Simple 555 PWM - disappointing performance

[Terry Pinnell]

Tony Williams <tonyw@ledelec.demon.co.uk> wrote:

In article <8eji91h3197k3a837hgcmuq7r63rcqovif@4ax.com>,
Terry Pinnell <terrypinDELETE@THESEdial.pipex.com> wrote:

The motor looks like this;

I Rmotor Lmotor
V-Atmotor +->----/\/\----))))---[Vback-emf]---+0v

Under steady state (dc) conditions the motor equation is.

V-Atmotor = I*Rmotor + Vback-emf.

Voltage (DC V)
Test description At source At motor Current (A)
---------------- --------- -------- -----------
- Stalled 0.98 0.34 4.4
- Stalled 1.67 0.67 8.5
- Stalled 2.19 0.95 11.5
- Stalled 2.77 1.26 14.1

When stalled, Vback-emf = Zero.

So V-Atmotor = I*Rmotor, or Rmotor = V-Atmotor/I.

So Rmotor = .34/4.4 or .67/8.5 or 0.95/11.5, or 1.26/14.1
which nicely averages out to Rmotor= 0.08 ohms.
~~~~~~~~~~~~~~~~~

- Motor unloaded 1.18 0.96 1.8 (0.816)
- Motor unloaded 2.35 2.12 2.1 (1.952)
- Motor unloaded 3.64 3.41 2.3 (3.226)
- Motor unloaded 4.97 4.74 2.5 (4.54 )

When running, the equation is V-Atmotor = I*Rmotor + Vback-emf.

eg, 0.96 = 1.8A*0.08ohms + (Vbemf at that speed, = 0.816V).

The added things in brackets are the calc'd internal Vbemfs
that the motor generates at the various rpm's.

That gearbox-only load will have a fixed current representing
the fixed friction, in parallel with a speed-dependant load,
ie, proportional to the Vbemf.

It roughly looks like Imotor = 1.7A + Vbemf/5.3ohms in the
unloaded condition. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I 0.08R Lmotor
V-Atmotor +->----/\/\----))))-+-[Vback-emf]-+-+0v
| |
Fixed loss--> +--[ 1.7A ]---+
| |
Speed-dependant loss--> +----/\/\/----+
| 5.3R |
| |
. .
. .
+---[I-Ext]---+

An external torque load will be another current in parallel
with Vbemf.

Excellent, thanks. Will get stuck into that asap.

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[Ban]

Terry Pinnell wrote:

"Ban" <bansuri@web.de> wrote:



To learn something needs thorough understanding of the proper
function. The same is valid for measuring, especially on a motor. A
motor has an inductance, which can be measured at stall, but when
turning an additional voltage source comes in play(Back EMF), so the
only way to determine the force is to measure the current through
the motor, which Terry so far has not managed to do. Exept a few
static values. So we do not know if the repetition rate of the PWM
is too high.

You're right that I've not so far managed to do that. I'm still
working at it though, as it interests me. What practical further
advice do you have on measuring this motor's dynamic torque?

He also seems to insist to use the
"quarter-bridge" he has found somewhere on the web together with the
3055. I do not see any insight here, so whatever anybody suggests
takes a long time to be evaluated and the final result will be
accordingly.

You really do have a bee in your bonnet about this, don't you, Ban!
Are you on some sort of worldwide commission scheme for promoting the
bridge approach? I explained this at least once up-thread. One more
time: the project was finished a YEAR ago. Past tense. So, with due
allowance for your english, it makes no sense to say 'he also seems to
insist to use the "quarter-bridge"...'

If you want to criticise a decision I took a year ago, fine, but
that's OT from this thread. FWIW, I was fully aware then of the
relative advantages and disadvantages of different approaches,
including the half and full bridges. Ever think there might have been
reasons for going the relay-based route? Ever think there might be
practical reasons for not now scrapping a successfully working project
and starting from scratch, to achieve a minor improvement?

And what's this all about: "he has found somewhere on the web together
with the 3055."? Have you got something against 'the web', BTW? And
what makes you think I 'found' that trivially simple 555/3055 PWM
circuit anywhere? I've been building circuits with 555s and/or 2N3055s
for over 25 years, so why would I need to 'find' it?

I do not see any insight here, so whatever anybody suggests takes a
long time to be evaluated and the final result will be accordingly.

English may not be your first language, but you're not bad on the
personal insults! Lighten up, Ban, you're a grandad now, remember? And
maybe read the threads a little more carefully, and consider the
background, before taking such a patronising and sanctimonious tone.

BTW, I'm curious what 'the final result' is supposed to mean? Do you
mean the improvement of my curtain controller - i.e. slowing it down
while maintaining adequate torque, the stated objective? Or my
ultimate personal destiny as an electronics hobbyist? <g

Have a nice Sunday!

Yeah, your last measurements confirm what almost everyone has noticed, the
555 together with the 3055 are not up to the job. It was not only the power
supply as you thought, but the whole concept.
When making a motor controller, you first collect data on the motor, so 0.08
ohms DC-resistance is the most important, because only with at least 25A you
will reach the desired starting torque, no need to measure this
mechanically. The number comes from your desciption of 2.4V supply with
2NiCads in series.
Just this single number will already give the amount of inadaequacy of your
concept. No need to solder and measure much.
It is a pity when so much thinking soldering and measuring is wasted to get
an unusable circuit.
When I make a little circuit like this, I want to have a better result than
what can be commercially bought or at least equal, otherwise it would be
smarter to buy a RC-racing car motor controller or some industrial circuit.
How can a hobby be fun, if you arrive at the situation that you better had
left this alone?
Your other responses also show that despite all the help here, you have not
understood the function of a motor let alone the idea of PWM.
It is not an insult if somebody expresses this, or do you want to become a
Burridge? Better to read up a bit, with the Net giving so much useful
information. When you feel insulted than just because I expressed what
everyone else thought too, and I happened to press the button. )
What I consider insulting is when someone writes: His stupidity is so gross,
that also here he chose a completly wrong concept, a waste of bandwidth
etc.etc. compare that with my words above.
I apologize for suspecting that you found the circuit "on the Net", when you
made it up yourself. Maybe a smaller motor will even do with it, despite the
fact that there is no regulation, which requires some sort of feedback.
Terry, take it easy. I just felt to express a bit of critics and you feel
insulted? Have a good laugh and accept that, next time "JT" might press the
button... I have heard the English humour is famous, look how Mr Atkinson
can express it by making fun with himself. Nevertheless I can appreciate
your contribution to this group.
I'm going swimming now, so I will have a nice sunday. You will see me
passing on the left webcam in a couple of minutes.
http://www.bordighera.it/Telecamere/tlc.html
--
ciao Ban
Bordighera, Italy

 

[John Fields]

On Sun, 29 May 2005 14:43:11 GMT, "Ban" <bansuri@web.de> wrote:

Yeah, your last measurements confirm what almost everyone has noticed, the
555 together with the 3055 are not up to the job. It was not only the power
supply as you thought, but the whole concept.
When making a motor controller, you first collect data on the motor, so 0.08
ohms DC-resistance is the most important, because only with at least 25A you
will reach the desired starting torque, no need to measure this
mechanically. The number comes from your desciption of 2.4V supply with
2NiCads in series.
Just this single number will already give the amount of inadaequacy of your
concept. No need to solder and measure much.
It is a pity when so much thinking soldering and measuring is wasted to get
an unusable circuit.
When I make a little circuit like this, I want to have a better result than
what can be commercially bought or at least equal, otherwise it would be
smarter to buy a RC-racing car motor controller or some industrial circuit.
How can a hobby be fun, if you arrive at the situation that you better had
left this alone?

---
It's called experimentation, and the fun is often in the journey, not
necessarily in just getting to the destination with a cut-and-dried
solution.
---

Your other responses also show that despite all the help here, you have not
understood the function of a motor let alone the idea of PWM.
It is not an insult if somebody expresses this, or do you want to become a
Burridge? Better to read up a bit, with the Net giving so much useful
information. When you feel insulted than just because I expressed what
everyone else thought too, and I happened to press the button. )

---
Are you so thick that you can't understand that using derisive
language can't be construed as anything but an insult? Or gross
insensitivity? Go back and read what you wrote and try to think how
it would make you feel if you were in Terry's place and he was
pontificating about the inadequacy of your concept, your ignorance,
and about how you should be conducting _your_ hobby.
---

What I consider insulting is when someone writes: His stupidity is so gross,
that also here he chose a completly wrong concept, a waste of bandwidth
etc.etc. compare that with my words above.

---
OK.

"Just this single number will already give the amount of inadaequacy
of your concept."

sounds to me to be very close to

"His stupidity is so gross, that also here he chose a completly wrong
concept."

Don't you agree?
---

I apologize for suspecting that you found the circuit "on the Net", when you
made it up yourself. Maybe a smaller motor will even do with it, despite the
fact that there is no regulation, which requires some sort of feedback.
Terry, take it easy. I just felt to express a bit of critics and you feel
insulted? Have a good laugh and accept that, next time "JT" might press the
button... I have heard the English humour is famous, look how Mr Atkinson
can express it by making fun with himself.

---
Yes, but that's Mr. Atkinson poking fun at _himself_, not someone else
poking fun at or trying to ridicule him.

Something like if I wrote, "What's the thinnest book in the world?"
and _you_ replied, "Italian War Heroes" before _I_ did.
---

Nevertheless I can appreciate
your contribution to this group.
I'm going swimming now, so I will have a nice sunday. You will see me
passing on the left webcam in a couple of minutes.
http://www.bordighera.it/Telecamere/tlc.html

--
John Fields
Professional Circuit Designer

 

[Terry Pinnell]

Tony Williams <tonyw@ledelec.demon.co.uk> wrote:

In article <8eji91h3197k3a837hgcmuq7r63rcqovif@4ax.com>,
Terry Pinnell <terrypinDELETE@THESEdial.pipex.com> wrote:

The motor looks like this;

I Rmotor Lmotor
V-Atmotor +->----/\/\----))))---[Vback-emf]---+0v

Under steady state (dc) conditions the motor equation is.

V-Atmotor = I*Rmotor + Vback-emf.

Voltage (DC V)
Test description At source At motor Current (A)
---------------- --------- -------- -----------
- Stalled 0.98 0.34 4.4
- Stalled 1.67 0.67 8.5
- Stalled 2.19 0.95 11.5
- Stalled 2.77 1.26 14.1

When stalled, Vback-emf = Zero.

So V-Atmotor = I*Rmotor, or Rmotor = V-Atmotor/I.

So Rmotor = .34/4.4 or .67/8.5 or 0.95/11.5, or 1.26/14.1
which nicely averages out to Rmotor= 0.08 ohms.
~~~~~~~~~~~~~~~~~

- Motor unloaded 1.18 0.96 1.8 (0.816)
- Motor unloaded 2.35 2.12 2.1 (1.952)
- Motor unloaded 3.64 3.41 2.3 (3.226)
- Motor unloaded 4.97 4.74 2.5 (4.54 )

When running, the equation is V-Atmotor = I*Rmotor + Vback-emf.

eg, 0.96 = 1.8A*0.08ohms + (Vbemf at that speed, = 0.816V).

The added things in brackets are the calc'd internal Vbemfs
that the motor generates at the various rpm's.

That gearbox-only load will have a fixed current representing
the fixed friction, in parallel with a speed-dependant load,
ie, proportional to the Vbemf.

It roughly looks like Imotor = 1.7A + Vbemf/5.3ohms in the
unloaded condition. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I 0.08R Lmotor
V-Atmotor +->----/\/\----))))-+-[Vback-emf]-+-+0v
| |
Fixed loss--> +--[ 1.7A ]---+
| |
Speed-dependant loss--> +----/\/\/----+
| 5.3R |
| |
. .
. .
+---[I-Ext]---+

An external torque load will be another current in parallel
with Vbemf.

Thanks again for that model, Tony, and your worked examples. I've made
real progress as a result of your practical help. Using the second
set, here's my first attempt at a simulation, with some explanatory
notes.
http://www.terrypin.dial.pipex.com/Images/PWM-Sim2.gif

As you see, quite a gap to resolve though! Had to finish this in
rather a hurry (my wife just asked me in frosty tones why I didn't
marry my computer - always a bad sign!), so I suspect an error. Can
you or anyone else see any obvious slip please?

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[Bob Monsen]

Terry Pinnell wrote:

Bob Monsen <rcsurname@comcast.net> wrote:



I've been playing with the motor model that was suggested by someone
upthread. However, the parameters required for the model are things like
motor inductance and resistance, a 'back emf' constant, and torque at a
particular current. I tried varying the particular values to match your
dataset, but was unsuccessful. You may want to give it a try, though. It
was here:

http://www.ecircuitcenter.com/Circuits/dc_motor_model/DCmotor_model.htm

Here is an LTSpice implementation of their model:


snipped very useful LTSpice circuit

That's great, thanks. I had seen the model and article (linked by Mike
Monett) but not had time to study it. Your LTSpice circuit will make
that easier, although I'm not very skilled at working with LTSpice.
Have you now switched from Circuitmaker, BTW?

I have switched. I was using the student version of CM, and was tired of
not being able to do temperature simulations. I admit to liking the user
interface of CM a bit more, but LTS isn't bad once you get used to it.
It's also far more flexible, and allows bigger circuits.

Did you also see Tony's earlier LTSpice model up-thread?

I did see it, but I haven't tried it out.

---
Regards,
Bob Monsen

 

[John Fields]

On Sun, 29 May 2005 18:30:50 +0100, Terry Pinnell
<terrypinDELETE@THESEdial.pipex.com> wrote:

Had to finish this in
rather a hurry (my wife just asked me in frosty tones why I didn't
marry my computer - always a bad sign!), so I suspect an error. Can
you or anyone else see any obvious slip please?

---
You married her before you knew what she was really like?^)

--
John Fields
Professional Circuit Designer

 

[Terry Pinnell]

John Fields <jfields@austininstruments.com> wrote:

On Sun, 29 May 2005 18:30:50 +0100, Terry Pinnell
terrypinDELETE@THESEdial.pipex.com> wrote:

Had to finish this in
rather a hurry (my wife just asked me in frosty tones why I didn't
marry my computer - always a bad sign!), so I suspect an error. Can
you or anyone else see any obvious slip please?

---
You married her before you knew what she was really like?^)

Something like that! Or maybe the reverse... <g>

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[Tony Williams]

In article <2luj91t5favbeoaknmk1nkqcqbutiv474n@4ax.com>,
Terry Pinnell <terrypinDELETE@THESEdial.pipex.com> wrote:

http://www.terrypin.dial.pipex.com/Images/PWM-Sim2.gif

You have a slight misunderstanding Terry.

Vbemf is a *result* of what happens when the motor
spins, so you can't force a value for it. You can
only apply power to the motor and use the resultant
Vbemf to see what speed the motor runs at, at that
torque load.

Suggest: Remove Vback entirely.
Increase C1 to 100,000uF.
Reduce L1 to 100uH.
Raise Vsupply to 4v at least, and give it
an R-source of 0.1 to 0.2 ohms.

Run the simulation again, and you should see something
rather nasty happening to the 2N3055. At that low base
current it operates as a constant current sink of about
3A and will be pulled out of saturation above that.

It means that you are on a hiding to nothing with the
circuit at present.

--
Tony Williams.

 

[Terry Pinnell]

Tony Williams <tonyw@ledelec.demon.co.uk> wrote:

In article <2luj91t5favbeoaknmk1nkqcqbutiv474n@4ax.com>,
Terry Pinnell <terrypinDELETE@THESEdial.pipex.com> wrote:

http://www.terrypin.dial.pipex.com/Images/PWM-Sim2.gif

You have a slight misunderstanding Terry.

Vbemf is a *result* of what happens when the motor
spins, so you can't force a value for it. You can
only apply power to the motor and use the resultant
Vbemf to see what speed the motor runs at, at that
torque load.

Suggest: Remove Vback entirely.
Increase C1 to 100,000uF.
Reduce L1 to 100uH.
Raise Vsupply to 4v at least, and give it
an R-source of 0.1 to 0.2 ohms.

Run the simulation again, and you should see something
rather nasty happening to the 2N3055. At that low base
current it operates as a constant current sink of about
3A and will be pulled out of saturation above that.

Thanks, Tony, appreciate your patience. I'd assumed I could plug that
previously calculated value of V back into the sima, so clearly I had
indeed misunderstood.

Do you mean like this?
http://www.terrypin.dial.pipex.com/Images/PWM3.gif

It means that you are on a hiding to nothing with the
circuit at present.

Understood, but my immediate interest is seeing how closely I can
simulate the circuit's actual waveform, channel B (red) in
http://www.terrypin.dial.pipex.com/Images/PWM555-10.gif

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[Tony Williams]

In article <ehll91lif42mjh7its0s317jh3gv00d9cb@4ax.com>,
Terry Pinnell <terrypinDELETE@THESEdial.pipex.com> wrote:

Do you mean like this?
http://www.terrypin.dial.pipex.com/Images/PWM3.gif

Re-run the simulation with R1= 10 ohms, get some
base current into Q1.

Make sure the simulation runs long enough (5-10 secs)
for the 'motor' to run up and reach a steady speed.

It means that you are on a hiding to nothing with the
circuit at present.

Understood, but my immediate interest is seeing how closely I can
simulate the circuit's actual waveform, channel B (red) in
http://www.terrypin.dial.pipex.com/Images/PWM555-10.gif

That collector waveform is odd.

When On; that 2N3055, (with only 150mA of base current)
bottoms nicely at about 0.5V. It suggests either a super
2N3055 or a collector current of less than 3A.

When Off is the real oddity though. Your scope picture
shows the collector voltage going to a nice flat Vsupply,
which suggest no Vbemf. If there were a back-emf the
Off collector voltage should initially rise from the
saturation voltage to (Vsupply - Vbemf). Then it should
ramp up slightly as Vbemf falls (as the torque slows a
motor without power).

--
Tony Williams.

 

[Terry Pinnell]

Tony Williams <tonyw@ledelec.demon.co.uk> wrote:

In article <ehll91lif42mjh7its0s317jh3gv00d9cb@4ax.com>,
Terry Pinnell <terrypinDELETE@THESEdial.pipex.com> wrote:

Do you mean like this?
http://www.terrypin.dial.pipex.com/Images/PWM3.gif

Re-run the simulation with R1= 10 ohms, get some
base current into Q1.

Make sure the simulation runs long enough (5-10 secs)
for the 'motor' to run up and reach a steady speed.

OK, thanks. I've made both those changes and they do make a
significant difference to the simulated waveform, as you see here:
http://www.terrypin.dial.pipex.com/Images/PWM4-5s.gif

It means that you are on a hiding to nothing with the
circuit at present.

Understood, but my immediate interest is seeing how closely I can
simulate the circuit's actual waveform, channel B (red) in
http://www.terrypin.dial.pipex.com/Images/PWM555-10.gif

That collector waveform is odd.

When On; that 2N3055, (with only 150mA of base current)
bottoms nicely at about 0.5V. It suggests either a super
2N3055 or a collector current of less than 3A.

When Off is the real oddity though. Your scope picture
shows the collector voltage going to a nice flat Vsupply,
which suggest no Vbemf. If there were a back-emf the
Off collector voltage should initially rise from the
saturation voltage to (Vsupply - Vbemf). Then it should
ramp up slightly as Vbemf falls (as the torque slows a
motor without power).

During a repeat experiment today I was apparently getting a minimum of
only around 1.8-2V at 2N3055 collector, so now unsure about that
previous waveform. Tomorrow I'll repeat yet again, more carefully. I
suspect positioning of 'scope return probe was not correct.

BTW, are those 'spikes' on the leading and trailing edge of the base
voltage (yellow plot) OK?

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[Tony Williams]

In article <v8em911ar13rrjbutkfshf5duhms685ma8@4ax.com>,
Terry Pinnell <terrypinDELETE@THESEdial.pipex.com> wrote:

OK, thanks. I've made both those changes and they do make a
significant difference to the simulated waveform, as you see here:
http://www.terrypin.dial.pipex.com/Images/PWM4-5s.gif

That's it Terry. When the transistor goes Off there
is the initial big spike from the inductor, clamped
by the flyback diode. Then the collector settles at
about 1.4V, (Vsupply - Vbemf), followed by a slight
upward ramp as the gearbox load torque slows the
rotor slightly. The Vbemf voltage across C1 should
should show a triangular voltage with about 2.5Vdc
average value.

Simulate an external load by increasing that 1uA
second cc sink, and watch the Vbemf reduce as the
'motor' slows down. You can go up to 2A before my
model goes wrong.

During a repeat experiment today I was apparently getting a
minimum of only around 1.8-2V at 2N3055 collector.....
[snip]

That's it. That 2N3055 is being pulled out of saturation.

It means that you either have to drastically increase
the base current or move to a low Rds-on MOSFET.

BTW, are those 'spikes' on the leading and trailing edge of the
base voltage (yellow plot) OK?

I think so, yes. The inductor means that the
collector voltage has fast edges and these may
be being capacitively coupled onto the base.

--
Tony Williams.

 

[Terry Pinnell]

Tony Williams <tonyw@ledelec.demon.co.uk> wrote:

In article <v8em911ar13rrjbutkfshf5duhms685ma8@4ax.com>,
Terry Pinnell <terrypinDELETE@THESEdial.pipex.com> wrote:

OK, thanks. I've made both those changes and they do make a
significant difference to the simulated waveform, as you see here:
http://www.terrypin.dial.pipex.com/Images/PWM4-5s.gif

That's it Terry. When the transistor goes Off there
is the initial big spike from the inductor, clamped
by the flyback diode. Then the collector settles at
about 1.4V, (Vsupply - Vbemf), followed by a slight
upward ramp as the gearbox load torque slows the
rotor slightly. The Vbemf voltage across C1 should
should show a triangular voltage with about 2.5Vdc
average value.

Simulate an external load by increasing that 1uA
second cc sink, and watch the Vbemf reduce as the
'motor' slows down. You can go up to 2A before my
model goes wrong.


During a repeat experiment today I was apparently getting a
minimum of only around 1.8-2V at 2N3055 collector.....
[snip]

That's it. That 2N3055 is being pulled out of saturation.

It means that you either have to drastically increase
the base current or move to a low Rds-on MOSFET.

BTW, are those 'spikes' on the leading and trailing edge of the
base voltage (yellow plot) OK?

I think so, yes. The inductor means that the
collector voltage has fast edges and these may
be being capacitively coupled onto the base.

At last, a result! Much appreciated your help on this, Tony. I'll
experiment a bit further along the lines you suggest.

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[Tony Williams]

In article <qv0o91t03i775dq33e1logjsgp3e39rfkp@4ax.com>,
Terry Pinnell <terrypinDELETE@THESEdial.pipex.com> wrote:

BTW, are those 'spikes' on the leading and trailing edge of the
base voltage (yellow plot) OK?

I think so, yes. The inductor means that the
collector voltage has fast edges and these may
be being capacitively coupled onto the base.

At last, a result! Much appreciated your help on this, Tony. I'll
experiment a bit further along the lines you suggest.

By the way. Your actual scoped spikes are much shorter
than the simulated spikes (with Lm = 100uH). That does
suggest that your 12uH measurement may have been in
the right ballpark.

With such a low L/R time constant, clocking the PWM at
300Hz is not good news. The motor runs in discontinuous
current mode, (poor torque-speed characteristic), and
the repetitive high peak currents results in higher
power losses in all resistive components (Rmotor!).

For smooth and efficient running the PWM should be clocked
at a period which is small compared to the L/R.

12uH/0.08R is 150uS, and 1/5th of that is 30us. So the
PWM should be clocked at about 30KHz for this motor.

This is reminiscent of another dc motor thread some months
ago, where the OP was using a 24Vdc Maxon motor, which was
running hot when clocked at 7.8KHz. The motor had an L/R
of 0.08mH/0.318R, which suggests a clock of not less than
20KHz, and (afair) Maxon's own driver did turn out to be
at 50KHz.

--
Tony Williams.

 

[Terry Pinnell]

Tony Williams <tonyw@ledelec.demon.co.uk> wrote:

In article <429739F1.199B@spam.com>,
Mike Monett <no@spam.com> wrote:

Tony, I just popped in briefly and probably missed a lot of
important stuff. But I came across this SPICE model that may
have some bearing on the problem:

http://www.ecircuitcenter.com/Circuits/dc_motor_model/DCmotor_model.htm

Nice one Mike, thank you. That looks like it will
enable a far better modelling of the generated Vbemf.

For my own education, I ran that example in CircuitMaker and added a
few explanatory notes:
http://www.terrypin.dial.pipex.com/Images/DCMotorExplanation.gif

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[Terry Pinnell]

Tony Williams <tonyw@ledelec.demon.co.uk> wrote:

In article <qv0o91t03i775dq33e1logjsgp3e39rfkp@4ax.com>,
Terry Pinnell <terrypinDELETE@THESEdial.pipex.com> wrote:

BTW, are those 'spikes' on the leading and trailing edge of the
base voltage (yellow plot) OK?

I think so, yes. The inductor means that the
collector voltage has fast edges and these may
be being capacitively coupled onto the base.

At last, a result! Much appreciated your help on this, Tony. I'll
experiment a bit further along the lines you suggest.

By the way. Your actual scoped spikes are much shorter
than the simulated spikes (with Lm = 100uH). That does
suggest that your 12uH measurement may have been in
the right ballpark.

With such a low L/R time constant, clocking the PWM at
300Hz is not good news. The motor runs in discontinuous
current mode, (poor torque-speed characteristic), and
the repetitive high peak currents results in higher
power losses in all resistive components (Rmotor!).

For smooth and efficient running the PWM should be clocked
at a period which is small compared to the L/R.

12uH/0.08R is 150uS, and 1/5th of that is 30us. So the
PWM should be clocked at about 30KHz for this motor.

Thanks, I'll try that. BTW, it was 14uH. (The Atlas LCR consistently
read 14.2 uH on several readings with two motors.) So I'll try 35kHz
for starters.

This is reminiscent of another dc motor thread some months
ago, where the OP was using a 24Vdc Maxon motor, which was
running hot when clocked at 7.8KHz. The motor had an L/R
of 0.08mH/0.318R, which suggests a clock of not less than
20KHz, and (afair) Maxon's own driver did turn out to be
at 50KHz.

Lots more complexity to modeling a small DC motor than I'd expected!

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[Terry Pinnell]

Terry Given <my_name@ieee.org> wrote:

Two comments re. your circuits.

Firstly, you brute! 100nF slapped on the output of poor old CMOS gates.
Still, rise time is slow, around 300ns IIRC, so I = C*dV/dt =
0.1uF*13V/0.3us = 4A, IOW it'll take a month of sundays to rise, and
frighten the pants off of the cmos gate. I would have slicer R3,R4 in
half, and popped the caps in there.

Hadn't thought about those sort of implications - thanks.

Secondly, the caps across the 2 switches. These will, at turn-on, pull
4001-6 and 4001-13 high (actually it will be an exponential spike, time
constant 10ms). Whether or not this is an issue, I leave as an exercise
to the reader (IOW I didnt check) but its something to be aware of.

Not sure I follow you; could you amplify please?. The idea of both
those RC filters was twofold:

- reduce switch bounce (although, in theory, it shouldn't really
matter, as the bistables should latch on only the very first pulse).

- reduce noise, as the wiring to the main unit was likely to be quite
long.

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[Terry Given]

Terry Pinnell wrote:

Terry Given <my_name@ieee.org> wrote:


Two comments re. your circuits.

Firstly, you brute! 100nF slapped on the output of poor old CMOS gates.
Still, rise time is slow, around 300ns IIRC, so I = C*dV/dt =
0.1uF*13V/0.3us = 4A, IOW it'll take a month of sundays to rise, and
frighten the pants off of the cmos gate. I would have slicer R3,R4 in
half, and popped the caps in there.


Hadn't thought about those sort of implications - thanks.


Secondly, the caps across the 2 switches. These will, at turn-on, pull
4001-6 and 4001-13 high (actually it will be an exponential spike, time
constant 10ms). Whether or not this is an issue, I leave as an exercise
to the reader (IOW I didnt check) but its something to be aware of.


Not sure I follow you; could you amplify please?. The idea of both
those RC filters was twofold:

- reduce switch bounce (although, in theory, it shouldn't really
matter, as the bistables should latch on only the very first pulse).

- reduce noise, as the wiring to the main unit was likely to be quite
long.

that they do.

Initially, these capacitors have no charge. that means Vcap = 0. At the
"instant" of turn on, the +13V supply rises (however fast or slow) to
+13V, but there is 0V across each of the debounce/filter caps. for the
caps to 0V, this means a logic zero. for the caps to +13V, (open/stop
and close/stop switches) the resistor is *initially* +13V and hence
logic level 1. Then the cap charges (to +13V) thru the 100k resistor,
time constant 10ms, resulting in an exponential decay to 0V, at which
point (really depends on gate Vil) the logic level becomes the zero you
expect..

If the +13V supply rises much slower than the RC time constant, this
effect can be masked. its quite common when using lab supplies,
especially if you turn them "off" with the current limit dial, like I do

I've shot myself in the foot just recently with exactly that - a
TUSB3210 based system, the micro's oscillator *wont start* if the supply
voltage ramps up too slow! I spent a few hours trying to figure out why
my programmed micro lit up LEDs it shouldnt....doh.

Cheers
Terry

 

[Terry Pinnell]

Terry Given <my_name@ieee.org> wrote:

Terry Pinnell wrote:
Terry Given <my_name@ieee.org> wrote:


Two comments re. your circuits.

Firstly, you brute! 100nF slapped on the output of poor old CMOS gates.
Still, rise time is slow, around 300ns IIRC, so I = C*dV/dt =
0.1uF*13V/0.3us = 4A, IOW it'll take a month of sundays to rise, and
frighten the pants off of the cmos gate. I would have slicer R3,R4 in
half, and popped the caps in there.


Hadn't thought about those sort of implications - thanks.


Secondly, the caps across the 2 switches. These will, at turn-on, pull
4001-6 and 4001-13 high (actually it will be an exponential spike, time
constant 10ms). Whether or not this is an issue, I leave as an exercise
to the reader (IOW I didnt check) but its something to be aware of.


Not sure I follow you; could you amplify please?. The idea of both
those RC filters was twofold:

- reduce switch bounce (although, in theory, it shouldn't really
matter, as the bistables should latch on only the very first pulse).

- reduce noise, as the wiring to the main unit was likely to be quite
long.


that they do.

Initially, these capacitors have no charge. that means Vcap = 0. At the
"instant" of turn on, the +13V supply rises (however fast or slow) to
+13V, but there is 0V across each of the debounce/filter caps. for the
caps to 0V, this means a logic zero. for the caps to +13V, (open/stop
and close/stop switches) the resistor is *initially* +13V and hence
logic level 1. Then the cap charges (to +13V) thru the 100k resistor,
time constant 10ms, resulting in an exponential decay to 0V, at which
point (really depends on gate Vil) the logic level becomes the zero you
expect..

If the +13V supply rises much slower than the RC time constant, this
effect can be masked. its quite common when using lab supplies,
especially if you turn them "off" with the current limit dial, like I do

I've shot myself in the foot just recently with exactly that - a
TUSB3210 based system, the micro's oscillator *wont start* if the supply
voltage ramps up too slow! I spent a few hours trying to figure out why
my programmed micro lit up LEDs it shouldnt....doh.

Cheers
Terry

Thanks for the follow-up.

--
Terry, West Sussex, UK

 

[Tony Williams]

In article <s6do91l38rhl08opvd9f4jdr80j31ft27o@4ax.com>,
Terry Pinnell <terrypinDELETE@THESEdial.pipex.com> wrote:

Thanks, I'll try that. BTW, it was 14uH. (The Atlas LCR
consistently read 14.2 uH on several readings with two motors.)
So I'll try 35kHz for starters.

Put this crash-bang suggestion on the shelf Terry,
(just in case your BUZ11's lose their smoke).

5v +---------+ 0.3ohms 1.8V?
Your 5V PC +---| Your |--/\/\--+----+-----+
supply? | Relay | _|_ _|_ |
+---|Switchery|--+ \_/ /_\ |+
0v +---------+ | | | [Motor]
| _|_ _|_ |-
| \_/ /_\ |
| | | |
+-----+----+-----+

Use a 5v supply, shunt diodes to regulate a lower voltage
across the Motor, and a series resistance to limit the
stall current to 10A (guaranteed).

You already seem to have a total loop resistance of
about 0.2R, of which 0.08R is known to be the motor.

Resistance budget: Relays+cabling...... 0.07
New resistor........ 0.30 (50W)
Motor cabling....... 0.05
Motor resistance.... 0.08
------
Total= 0.5R
------

This gives the req'd 10A current at stall.

The diodes will be pairs of Schottky+Silicon to get
about 1.8v shunt regulation, and they will need to
carry about 7A when the motor is running off-load.

Stick a thermal sensing switch onto the 0.3R, as an
emergency relay unlatcher if either of the limit
switches fails to operate.

--
Tony Williams.