@4ax.com...

On Tue, 27 Jan 2009 13:38:51 -0000, "Rich" <notty@emailo.com> wrote:

Can anyone please tell me what passes as PWM in Motor Control?

Is it limited to supplying the motor with a square wave, where a set and
constant level of voltage is switched on or off to the motor?

---
Yes, but not necessarily a square wave, which would represent a 50% duty
cycle PWM signal.

What's done is that the duty cycle of a constant voltage signal is
varied in order to change the speed of the motor.
---

Hi.

So PWM is not just simply switching on and off of a steady dc voltage.

If you employed a trangle wave, or sawtooth wave, or some other waveform,
these waveforms require something other than a simple switch action. And
therefore they cannot theoretically be as efficient as square-wave PWM.

But as long as there is no constant "pedastal" dc voltage in the dc supply
to the motor, such waveforms would pass a PWM.

This is the logic behind what you said.

 

[John Fields]

On Tue, 27 Jan 2009 14:22:29 -0000, "Rich" <notty@emailo.com> wrote:

"John Fields" <jfields@austininstruments.com> wrote in message
news:n84un4pdjlplqlk0uoapb4ppdpue7803b5@4ax.com...
On Tue, 27 Jan 2009 13:38:51 -0000, "Rich" <notty@emailo.com> wrote:

Can anyone please tell me what passes as PWM in Motor Control?

Is it limited to supplying the motor with a square wave, where a set and
constant level of voltage is switched on or off to the motor?

---
Yes, but not necessarily a square wave, which would represent a 50% duty
cycle PWM signal.

What's done is that the duty cycle of a constant voltage signal is
varied in order to change the speed of the motor.
---

Hi.

So PWM is not just simply switching on and off of a steady dc voltage.

---
Yes, it is.

Pulse Width Modulation = constant amplitude, variable pulse width:

_ _
10%__| |_________________________________| |_______SLOW

__________________ _________MEDIUM
50%__| |________________|

_________________________________ _________FAST
90%__| |_|

---

If you employed a trangle wave, or sawtooth wave, or some other waveform,
these waveforms require something other than a simple switch action. And
therefore they cannot theoretically be as efficient as square-wave PWM.

---
Who said anything about efficiency?

What you asked about is what is considered PWM, and I told you/

But as long as there is no constant "pedastal" dc voltage in the dc supply
to the motor, such waveforms would pass a PWM.

---
Nonsense.
---

This is the logic behind what you said.

---
Read it again.

JF

 

[Rich]

"John Fields" <jfields@austininstruments.com> wrote in message
news:n84un4pdjlplqlk0uoapb4ppdpue7803b5@4ax.com...

On Tue, 27 Jan 2009 13:38:51 -0000, "Rich" <notty@emailo.com> wrote:

Can anyone please tell me what passes as PWM in Motor Control?

Is it limited to supplying the motor with a square wave, where a set and
constant level of voltage is switched on or off to the motor?

---
Yes, but not necessarily a square wave, which would represent a 50% duty
cycle PWM signal.

This is confusing actually when I think about it.

The question is asking is, is PWM limited to the switching on and off of a
contant dc voltage.

You say "Yes,..."

Okay.

But then you say, "...but not necessarily a square wave, which would
represent a 50% duty cycle"

I know about duty cycle, so I'm thinking JF is saying the dc voltage can be
some other waveform other than square.

Which is a bit confusing because you said that PWM is limited to simple
swithing on and off of a constant dc voltage.

What's done is that the duty cycle of a constant voltage signal is
varied in order to change the speed of the motor.
---

Or can other waveforms consitutute PWM control of a dc motor? Such as
where
voltage does not drop to zero, but has periodic peaks having square,
triangular, sawtooth or some other waveform?

---
No.
---

"No." just tells me the dc voltage must go to zero at some point.

Can PWM control consist of dc pulses that rise and fall in voltage in a
sinusoidal fashion?

---
No.
---

At the moment I am believing that PWM is just on/off of a contant
level of dc voltage. But wonder if that belief is correct or not. Thanks.

---
It is.

JF

You confirmed that PWM *is" the switching on and off of a constant dc
voltage.

What it was is that you saying above, "Yes, but not necessarily a square
wave,.." Probably illogical, but it sent me off in a direction to think any
dc waveform that was turned on and off constituted PWM.

Anyway gotcha. PWM is the on//off switching of a dc voltage that remains
*constant* in time.

Thanks.

 

[Ecnerwal]

In article <6u8opoFd1toiU1@mid.individual.net>,
"Rich" <notty@emailo.com> wrote:

"John Fields" <jfields@austininstruments.com> wrote in message
news:n84un4pdjlplqlk0uoapb4ppdpue7803b5@4ax.com...
On Tue, 27 Jan 2009 13:38:51 -0000, "Rich" <notty@emailo.com> wrote:

Can anyone please tell me what passes as PWM in Motor Control?

Is it limited to supplying the motor with a square wave, where a set and
constant level of voltage is switched on or off to the motor?

---
Yes, but not necessarily a square wave, which would represent a 50% duty
cycle PWM signal.

This is confusing actually when I think about it.

The question is asking is, is PWM limited to the switching on and off of a
contant dc voltage.

You say "Yes,..."

Okay.

But then you say, "...but not necessarily a square wave, which would
represent a 50% duty cycle"

I know about duty cycle, so I'm thinking JF is saying the dc voltage can be
some other waveform other than square.

Which is a bit confusing because you said that PWM is limited to simple
swithing on and off of a constant dc voltage.

Square defined as 50% duty cycle. Other duty cycles are perceived as
"rectangular" in this definition. Short on, long off. Long on, short
off. Still going to constant voltage when switched on/off. Clearer now?

--
Cats, coffee, chocolate...vices to live by

 

[John Fields]

On Tue, 27 Jan 2009 15:50:19 -0000, "Rich" <notty@emailo.com> wrote:

"John Fields" <jfields@austininstruments.com> wrote in message
news:n84un4pdjlplqlk0uoapb4ppdpue7803b5@4ax.com...
On Tue, 27 Jan 2009 13:38:51 -0000, "Rich" <notty@emailo.com> wrote:

Can anyone please tell me what passes as PWM in Motor Control?

Is it limited to supplying the motor with a square wave, where a set and
constant level of voltage is switched on or off to the motor?

---
Yes, but not necessarily a square wave, which would represent a 50% duty
cycle PWM signal.

This is confusing actually when I think about it.

The question is asking is, is PWM limited to the switching on and off of a
contant dc voltage.

---
Yes, it is.

The problem seems to lie in the terminology, where a "square wave" is a
special case of a rectangular wave and is defined as being a signal
which has an ON time equal to its OFF time and, consequently, a duty
cycle of 50%.

Other rectangular waves with duty cycles not equal to 50% aren't, then,
square waves.

JF

 

[Jon Slaughter]

"Rich" <notty@emailo.com> wrote in message
news:6u8h38Fe4fkpU1@mid.individual.net...

Can anyone please tell me what passes as PWM in Motor Control?

Is it limited to supplying the motor with a square wave, where a set and
constant level of voltage is switched on or off to the motor?

Or can other waveforms consitutute PWM control of a dc motor? Such as
where
voltage does not drop to zero, but has periodic peaks having square,
triangular, sawtooth or some other waveform?

Can PWM control consist of dc pulses that rise and fall in voltage in a
sinusoidal fashion?

At the moment I am believing that PWM is just on/off of a contant level of
dc voltage. But wonder if that belief is correct or not. Thanks.

Well, I think technically PWM is "square" but I suppose in general anything
could work.

The point here is that one can vary the function so that an average can be
computed.

The time average is

1/(t1 - t0)*int(f(t),t=t0..t1) which is over one period but generally is
taken to be infinite(they end up being the same though unless f is not
periodic).

So the idea is that we have some parameter that we can adjust to modify the
average.

The "standard" PWM scheme is to use rectangular pulses. It is defined as

f(t) = 1 if 0 < t - k*P < d for some integer k, else 0

P = period, d = "duty cycle"
If you take the time average you get d.

This means that if f is a voltage then we can easily control the average
voltage.

But we can actually choose f to be many things. The problem is that the
relationship between the "control parameter" and the output becomes more
complex.

You could use a triangular function such as

f(t) = 2t/d if 0 < t < d/2, -2t/d + 2 if d/2 < t < 2, else 0

then d controls the "base width" of the triangle.

Integrating over a period P gives

1/P*int(f(t),t=0..P) = d/2

(it's pretty easy to see because it's 1/2 the area of the rectangular pulse)

Now this is relatively simple but it's much harder to generate a triangular
pulse than a rectangular pulse.

Essentially though any function will end up simply a*d where a is some
constant <= 1.

For rectangle a = 1, for triangle a = 1/2. (here I assume the pulse is
normalized to 1 so that it's max value is 1)

This also assumes the pulse is "one" for d% of the period and off the rest.
(although a similar analysis will work for more complicated function)

If you understood all that then you should realize that other schemes don't
really have any benefit as ultimately they all will just be some constant
factor times the duty cycle. Also, in general the rectangular wave isn't
perfectly rectangular but exponential(depending on the capacitance and
switching frequency).

Now, it might be true that there is some optimal "function" to use that will
minimize power dissipation but I've not seen any research into things.
(probably the rectangle is simply the best when one considers all aspects)

 

[Rich]

"John Fields" <jfields@austininstruments.com> wrote in message
news:atbun4pcc7fjlhunpatif1l3hrjpvecmn4@4ax.com...

On Tue, 27 Jan 2009 15:50:19 -0000, "Rich" <notty@emailo.com> wrote:


"John Fields" <jfields@austininstruments.com> wrote in message
news:n84un4pdjlplqlk0uoapb4ppdpue7803b5@4ax.com...
On Tue, 27 Jan 2009 13:38:51 -0000, "Rich" <notty@emailo.com> wrote:

Can anyone please tell me what passes as PWM in Motor Control?

Is it limited to supplying the motor with a square wave, where a set and
constant level of voltage is switched on or off to the motor?

---
Yes, but not necessarily a square wave, which would represent a 50% duty
cycle PWM signal.

This is confusing actually when I think about it.

The question is asking is, is PWM limited to the switching on and off of a
contant dc voltage.

---
Yes, it is.

The problem seems to lie in the terminology, where a "square wave" is a
special case of a rectangular wave and is defined as being a signal
which has an ON time equal to its OFF time and, consequently, a duty
cycle of 50%.

Other rectangular waves with duty cycles not equal to 50% aren't, then,
square waves.

JF

Hi. I thought that's where the problem lies. "Square-wave" to you was
meaning the on/off waveform where the duty cycle is 50%. It really
is square when that pertains.

I was simply meaning a waveform that had a very steep rise and fall.

All sorted I think. :c)

 

[Tim Wescott]

On Tue, 27 Jan 2009 07:58:23 -0600, John Fields wrote:

On Tue, 27 Jan 2009 13:38:51 -0000, "Rich" <notty@emailo.com> wrote:

Can anyone please tell me what passes as PWM in Motor Control?

Is it limited to supplying the motor with a square wave, where a set and
constant level of voltage is switched on or off to the motor?

---
Yes, but not necessarily a square wave, which would represent a 50% duty
cycle PWM signal.

What's done is that the duty cycle of a constant voltage signal is
varied in order to change the speed of the motor. ---

Or can other waveforms consitutute PWM control of a dc motor? Such as
where voltage does not drop to zero, but has periodic peaks having
square, triangular, sawtooth or some other waveform?

---
No.
---

Can PWM control consist of dc pulses that rise and fall in voltage in a
sinusoidal fashion?

---
No.
---

At the moment I am believing that PWM is just on/off of a contant level
of dc voltage. But wonder if that belief is correct or not. Thanks.

---
It is.

JF

Switching between a supply voltage and open circuit is also called "PWM",
which is a bit of a misnomer but is in common use.

Otherwise, what John said.

--
http://www.wescottdesign.com

 

[Rich]

"Tim Wescott" <tim@seemywebsite.com> wrote in message
news:jpCdne2EGKyZp-LUnZ2dnUVZ_sLinZ2d@web-ster.com...

On Tue, 27 Jan 2009 07:58:23 -0600, John Fields wrote:

On Tue, 27 Jan 2009 13:38:51 -0000, "Rich" <notty@emailo.com> wrote:

Can anyone please tell me what passes as PWM in Motor Control?

Is it limited to supplying the motor with a square wave, where a set and
constant level of voltage is switched on or off to the motor?

---
Yes, but not necessarily a square wave, which would represent a 50% duty
cycle PWM signal.

What's done is that the duty cycle of a constant voltage signal is
varied in order to change the speed of the motor. ---

Or can other waveforms consitutute PWM control of a dc motor? Such as
where voltage does not drop to zero, but has periodic peaks having
square, triangular, sawtooth or some other waveform?

---
No.
---

Can PWM control consist of dc pulses that rise and fall in voltage in a
sinusoidal fashion?

---
No.
---

At the moment I am believing that PWM is just on/off of a contant level
of dc voltage. But wonder if that belief is correct or not. Thanks.

---
It is.

JF

Switching between a supply voltage and open circuit is also called "PWM",
which is a bit of a misnomer but is in common use.

Otherwise, what John said.

--
http://www.wescottdesign.com

I think you might say a sawtooth waveform or any waveform where there is a
period of voltage cutoff, fails to qualify if there is no *sharp* cut-off of
voltage.

It's the sudden "chopping action" that establishes a PWM voltage supply.

What I was trying to establish is what would pass as PWM motor control in
*DC motor control*.

Seems to me, that you make up your own mind, but many might say switching
between a constant voltage supply and open circuit is normally what is meant
by PWM in DC motor control.

Where the supply has a "pedestal" with peaks, no matter if the peaks are
square-wave, that's clearly not PWM.

Some might think, as long as there is a sudden chopping of the supply
voltage, irrespective of the waveform of the supply, be it DC , triangular,
sawtooth, whatever, passes as PWM in DC motor control.

I think you will think it does. Rich

 

[Jon Slaughter]

I think you might say a sawtooth waveform or any waveform where there is a
period of voltage cutoff, fails to qualify if there is no *sharp* cut-off
of
voltage.

It's the sudden "chopping action" that establishes a PWM voltage supply.

No, it is the averaging. It's a way to reduce an input's value. PWM is one
way, there are others such as PCM.

PWM is efficient because it achieves the reduction without wasting power. It
only wastes it during the transition from high to low or on to off.

Because PWM is generallly done by using mosfets, which are very efficient
switches... probably closest thing we have to mechanical switches, the best
method is simply your rectangular pulses. This is the most efficient with
method and generally the easiest.

Now, you can use triangular pulses too but you need to realize that it is
much more complex to do and probably is not as efficient.

I think your trying to understand it back-asswards. PWM is a techinique for
reducing the maximum output of something(usually voltage from a power
source). There as many other ways such as a resistor, linear regulator, SMPS
which uses PWM but makes it more useful for devices that need continuous
power).

Check out

http://en.wikipedia.org/wiki/Pulse-width_modulation

There might be a technique definition for PWM but generally it shouldn't
matter to much in practice. Rectangular pulses are almost always used
because it is the easiest and most efficient method.

It seems you are failing to understand why PWM is actually used for motor
control? (besides the obvious stuff you read) PWM is basically the idea of
averaging. A solar sail has many photons that hit the sail. The average
effect is force acting on the sail even though at any given moment no
significant force is acting on it.

An engine has the same effect. The pistions are fired by "pulses" from the
fuel igniting. At any given instance there is no force being created but on
average there is and it is felt as if it were a continuous force. This is a
form of "PWM"... at least abstractly.

V-----Motor----GND

If we just connect power to the motor it will run at full force. We cannot
control it's speed. It is pre-determined.

But suppose we put a switch in there

V---S---Motor---GND

If we turn the switch on and off then we reduce the average current to the
motor that simply depends on the off time to on time.

e.g., if we turned on the switch for 1 sec then off for 1 sec then we would
get only 50 of the power to the motor(ideally). If it was 1:2 then it would
be 33% of the power.

Since the power delivered is proportional to the speed(ideally) and hence
proprotional to the current, we would expect the speed to be reduced!!!

this is quite amazing!!!!! We can reduce the speed without costing any
power!!! If we used a resistor we would waste power in the resistor!!!!!

The method described is exactly what is done except a voltage controlled
switch, e.g. a mosfet, is used instead of our hand.

Note that if we do it too slow the motor will be jerky but if we do it
really fast it will seem continuous. This is because the motor has inertia
and for the switch was off it will still move as fast assuming it is not off
for too long.

 

[John Fields]

On Tue, 27 Jan 2009 19:05:18 -0000, "Rich" <notty@emailo.com> wrote:

"Tim Wescott" <tim@seemywebsite.com> wrote in message
news:jpCdne2EGKyZp-LUnZ2dnUVZ_sLinZ2d@web-ster.com...
On Tue, 27 Jan 2009 07:58:23 -0600, John Fields wrote:

On Tue, 27 Jan 2009 13:38:51 -0000, "Rich" <notty@emailo.com> wrote:

Can anyone please tell me what passes as PWM in Motor Control?

Is it limited to supplying the motor with a square wave, where a set and
constant level of voltage is switched on or off to the motor?

---
Yes, but not necessarily a square wave, which would represent a 50% duty
cycle PWM signal.

What's done is that the duty cycle of a constant voltage signal is
varied in order to change the speed of the motor. ---

Or can other waveforms consitutute PWM control of a dc motor? Such as
where voltage does not drop to zero, but has periodic peaks having
square, triangular, sawtooth or some other waveform?

---
No.
---

Can PWM control consist of dc pulses that rise and fall in voltage in a
sinusoidal fashion?

---
No.
---

At the moment I am believing that PWM is just on/off of a contant level
of dc voltage. But wonder if that belief is correct or not. Thanks.

---
It is.

JF

Switching between a supply voltage and open circuit is also called "PWM",
which is a bit of a misnomer but is in common use.

Otherwise, what John said.

--
http://www.wescottdesign.com

I think you might say a sawtooth waveform or any waveform where there is a
period of voltage cutoff, fails to qualify if there is no *sharp* cut-off of
voltage.

It's the sudden "chopping action" that establishes a PWM voltage supply.

What I was trying to establish is what would pass as PWM motor control in
*DC motor control*.

Seems to me, that you make up your own mind, but many might say switching
between a constant voltage supply and open circuit is normally what is meant
by PWM in DC motor control.

---
Interesting.

You know nothing about PWM and all of a sudden you're telling us what
"normally" is meant by 'PWM' in DC motor control?

Switching a load between a constant voltage supply and an open circuit
is _not_ the same as switching a load between a constant voltage supply
and ground, the former being the bit of a misnomer that Tim was
referring to.
---

Where the supply has a "pedestal" with peaks, no matter if the peaks are
square-wave, that's clearly not PWM.

Some might think, as long as there is a sudden chopping of the supply
voltage, irrespective of the waveform of the supply, be it DC , triangular,
sawtooth, whatever, passes as PWM in DC motor control.

I think you will think it does. Rich

---
Why would you think that?

Here, read this:

http://en.wikipedia.org/wiki/Pulse-width_modulation

JF

 

[John Fields]

On Tue, 27 Jan 2009 14:08:30 -0600, "Jon Slaughter"
<Jon_Slaughter@Hotmail.com> wrote:

Since the power delivered is proportional to the speed(ideally) and hence
proprotional to the current, we would expect the speed to be reduced!!!

this is quite amazing!!!!! We can reduce the speed without costing any
power!!! If we used a resistor we would waste power in the resistor!!!!!

The method described is exactly what is done except a voltage controlled
switch, e.g. a mosfet, is used instead of our hand.

Note that if we do it too slow the motor will be jerky but if we do it
really fast it will seem continuous. This is because the motor has inertia
and for the switch was off it will still move as fast assuming it is not off
for too long.

---
Nice post.

The other thing that's nice about PWM'ing a motor is that there's no
loss in torque as speed is lowered as it is when voltage is lowered.

JF

 

[Rich]

"Jon Slaughter" <Jon_Slaughter@Hotmail.com> wrote in message
news:7RJfl.14687$YU2.88@nlpi066.nbdc.sbc.com...

I think you might say a sawtooth waveform or any waveform where there is
a
period of voltage cutoff, fails to qualify if there is no *sharp* cut-off
of
voltage.

It's the sudden "chopping action" that establishes a PWM voltage supply.


No, it is the averaging. It's a way to reduce an input's value. PWM is one
way, there are others such as PCM.

PWM is efficient because it achieves the reduction without wasting power.
It only wastes it during the transition from high to low or on to off.

Because PWM is generallly done by using mosfets, which are very efficient
switches... probably closest thing we have to mechanical switches, the
best method is simply your rectangular pulses. This is the most efficient
with method and generally the easiest.

Now, you can use triangular pulses too but you need to realize that it is
much more complex to do and probably is not as efficient.

I think your trying to understand it back-asswards. PWM is a techinique
for reducing the maximum output of something(usually voltage from a power
source). There as many other ways such as a resistor, linear regulator,
SMPS which uses PWM but makes it more useful for devices that need
continuous power).

Check out

http://en.wikipedia.org/wiki/Pulse-width_modulation

There might be a technique definition for PWM but generally it shouldn't
matter to much in practice. Rectangular pulses are almost always used
because it is the easiest and most efficient method.

It seems you are failing to understand why PWM is actually used for motor
control? (besides the obvious stuff you read) PWM is basically the idea
of averaging. A solar sail has many photons that hit the sail. The average
effect is force acting on the sail even though at any given moment no
significant force is acting on it.

An engine has the same effect. The pistions are fired by "pulses" from the
fuel igniting. At any given instance there is no force being created but
on average there is and it is felt as if it were a continuous force. This
is a form of "PWM"... at least abstractly.

V-----Motor----GND

If we just connect power to the motor it will run at full force. We cannot
control it's speed. It is pre-determined.

But suppose we put a switch in there

V---S---Motor---GND

If we turn the switch on and off then we reduce the average current to the
motor that simply depends on the off time to on time.

e.g., if we turned on the switch for 1 sec then off for 1 sec then we
would get only 50 of the power to the motor(ideally). If it was 1:2 then
it would be 33% of the power.

Since the power delivered is proportional to the speed(ideally) and hence
proprotional to the current, we would expect the speed to be reduced!!!

this is quite amazing!!!!! We can reduce the speed without costing any
power!!! If we used a resistor we would waste power in the resistor!!!!!

The method described is exactly what is done except a voltage controlled
switch, e.g. a mosfet, is used instead of our hand.

Note that if we do it too slow the motor will be jerky but if we do it
really fast it will seem continuous. This is because the motor has inertia
and for the switch was off it will still move as fast assuming it is not
off for too long.

Hi.

What I meant by "It's the sudden "chopping action" that establishes a PWM
voltage supply." was that turning off and on, in a mechanical switch-like
manner, of any waveform, is a form of PWM. Because whatever the waveform is,
it's being averaged by the switching on and off in a switch-like manner.

Therefore I agree with what you said: "Now, you can use triangular pulses
too but you need to realize that it is much more complex to do and probably
is not as efficient."

My understanding is that you probably would not want to go for anything
other than turning on and off a steady dc voltage to achieve the efficiency
possible with PWM motor control.

I did in fact wonder if the following circuit was a curious case where a
waveform other than steady dc was being switched on and off. I even wondered
whether there was any full turn off of the voltage to the motor.

http://electronicdesign.com/Articles/Print.cfm?ArticleID=6315

I was having trouble getting a hang of what kind of reference voltage the
circuit was meant to be fed with, and what the sawtooth wave did to
proceedings.

No-one yet seems to have explained this circuit and whether it is a PWM
motor controller.

 

[Nobody]

On Tue, 27 Jan 2009 13:38:51 +0000, Rich wrote:

Can anyone please tell me what passes as PWM in Motor Control?

Is it limited to supplying the motor with a square wave, where a set and
constant level of voltage is switched on or off to the motor?

Or can other waveforms consitutute PWM control of a dc motor? Such as where
voltage does not drop to zero, but has periodic peaks having square,
triangular, sawtooth or some other waveform?

Can PWM control consist of dc pulses that rise and fall in voltage in a
sinusoidal fashion?

At the moment I am believing that PWM is just on/off of a contant
level of dc voltage. But wonder if that belief is correct or not. Thanks.

I would say that PWM has to be bi-level, i.e. with negligible rise/fall
times. One of those levels is usually zero, but I wouldn't exclude
switching between two fixed, non-zero levels from the definition. After
all, they're still pulses, and their width is being modulated.

E.g. for a analogue signalling circuit, if you have "off" = 1V (so you
can distinguish "off" from a broken cable) and "on" = 10V, and the level
is represented by the width of the pulse, it's still PWM.

Use of the term "PWM" in power conversion is a bit misleading to start
with, as it isn't actually the width of the pulses which matters, but the
mark-space ratio. Keeping the pulse-width (mark) fixed while changing the
spacing will change the power.

 

[Phil Allison]

"John Fields"

The other thing that's nice about PWM'ing a motor is that there's no
loss in torque as speed is lowered as it is when voltage is lowered.

** Completely wrong.

PWM of the DC supply to a motor has the same effect on available torque and
free running rpm as varying the DC voltage to that motor.




....... Phil

 

[Jon Slaughter]

Hi.

What I meant by "It's the sudden "chopping action" that establishes a PWM
voltage supply." was that turning off and on, in a mechanical switch-like
manner, of any waveform, is a form of PWM. Because whatever the waveform
is, it's being averaged by the switching on and off in a switch-like
manner.

The chopping action is just a convienient method because it's easy to do.
Mosfets are almost ideal switches and generally it's not a good idea to
operate them in the linear region. There reason is because they dissipate
more power(they act almost as a resistor).

Essentially it goes from 0 resistance to infinite resistance just like a
normal switch but it has a resistance inbetween. R = 0 ==> no power
dissipation. R = inf ==> no power dissipation. Else There is power
dissipation. Generally this is not a good idea for motors because of the
large currents. (I'm speaking in ideal terms here because in reality a
mosfet takes time to switch on)

Therefore I agree with what you said: "Now, you can use triangular pulses
too but you need to realize that it is much more complex to do and
probably is not as efficient."

My understanding is that you probably would not want to go for anything
other than turning on and off a steady dc voltage to achieve the
efficiency possible with PWM motor control.

Yes, this is generally the case.

I did in fact wonder if the following circuit was a curious case where a
waveform other than steady dc was being switched on and off. I even
wondered whether there was any full turn off of the voltage to the motor.

http://electronicdesign.com/Articles/Print.cfm?ArticleID=6315

I was having trouble getting a hang of what kind of reference voltage the
circuit was meant to be fed with, and what the sawtooth wave did to
proceedings.

No-one yet seems to have explained this circuit and whether it is a PWM
motor controller.

Note they are not using mosfets!!! This should tell you that they probably
are not using PWM. (rarely would you use anything but mosfets for PWM)

They are simply using a current driver for the motor. They are using
feedback to determine how much current to dump into the motor using the
bjt's. The bjt's are current controlled current sources. They dump so much
current it, if the motor is going to fast they reduce the current.

It's not PWM but a simple current driven system with feedback. Think of it
as a current power source supplying the motor.

This is not the best method in general but maybe for some specific
application it is good.

So here you are getting confused in thinking that they are using PWM when
there is no pulsing.

In some sense this is just an active "resistor" where the resistance is
proportional the motor speed. 0 rpm's means 0 resistance => causes motor to
spin faster. large rpm's means large resistance => cause the motor to spin
slower.

PWM works differently but abstractly it is the same. I wouldn't say it is
PWM because it is not pulsing anything. There is no "duty cycle".

 

[Rich]

"Jon Slaughter" <Jon_Slaughter@Hotmail.com> wrote in message
news:2QMfl.9562$8_3.4563@flpi147.ffdc.sbc.com...

Hi.

What I meant by "It's the sudden "chopping action" that establishes a PWM
voltage supply." was that turning off and on, in a mechanical switch-like
manner, of any waveform, is a form of PWM. Because whatever the waveform
is, it's being averaged by the switching on and off in a switch-like
manner.


The chopping action is just a convienient method because it's easy to do.
Mosfets are almost ideal switches and generally it's not a good idea to
operate them in the linear region. There reason is because they dissipate
more power(they act almost as a resistor).

Essentially it goes from 0 resistance to infinite resistance just like a
normal switch but it has a resistance inbetween. R = 0 ==> no power
dissipation. R = inf ==> no power dissipation. Else There is power
dissipation. Generally this is not a good idea for motors because of the
large currents. (I'm speaking in ideal terms here because in reality a
mosfet takes time to switch on)



Therefore I agree with what you said: "Now, you can use triangular pulses
too but you need to realize that it is much more complex to do and
probably is not as efficient."

My understanding is that you probably would not want to go for anything
other than turning on and off a steady dc voltage to achieve the
efficiency possible with PWM motor control.


Yes, this is generally the case.

I did in fact wonder if the following circuit was a curious case where a
waveform other than steady dc was being switched on and off. I even
wondered whether there was any full turn off of the voltage to the motor.

http://electronicdesign.com/Articles/Print.cfm?ArticleID=6315

I was having trouble getting a hang of what kind of reference voltage the
circuit was meant to be fed with, and what the sawtooth wave did to
proceedings.

No-one yet seems to have explained this circuit and whether it is a PWM
motor controller.


Note they are not using mosfets!!! This should tell you that they probably
are not using PWM. (rarely would you use anything but mosfets for PWM)

They are simply using a current driver for the motor. They are using
feedback to determine how much current to dump into the motor using the
bjt's. The bjt's are current controlled current sources. They dump so much
current it, if the motor is going to fast they reduce the current.

It's not PWM but a simple current driven system with feedback. Think of it
as a current power source supplying the motor.

This is not the best method in general but maybe for some specific
application it is good.

So here you are getting confused in thinking that they are using PWM when
there is no pulsing.

In some sense this is just an active "resistor" where the resistance is
proportional the motor speed. 0 rpm's means 0 resistance => causes motor
to spin faster. large rpm's means large resistance => cause the motor to
spin slower.

PWM works differently but abstractly it is the same. I wouldn't say it is
PWM because it is not pulsing anything. There is no "duty cycle".

Thanks. I think what you wrote is beginng to help me understand that
particular circuit. I need to ponder what you said until I fully grasp
things.

As a novice, I think I just happened to pick on a tricky circuit to explain.
I was looking for a PWM circuit that had speed regulation and I thought that
perhaps that circuit was such a circuit. It now seem I have the answer I
have been seeking - this is not a PWM controller that will drive well at
slow speeds, unlike PWM that tends to do that quite well.

Just one more thing: What do you think the reference input is meant to be
where it is marked "Input voltage from computer"?

 

[Jamie]

Rich wrote:

Can anyone please tell me what passes as PWM in Motor Control?

Is it limited to supplying the motor with a square wave, where a set and
constant level of voltage is switched on or off to the motor?

Or can other waveforms consitutute PWM control of a dc motor? Such as where
voltage does not drop to zero, but has periodic peaks having square,
triangular, sawtooth or some other waveform?

Can PWM control consist of dc pulses that rise and fall in voltage in a
sinusoidal fashion?

At the moment I am believing that PWM is just on/off of a contant level
of dc voltage. But wonder if that belief is correct or not. Thanks.
Yes, its an on/off pulse. The duty cycle dictates the resulting

voltage through a filter.

Many circuits can filter a PWM signal to produce useable DC control
even though the circuit wasn't designed with that in mind.


http://webpages.charter.net/jamie_5"