[EE] DC Motor PWM, Average Voltage?

[Jason White]

Hello PICList!

I am currently design my first piece of avionics hardware. For my
application I am driving a DC motor with an H-Bridge. I aim to (very
loosely) regulate the average voltage supplied to the motor to 20V over a
supply voltage of 20V to 40V using open-loop PWM.

I would like to know if my thinking is correct.

The H-Bridge contains two "Off" states: High impedance and braking.

With a 40V supply: If I PWM the H-Bridge between "on" and high impedance at
a 50% duty cycle the average voltage will not be 20V because of the back
emf of the DC motor.

Instead to achieve an average of 20V with a 50% duty cycle the H-Bridge
must be toggled between "on" and braking. This will eliminate the effect of
back-EMF on the average applied motor voltage.

Correct thinking?

Thanks,
Jason White
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[Bob Blick]

Hi Jason,

At high frequencies and <<50% duty cycle, yes, the ON/Hi-Z cycle will average 0 volts.

It depends on duty cycle, your PWM frequency and the motor inductance.

The motor is an imperfect inductor.

But generally what you say is entirely accurate.

Friendly regards, Bob

________________________________________
From: piclist-***@mit.edu <piclist-***@mit.edu> on behalf of Jason White
Sent: Tuesday, July 3, 2018 12:48 PM
To: Microcontroller discussion list - Public.
Subject: [EE] DC Motor PWM, Average Voltage?

Hello PICList!

I am currently design my first piece of avionics hardware. For my
application I am driving a DC motor with an H-Bridge. I aim to (very
loosely) regulate the average voltage supplied to the motor to 20V over a
supply voltage of 20V to 40V using open-loop PWM.

I would like to know if my thinking is correct.

The H-Bridge contains two "Off" states: High impedance and braking.

With a 40V supply: If I PWM the H-Bridge between "on" and high impedance at
a 50% duty cycle the average voltage will not be 20V because of the back
emf of the DC motor.

Instead to achieve an average of 20V with a 50% duty cycle the H-Bridge
must be toggled between "on" and braking. This will eliminate the effect of
back-EMF on the average applied motor voltage.

Correct thinking?

Thanks,
Jason White


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[Jason White]

Supplementary information:

PWM frequency = 25kHz
Motor Wattage 15W, 20V (I suppose low wattage could imply lowish
inductance, I don't know the actual motor)
Duty Cycle is always between 50% and 100%.

-Jason White

On Tue, Jul 3, 2018 at 4:17 PM, Bob Blick <***@outlook.com> wrote:

> Hi Jason,
>
> At high frequencies and <<50% duty cycle, yes, the ON/Hi-Z cycle will
> average 0 volts.
>
> It depends on duty cycle, your PWM frequency and the motor inductance.
>
> The motor is an imperfect inductor.
>
> But generally what you say is entirely accurate.
>
> Friendly regards, Bob
>
> ________________________________________
> From: piclist-***@mit.edu <piclist-***@mit.edu> on behalf of
> Jason White
> Sent: Tuesday, July 3, 2018 12:48 PM
> To: Microcontroller discussion list - Public.
> Subject: [EE] DC Motor PWM, Average Voltage?
>
> Hello PICList!
>
> I am currently design my first piece of avionics hardware. For my
> application I am driving a DC motor with an H-Bridge. I aim to (very
> loosely) regulate the average voltage supplied to the motor to 20V over a
> supply voltage of 20V to 40V using open-loop PWM.
>
> I would like to know if my thinking is correct.
>
> The H-Bridge contains two "Off" states: High impedance and braking.
>
> With a 40V supply: If I PWM the H-Bridge between "on" and high impedance at
> a 50% duty cycle the average voltage will not be 20V because of the back
> emf of the DC motor.
>
> Instead to achieve an average of 20V with a 50% duty cycle the H-Bridge
> must be toggled between "on" and braking. This will eliminate the effect of
> back-EMF on the average applied motor voltage.
>
> Correct thinking?
>
> Thanks,
> Jason White
>
>
> --
> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>



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[Bob Blick]

Your frequency is not low, so what happens is this. When the h-bridge goes HI-Z, the energy stored in the inductance dumps back into the supply rails and your net motor voltage is 0 and the current waveform is based on the inductance and frequency. Current means you have some motor torque, but no voltage means no speed.

But when you raise the duty cycle to 50% and above, remember your supply rails clamp the voltage through those back diodes. The voltage is the same(you use lossless transistors and diodes, right?) as when ON, just the polarity is now reversed. Being an inductor, when you go Hi-Z the current can't increase, it ramps down. There isn't enough time before the next ON cycle for the inductor current to go to zero, and current will build up over subsequent cycles.

In order for things to be happy, the average motor voltage will move away from 0, and your motor will actually spin.

So what you have is a voltage profile that is not a straight line starting from 0% duty cycle such as you would have with an ON/shorted PWM cycle train.

Either way works, it's just the result that's quite different. You need good strong caps across the supply rails close to the H-bridge either way.

The third way is to not use Hi-Z or braking, just drive the H-bridge for AC out and then you get 0 voltage at 50% duty cycle. That method is easy to calculate, but losses are continuous so you will want to shut off the H-bridge when you are not running the motor.

Hope this helps.

Bob


________________________________________
From: piclist-***@mit.edu <piclist-***@mit.edu> on behalf of Jason White
Sent: Tuesday, July 3, 2018 1:26 PM
To: Microcontroller discussion list - Public.
Subject: Re: [EE] DC Motor PWM, Average Voltage?

Supplementary information:

PWM frequency = 25kHz
Motor Wattage 15W, 20V (I suppose low wattage could imply lowish
inductance, I don't know the actual motor)
Duty Cycle is always between 50% and 100%.

-Jason White

On Tue, Jul 3, 2018 at 4:17 PM, Bob Blick wrote:

> Hi Jason,
>
> At high frequencies and <<50% duty cycle, yes, the ON/Hi-Z cycle will
> average 0 volts.
>
> It depends on duty cycle, your PWM frequency and the motor inductance.
>
> The motor is an imperfect inductor.
>
> But generally what you say is entirely accurate.
>
> Friendly regards, Bob
>
> ________________________________________
> From: piclist-***@mit.edu <piclist-***@mit.edu> on behalf of
> Jason White
> Sent: Tuesday, July 3, 2018 12:48 PM
> To: Microcontroller discussion list - Public.
> Subject: [EE] DC Motor PWM, Average Voltage?
>
> Hello PICList!
>
> I am currently design my first piece of avionics hardware. For my
> application I am driving a DC motor with an H-Bridge. I aim to (very
> loosely) regulate the average voltage supplied to the motor to 20V over a
> supply voltage of 20V to 40V using open-loop PWM.
>
> I would like to know if my thinking is correct.
>
> The H-Bridge contains two "Off" states: High impedance and braking.
>
> With a 40V supply: If I PWM the H-Bridge between "on" and high impedance at
> a 50% duty cycle the average voltage will not be 20V because of the back
> emf of the DC motor.
>
> Instead to achieve an average of 20V with a 50% duty cycle the H-Bridge
> must be toggled between "on" and braking. This will eliminate the effect of
> back-EMF on the average applied motor voltage.
>
> Correct thinking?
>
> Thanks,
> Jason White


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[Jason White]

Bob, I am curious about method (3). Would the efficiency/load/stress be any
different than the On-Braking case?

This has been very helpful.

Thanks,
-Jason White

On Tuesday, July 3, 2018, Bob Blick <***@outlook.com> wrote:

> Your frequency is not low, so what happens is this. When the h-bridge goes
> HI-Z, the energy stored in the inductance dumps back into the supply rails
> and your net motor voltage is 0 and the current waveform is based on the
> inductance and frequency. Current means you have some motor torque, but no
> voltage means no speed.
>
> But when you raise the duty cycle to 50% and above, remember your supply
> rails clamp the voltage through those back diodes. The voltage is the
> same(you use lossless transistors and diodes, right?) as when ON, just the
> polarity is now reversed. Being an inductor, when you go Hi-Z the current
> can't increase, it ramps down. There isn't enough time before the next ON
> cycle for the inductor current to go to zero, and current will build up
> over subsequent cycles.
>
> In order for things to be happy, the average motor voltage will move away
> from 0, and your motor will actually spin.
>
> So what you have is a voltage profile that is not a straight line starting
> from 0% duty cycle such as you would have with an ON/shorted PWM cycle
> train.
>
> Either way works, it's just the result that's quite different. You need
> good strong caps across the supply rails close to the H-bridge either way.
>
> The third way is to not use Hi-Z or braking, just drive the H-bridge for
> AC out and then you get 0 voltage at 50% duty cycle. That method is easy to
> calculate, but losses are continuous so you will want to shut off the
> H-bridge when you are not running the motor.
>
> Hope this helps.
>
> Bob
>
>
> ________________________________________
> From: piclist-***@mit.edu <piclist-***@mit.edu> on behalf of
> Jason White
> Sent: Tuesday, July 3, 2018 1:26 PM
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE] DC Motor PWM, Average Voltage?
>
> Supplementary information:
>
> PWM frequency = 25kHz
> Motor Wattage 15W, 20V (I suppose low wattage could imply lowish
> inductance, I don't know the actual motor)
> Duty Cycle is always between 50% and 100%.
>
> -Jason White
>
> On Tue, Jul 3, 2018 at 4:17 PM, Bob Blick wrote:
>
> > Hi Jason,
> >
> > At high frequencies and <<50% duty cycle, yes, the ON/Hi-Z cycle will
> > average 0 volts.
> >
> > It depends on duty cycle, your PWM frequency and the motor inductance.
> >
> > The motor is an imperfect inductor.
> >
> > But generally what you say is entirely accurate.
> >
> > Friendly regards, Bob
> >
> > ________________________________________
> > From: piclist-***@mit.edu <piclist-***@mit.edu> on behalf of
> > Jason White
> > Sent: Tuesday, July 3, 2018 12:48 PM
> > To: Microcontroller discussion list - Public.
> > Subject: [EE] DC Motor PWM, Average Voltage?
> >
> > Hello PICList!
> >
> > I am currently design my first piece of avionics hardware. For my
> > application I am driving a DC motor with an H-Bridge. I aim to (very
> > loosely) regulate the average voltage supplied to the motor to 20V over a
> > supply voltage of 20V to 40V using open-loop PWM.
> >
> > I would like to know if my thinking is correct.
> >
> > The H-Bridge contains two "Off" states: High impedance and braking.
> >
> > With a 40V supply: If I PWM the H-Bridge between "on" and high impedance
> at
> > a 50% duty cycle the average voltage will not be 20V because of the back
> > emf of the DC motor.
> >
> > Instead to achieve an average of 20V with a 50% duty cycle the H-Bridge
> > must be toggled between "on" and braking. This will eliminate the effect
> of
> > back-EMF on the average applied motor voltage.
> >
> > Correct thinking?
> >
> > Thanks,
> > Jason White
>
>
> --
> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>


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[Bob Blick]

Hi Jason,

You're quite welcome, happy to help.

All three methods have losses when there is PWM of any sort.

Option 3 will heat up your motor even if you sit at 50% duty cycle, your losses are continuous because you are basically putting AC into a DC motor. At 50% PWM the DC voltage is 0, so the motor isn't running, but you're still pumping AC into it. Depending on your motor's losses, it may not be much or it might be a lot. That's why you'd want to shut off the h-bridge when you want the motor idle.

Filtering between h-bridge and motor (series inductors in both legs and then a cap in parallel with the motor) will really reduce the motor losses, no matter what choice of PWM you make. If you put the LC filtering right at the h-bridge and then a smaller cap at the motor it will reduce the EMI too.

PWM of any sort is stressful to a brushed DC motor. Filtering reduces the brush and commutator wear(and heat).

You might give all three PWM methods a try and see what works best for you.

Basically, for most motors, if you don't plan on filtering, stick with one of the first two methods.

Best regards, Bob

________________________________________
From: piclist-***@mit.edu <piclist-***@mit.edu> on behalf of Jason White
Sent: Tuesday, July 3, 2018 3:50 PM
To: Microcontroller discussion list - Public.
Subject: Re: [EE] DC Motor PWM, Average Voltage?

Bob, I am curious about method (3). Would the efficiency/load/stress be any
different than the On-Braking case?

This has been very helpful.

Thanks,
-Jason White

On Tuesday, July 3, 2018, Bob Blick wrote:

> Your frequency is not low, so what happens is this. When the h-bridge goes
> HI-Z, the energy stored in the inductance dumps back into the supply rails
> and your net motor voltage is 0 and the current waveform is based on the
> inductance and frequency. Current means you have some motor torque, but no
> voltage means no speed.
>
> But when you raise the duty cycle to 50% and above, remember your supply
> rails clamp the voltage through those back diodes. The voltage is the
> same(you use lossless transistors and diodes, right?) as when ON, just the
> polarity is now reversed. Being an inductor, when you go Hi-Z the current
> can't increase, it ramps down. There isn't enough time before the next ON
> cycle for the inductor current to go to zero, and current will build up
> over subsequent cycles.
>
> In order for things to be happy, the average motor voltage will move away
> from 0, and your motor will actually spin.
>
> So what you have is a voltage profile that is not a straight line starting
> from 0% duty cycle such as you would have with an ON/shorted PWM cycle
> train.
>
> Either way works, it's just the result that's quite different. You need
> good strong caps across the supply rails close to the H-bridge either way.
>
> The third way is to not use Hi-Z or braking, just drive the H-bridge for
> AC out and then you get 0 voltage at 50% duty cycle. That method is easy to
> calculate, but losses are continuous so you will want to shut off the
> H-bridge when you are not running the motor.
>
> Hope this helps.
>
> Bob
>
>
> ________________________________________
> From: piclist-***@mit.edu <piclist-***@mit.edu> on behalf of
> Jason White
> Sent: Tuesday, July 3, 2018 1:26 PM
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE] DC Motor PWM, Average Voltage?
>
> Supplementary information:
>
> PWM frequency = 25kHz
> Motor Wattage 15W, 20V (I suppose low wattage could imply lowish
> inductance, I don't know the actual motor)
> Duty Cycle is always between 50% and 100%.
>
> -Jason White

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[Bob Blick]

One other thing I forgot to mention, you can also have losses directly within the H-bridge itself, many have shoot-through current (internal short circuit) during transitions, due to turn-off usually being longer than turn-on. That happens with both ON/brake and ON/ON methods of PWM but not ON/Hi-Z. Most microcontrollers with multiple PWM outputs have deadband adjustable or programmed in to prevent this, and some H-bridge chips also have shoot-through protection.

Bob
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[Harold Hallikainen]

What is the advantage of using the H-bridge in this application over just
using a buck regulator? The buck regulator would use just one switching
transistor, a "catch diode," an inductor, and an output capacitor. This is
assuming the motor only needs to rotate in one direction (no voltage
polarity reversal).

Harold

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[Bob Blick]

Hi Harold,

Yes, basically a buck regulator, the h-bridge is to make it bipolar.

Friendly regards, Bob
________________________________________
From: piclist-***@mit.edu <piclist-***@mit.edu> on behalf of Harold Hallikainen
Sent: Wednesday, July 4, 2018 10:51 AM
To: Microcontroller discussion list - Public.
Subject: Re: [EE] DC Motor PWM, Average Voltage?

What is the advantage of using the H-bridge in this application over just
using a buck regulator? The buck regulator would use just one switching
transistor, a "catch diode," an inductor, and an output capacitor. This is
assuming the motor only needs to rotate in one direction (no voltage
polarity reversal).

Harold


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[Jason White]

I need reversal, it is a solenoid/valve controller used for hydraulics in
aircraft. Size is rather constrained so PWM on the bridge is preferred.

On Wednesday, July 4, 2018, Harold Hallikainen <***@mai.hallikainen.org>
wrote:

> What is the advantage of using the H-bridge in this application over just
> using a buck regulator? The buck regulator would use just one switching
> transistor, a "catch diode," an inductor, and an output capacitor. This is
> assuming the motor only needs to rotate in one direction (no voltage
> polarity reversal).
>
> Harold
>
> --
> FCC Rules Updated Daily at http://www.hallikainen.com
> Not sent from an iPhone.
> --
> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>


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[Sean Breheny]

I would definitely recommend considering PWM drive of motors in terms of
the motor current as a function of motor speed, PWM duty, and supply
voltage, rather than an effective voltage. This is because the transfer
function from duty to effective voltage is moderately nonlinear overall and
very nonlinear near zero duty. Taking this nonlinear voltage expression and
using it in a simplified voltage-driven motor model will result in complex
equations which are not very accurate in predicting motor behavior.

Is this a brushed or brushless motor?

A brushless motor driven by PWM is, as some have pointed out, effectively a
buck converter because the inductance of the motor acts to create a voltage
drop between the back EMF and the supply voltage. If you drive the motor
with a sinusoidal current and vary the phase of this current relative to
the back-EMF then you can also make it work like a boost converter (i.e.,
produce torque even when the back-EMF is higher than the supply voltage).
This is often called either phase advance or field weakening (this latter
term comes from the fact that the out-of-phase current in the coils
produces a field which opposes or weakens the permanent magnet field as
seen by the coils themselves).

Brushed motors have the additional property that the commutation transients
dump the stored magnetic energy in the motor coils (instead of 6-step
commutated brushless motors where most of the stored energy is fed back
into the supply via the body diodes of the driving MOSFETs or the
protection diodes added to the switching elements if they are BJTs or
IGBTs). This is slightly less efficient but it does reduce the effect which
the motor inductance has on the electrical dynamics of the motor.

If PWM is operated at a high enough frequency that the current flow is
continuous through the motor coil from PWM cycle to PWM cycle (which is
almost certainly what you want because it is more efficient and more linear
in duty to torque relationship) then there really are only two kinds of PWM
for a two-terminal (brushed) motor: on-reverse and on-shorted. on-reverse
means that you alternate between applying positive and negative voltage to
the motor during each portion of the PWM cycle (positive during the ON
portion and negative during the OFF portion, assuming positive effective
voltage is desired). on-shorted means that you short the motor during the
off portion of each PWM cycle. There is no true OFF because even if you
turn off all switching elements the protection diodes will cause current
flow from the motor inductance back into the power supply, which is the
same thing as on-reverse operation (but less efficient because of the diode
voltage drop). On-shorted prevents this backflow into the power supply but
it produces less maximum braking torque and prevents useful regenerative
braking.

Sean



On Wed, Jul 4, 2018 at 10:01 PM, Jason White <
***@gmail.com> wrote:

> I need reversal, it is a solenoid/valve controller used for hydraulics in
> aircraft. Size is rather constrained so PWM on the bridge is preferred.
>
> On Wednesday, July 4, 2018, Harold Hallikainen <***@mai.hallikainen.org
> >
> wrote:
>
> > What is the advantage of using the H-bridge in this application over just
> > using a buck regulator? The buck regulator would use just one switching
> > transistor, a "catch diode," an inductor, and an output capacitor. This
> is
> > assuming the motor only needs to rotate in one direction (no voltage
> > polarity reversal).
> >
> > Harold
> >
> > --
> > FCC Rules Updated Daily at http://www.hallikainen.com
> > Not sent from an iPhone.
> > --
> > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> > View/change your membership options at
> > http://mailman.mit.edu/mailman/listinfo/piclist
> >
>
>
> --
> Jason White
> --
> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>
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[Jason White]

Hello Sean,

This is a small brushed DC motor going into a ~180:1 gearbox to actuate a
On/Off hydraulic valve. 15W 20V. There are cams and limit switches that get
adjusted for each type of valve. Part-wise I am limited to discretes,
op-amps, comparators, and logic gates.

This is a fixed speed application, very size constrained. I need to limit
the max speed over 20-40V supply voltage from -50C to 100C. Too fast is
bad, too slow is acceptable. It would be "nice" to have a constant speed
but given the design constraints I'm not sure that I can actually do that.

Am I correct in thinking that a fixed (PWM output) voltage would produce a
relatively fixed max speed?

The load and motor characteristics almost certainly very with temperature -
but I don't have control over or data on that. I don't know the exact
motor. This controller is going to be used with multiple (very similar)
types of valve-gearbox-motor assemblies that do not exist yet.

I am pretty rusty on my control theory, I have the following worries about
a closed loop current feedback system: (1) it might oscillate or do
something unexpected due to my inexperience (2) it might consume more space
on the small PCB (3) different valves might require different currents.

If you could point me to an example/app note that goes into the control
theory, I will gladly study it. But for now a simple fixed voltage-variable
current scheme seems like a better fit

-Jason White


On Wed, Jul 4, 2018 at 11:43 PM, Sean Breheny <***@cornell.edu> wrote:

> I would definitely recommend considering PWM drive of motors in terms of
> the motor current as a function of motor speed, PWM duty, and supply
> voltage, rather than an effective voltage. This is because the transfer
> function from duty to effective voltage is moderately nonlinear overall and
> very nonlinear near zero duty. Taking this nonlinear voltage expression and
> using it in a simplified voltage-driven motor model will result in complex
> equations which are not very accurate in predicting motor behavior.
>
> Is this a brushed or brushless motor?
>
> A brushless motor driven by PWM is, as some have pointed out, effectively a
> buck converter because the inductance of the motor acts to create a voltage
> drop between the back EMF and the supply voltage. If you drive the motor
> with a sinusoidal current and vary the phase of this current relative to
> the back-EMF then you can also make it work like a boost converter (i.e.,
> produce torque even when the back-EMF is higher than the supply voltage).
> This is often called either phase advance or field weakening (this latter
> term comes from the fact that the out-of-phase current in the coils
> produces a field which opposes or weakens the permanent magnet field as
> seen by the coils themselves).
>
> Brushed motors have the additional property that the commutation transients
> dump the stored magnetic energy in the motor coils (instead of 6-step
> commutated brushless motors where most of the stored energy is fed back
> into the supply via the body diodes of the driving MOSFETs or the
> protection diodes added to the switching elements if they are BJTs or
> IGBTs). This is slightly less efficient but it does reduce the effect which
> the motor inductance has on the electrical dynamics of the motor.
>
> If PWM is operated at a high enough frequency that the current flow is
> continuous through the motor coil from PWM cycle to PWM cycle (which is
> almost certainly what you want because it is more efficient and more linear
> in duty to torque relationship) then there really are only two kinds of PWM
> for a two-terminal (brushed) motor: on-reverse and on-shorted. on-reverse
> means that you alternate between applying positive and negative voltage to
> the motor during each portion of the PWM cycle (positive during the ON
> portion and negative during the OFF portion, assuming positive effective
> voltage is desired). on-shorted means that you short the motor during the
> off portion of each PWM cycle. There is no true OFF because even if you
> turn off all switching elements the protection diodes will cause current
> flow from the motor inductance back into the power supply, which is the
> same thing as on-reverse operation (but less efficient because of the diode
> voltage drop). On-shorted prevents this backflow into the power supply but
> it produces less maximum braking torque and prevents useful regenerative
> braking.
>
> Sean
>
>
>
> On Wed, Jul 4, 2018 at 10:01 PM, Jason White <
> ***@gmail.com> wrote:
>
> > I need reversal, it is a solenoid/valve controller used for hydraulics in
> > aircraft. Size is rather constrained so PWM on the bridge is preferred.
> >
> > On Wednesday, July 4, 2018, Harold Hallikainen <
> ***@mai.hallikainen.org
> > >
> > wrote:
> >
> > > What is the advantage of using the H-bridge in this application over
> just
> > > using a buck regulator? The buck regulator would use just one switching
> > > transistor, a "catch diode," an inductor, and an output capacitor. This
> > is
> > > assuming the motor only needs to rotate in one direction (no voltage
> > > polarity reversal).
> > >
> > > Harold
> > >
> > > --
> > > FCC Rules Updated Daily at http://www.hallikainen.com
> > > Not sent from an iPhone.
> > > --
> > > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> > > View/change your membership options at
> > > http://mailman.mit.edu/mailman/listinfo/piclist
> > >
> >
> >
> > --
> > Jason White
> > --
> > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> > View/change your membership options at
> > http://mailman.mit.edu/mailman/listinfo/piclist
> >
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[AB Pearce - UKRI STFC]

http://www.microchip.com/design-centers/motor-control-and-drive/motor-types/brushed-dc-motor

Go down the bottom to 'documentation' tab and work through the app notes for brushed DC motors.


-----Original Message-----
From: piclist-***@mit.edu <piclist-***@mit.edu> On Behalf Of Jason White
Sent: 05 July 2018 13:04
To: Microcontroller discussion list - Public. <***@mit.edu>
Subject: Re: [EE] DC Motor PWM, Average Voltage?

Hello Sean,

This is a small brushed DC motor going into a ~180:1 gearbox to actuate a On/Off hydraulic valve. 15W 20V. There are cams and limit switches that get adjusted for each type of valve. Part-wise I am limited to discretes, op-amps, comparators, and logic gates.

This is a fixed speed application, very size constrained. I need to limit the max speed over 20-40V supply voltage from -50C to 100C. Too fast is bad, too slow is acceptable. It would be "nice" to have a constant speed but given the design constraints I'm not sure that I can actually do that.

Am I correct in thinking that a fixed (PWM output) voltage would produce a relatively fixed max speed?

The load and motor characteristics almost certainly very with temperature - but I don't have control over or data on that. I don't know the exact motor. This controller is going to be used with multiple (very similar) types of valve-gearbox-motor assemblies that do not exist yet.

I am pretty rusty on my control theory, I have the following worries about a closed loop current feedback system: (1) it might oscillate or do something unexpected due to my inexperience (2) it might consume more space on the small PCB (3) different valves might require different currents.

If you could point me to an example/app note that goes into the control theory, I will gladly study it. But for now a simple fixed voltage-variable current scheme seems like a better fit

-Jason White


On Wed, Jul 4, 2018 at 11:43 PM, Sean Breheny <***@cornell.edu> wrote:

> I would definitely recommend considering PWM drive of motors in terms
> of the motor current as a function of motor speed, PWM duty, and
> supply voltage, rather than an effective voltage. This is because the
> transfer function from duty to effective voltage is moderately
> nonlinear overall and very nonlinear near zero duty. Taking this
> nonlinear voltage expression and using it in a simplified
> voltage-driven motor model will result in complex equations which are not very accurate in predicting motor behavior.
>
> Is this a brushed or brushless motor?
>
> A brushless motor driven by PWM is, as some have pointed out,
> effectively a buck converter because the inductance of the motor acts
> to create a voltage drop between the back EMF and the supply voltage.
> If you drive the motor with a sinusoidal current and vary the phase of
> this current relative to the back-EMF then you can also make it work
> like a boost converter (i.e., produce torque even when the back-EMF is higher than the supply voltage).
> This is often called either phase advance or field weakening (this
> latter term comes from the fact that the out-of-phase current in the
> coils produces a field which opposes or weakens the permanent magnet
> field as seen by the coils themselves).
>
> Brushed motors have the additional property that the commutation
> transients dump the stored magnetic energy in the motor coils (instead
> of 6-step commutated brushless motors where most of the stored energy
> is fed back into the supply via the body diodes of the driving MOSFETs
> or the protection diodes added to the switching elements if they are
> BJTs or IGBTs). This is slightly less efficient but it does reduce the
> effect which the motor inductance has on the electrical dynamics of the motor.
>
> If PWM is operated at a high enough frequency that the current flow is
> continuous through the motor coil from PWM cycle to PWM cycle (which
> is almost certainly what you want because it is more efficient and
> more linear in duty to torque relationship) then there really are only
> two kinds of PWM for a two-terminal (brushed) motor: on-reverse and
> on-shorted. on-reverse means that you alternate between applying
> positive and negative voltage to the motor during each portion of the
> PWM cycle (positive during the ON portion and negative during the OFF
> portion, assuming positive effective voltage is desired). on-shorted
> means that you short the motor during the off portion of each PWM
> cycle. There is no true OFF because even if you turn off all switching
> elements the protection diodes will cause current flow from the motor
> inductance back into the power supply, which is the same thing as
> on-reverse operation (but less efficient because of the diode voltage
> drop). On-shorted prevents this backflow into the power supply but it
> produces less maximum braking torque and prevents useful regenerative braking.
>
> Sean
>
>
>
> On Wed, Jul 4, 2018 at 10:01 PM, Jason White <
> ***@gmail.com> wrote:
>
> > I need reversal, it is a solenoid/valve controller used for
> > hydraulics in aircraft. Size is rather constrained so PWM on the bridge is preferred.
> >
> > On Wednesday, July 4, 2018, Harold Hallikainen <
> ***@mai.hallikainen.org
> > >
> > wrote:
> >
> > > What is the advantage of using the H-bridge in this application
> > > over
> just
> > > using a buck regulator? The buck regulator would use just one
> > > switching transistor, a "catch diode," an inductor, and an output
> > > capacitor. This
> > is
> > > assuming the motor only needs to rotate in one direction (no
> > > voltage polarity reversal).
> > >
> > > Harold
> > >
> > > --
> > > FCC Rules Updated Daily at http://www.hallikainen.com Not sent
> > > from an iPhone.
> > > --
> > > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> > > View/change your membership options at
> > > http://mailman.mit.edu/mailman/listinfo/piclist
> > >
> >
> >
> > --
> > Jason White
> > --
> > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> > View/change your membership options at
> > http://mailman.mit.edu/mailman/listinfo/piclist
> >
> --
> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>



--
Jason White
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[Sean Breheny]

Hi Jason,

I understand your application better now and I do agree that if you have no
speed feedback (other than back-EMF) then the open-loop effective voltage
method does make sense in your case.

You can derive the relationship from the standard buck converter equations.

Sean


On Thu, Jul 5, 2018 at 8:03 AM, Jason White <
***@gmail.com> wrote:

> Hello Sean,
>
> This is a small brushed DC motor going into a ~180:1 gearbox to actuate a
> On/Off hydraulic valve. 15W 20V. There are cams and limit switches that get
> adjusted for each type of valve. Part-wise I am limited to discretes,
> op-amps, comparators, and logic gates.
>
> This is a fixed speed application, very size constrained. I need to limit
> the max speed over 20-40V supply voltage from -50C to 100C. Too fast is
> bad, too slow is acceptable. It would be "nice" to have a constant speed
> but given the design constraints I'm not sure that I can actually do that.
>
> Am I correct in thinking that a fixed (PWM output) voltage would produce a
> relatively fixed max speed?
>
> The load and motor characteristics almost certainly very with temperature -
> but I don't have control over or data on that. I don't know the exact
> motor. This controller is going to be used with multiple (very similar)
> types of valve-gearbox-motor assemblies that do not exist yet.
>
> I am pretty rusty on my control theory, I have the following worries about
> a closed loop current feedback system: (1) it might oscillate or do
> something unexpected due to my inexperience (2) it might consume more space
> on the small PCB (3) different valves might require different currents.
>
> If you could point me to an example/app note that goes into the control
> theory, I will gladly study it. But for now a simple fixed voltage-variable
> current scheme seems like a better fit
>
> -Jason White
>
>
> On Wed, Jul 4, 2018 at 11:43 PM, Sean Breheny <***@cornell.edu> wrote:
>
> > I would definitely recommend considering PWM drive of motors in terms of
> > the motor current as a function of motor speed, PWM duty, and supply
> > voltage, rather than an effective voltage. This is because the transfer
> > function from duty to effective voltage is moderately nonlinear overall
> and
> > very nonlinear near zero duty. Taking this nonlinear voltage expression
> and
> > using it in a simplified voltage-driven motor model will result in
> complex
> > equations which are not very accurate in predicting motor behavior.
> >
> > Is this a brushed or brushless motor?
> >
> > A brushless motor driven by PWM is, as some have pointed out,
> effectively a
> > buck converter because the inductance of the motor acts to create a
> voltage
> > drop between the back EMF and the supply voltage. If you drive the motor
> > with a sinusoidal current and vary the phase of this current relative to
> > the back-EMF then you can also make it work like a boost converter (i.e.,
> > produce torque even when the back-EMF is higher than the supply voltage).
> > This is often called either phase advance or field weakening (this latter
> > term comes from the fact that the out-of-phase current in the coils
> > produces a field which opposes or weakens the permanent magnet field as
> > seen by the coils themselves).
> >
> > Brushed motors have the additional property that the commutation
> transients
> > dump the stored magnetic energy in the motor coils (instead of 6-step
> > commutated brushless motors where most of the stored energy is fed back
> > into the supply via the body diodes of the driving MOSFETs or the
> > protection diodes added to the switching elements if they are BJTs or
> > IGBTs). This is slightly less efficient but it does reduce the effect
> which
> > the motor inductance has on the electrical dynamics of the motor.
> >
> > If PWM is operated at a high enough frequency that the current flow is
> > continuous through the motor coil from PWM cycle to PWM cycle (which is
> > almost certainly what you want because it is more efficient and more
> linear
> > in duty to torque relationship) then there really are only two kinds of
> PWM
> > for a two-terminal (brushed) motor: on-reverse and on-shorted. on-reverse
> > means that you alternate between applying positive and negative voltage
> to
> > the motor during each portion of the PWM cycle (positive during the ON
> > portion and negative during the OFF portion, assuming positive effective
> > voltage is desired). on-shorted means that you short the motor during the
> > off portion of each PWM cycle. There is no true OFF because even if you
> > turn off all switching elements the protection diodes will cause current
> > flow from the motor inductance back into the power supply, which is the
> > same thing as on-reverse operation (but less efficient because of the
> diode
> > voltage drop). On-shorted prevents this backflow into the power supply
> but
> > it produces less maximum braking torque and prevents useful regenerative
> > braking.
> >
> > Sean
> >
> >
> >
> > On Wed, Jul 4, 2018 at 10:01 PM, Jason White <
> > ***@gmail.com> wrote:
> >
> > > I need reversal, it is a solenoid/valve controller used for hydraulics
> in
> > > aircraft. Size is rather constrained so PWM on the bridge is preferred.
> > >
> > > On Wednesday, July 4, 2018, Harold Hallikainen <
> > ***@mai.hallikainen.org
> > > >
> > > wrote:
> > >
> > > > What is the advantage of using the H-bridge in this application over
> > just
> > > > using a buck regulator? The buck regulator would use just one
> switching
> > > > transistor, a "catch diode," an inductor, and an output capacitor.
> This
> > > is
> > > > assuming the motor only needs to rotate in one direction (no voltage
> > > > polarity reversal).
> > > >
> > > > Harold
> > > >
> > > > --
> > > > FCC Rules Updated Daily at http://www.hallikainen.com
> > > > Not sent from an iPhone.
> > > > --
> > > > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> > > > View/change your membership options at
> > > > http://mailman.mit.edu/mailman/listinfo/piclist
> > > >
> > >
> > >
> > > --
> > > Jason White
> > > --
> > > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> > > View/change your membership options at
> > > http://mailman.mit.edu/mailman/listinfo/piclist
> > >
> > --
> > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> > View/change your membership options at
> > http://mailman.mit.edu/mailman/listinfo/piclist
> >
>
>
>
> --
> Jason White
> --
> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>
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[Brooke Clarke]

Hi Jason:

Awhile back I looked into Permanent Magnet DC motors and discovered that the "K" value relating RPM/Volt can be directly
translated into a constant for Torque/Amp.
http://prc68.com/I/DCpmMotors.html
also had some flywheel - pulleys made for various shaft diameters where the circumference of the slot is exactly one
half foot to make horsepower calculations easier.
http://prc68.com/I/Flywheel.html
Used these for testing:
http://prc68.com/I/TestingDCMotors.html
http://prc68.com/I/TestingDCMotors.html#Videos

The time constant for an L-R circuit is counter intuitive, it's T = L/R, so unlike a C-R circuit where increasing either
C or R causes a longer time constant, the L-R circuit gets faster when R is increased.  This is a mandatory part of
teletype DC loops where a series carbon resistor is part of the loop to get the speed up.  My gut feeling is that a
similar thing may apply to any L-R circuit where you want fast operation.  The down side is that it takes higher
voltages and wastes power.

--
Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html
axioms:
1. The extent to which you can fix or improve something will be limited by how well you understand how it works.
2. Everybody, with no exceptions, holds false beliefs.

-------- Original Message --------
> Re: [EE] DC Motor PWM, Average Voltage?

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