OT; brushed DC motor speed measurement

Started by Peter Lissenburg June 19, 2005
Hi all,
and sorry about this off topic, but I need the collective wisdom of the group.
Is it OK, possible, wise, to use the noise spikes of the drive voltage of a
DC motor to measure it's speed?
Thanks for any guidance.
Peter L.



In a message dated 6/18/05 11:16:03 P.M. Eastern Daylight Time,
peter@pete... writes:

Is it OK, possible, wise, to use the noise spikes of the drive voltage of a
DC motor to measure it's speed?
====================
Sounds like a clever trick to me. Wish I'd have thought of it before I
bought that expensive tacho!


Peter,

Clearly, the spikes offer a way to very accurately measure the speed (e.g.
using an HC12 to measure the time between spikes). The key is where and how
does one pick up those noise spikes. The terminals of a DC motor driven by a
battery don't have much for spikes since the battery has such a low internal
impedance.

The big voltage spike takes place on the armature winding that has just broken
contact with the brushes. One might pick this up by stuffing a wire into the
motor to act as a capacitive pick up.

The model airplanes that use electric motors usually have a by-pass cap right
at the motor terminals to reduce radio interference, so this suggests rf
pickup is another possibility.

The current going into the motor does have a ripple. Some (actually many)
years ago I considered using this to measure the speed of the cars on a
track-car set my son had (he moved on to other things before that project got
far).

I would also expect a very large variation between makes of motors, some being
relatively "quiet," and motor load will also have a big affect.

On a DC motor speed is often sensed by the back emf in a bridge circuit
arrangement, however this is an analog solution, and more difficult to get
high accuracy.

Regards,

Donald E Haselwood

On Saturday 18 June 2005 23:12, Peter Lissenburg wrote:
> Hi all,
> and sorry about this off topic, but I need the collective wisdom of
> the group. Is it OK, possible, wise, to use the noise spikes of the drive
> voltage of a DC motor to measure it's speed?
> Thanks for any guidance.
> Peter L.



Thanks guys,
I'm looking at the CRO with a small motor running. I'm seeing
bursts of spikes which get more numerous and spread over a longer burst as
I increase the speed. Also I think I'm seeing asynchronous spikes as well.
Maybe carbon build up or other dirt under the brush. Sounding a bit hit and
miss, also with the variation between motors and load. This might not be
worth following.
I think it might be best to follow a more conventional line for the moment.
Thanks for your time.
Peter L. At 12:25 AM 19/06/2005 -0400, you wrote:
>Peter,
>
>Clearly, the spikes offer a way to very accurately measure the speed (e.g.
>using an HC12 to measure the time between spikes). The key is where and how
>does one pick up those noise spikes. The terminals of a DC motor driven by a
>battery don't have much for spikes since the battery has such a low internal
>impedance.
>
>The big voltage spike takes place on the armature winding that has just
>broken
>contact with the brushes. One might pick this up by stuffing a wire into the
>motor to act as a capacitive pick up.
>
>The model airplanes that use electric motors usually have a by-pass cap right
>at the motor terminals to reduce radio interference, so this suggests rf
>pickup is another possibility.
>
>The current going into the motor does have a ripple. Some (actually many)
>years ago I considered using this to measure the speed of the cars on a
>track-car set my son had (he moved on to other things before that project got
>far).
>
>I would also expect a very large variation between makes of motors, some
>being
>relatively "quiet," and motor load will also have a big affect.
>
>On a DC motor speed is often sensed by the back emf in a bridge circuit
>arrangement, however this is an analog solution, and more difficult to get
>high accuracy.
>
>Regards,
>
>Donald E Haselwood >
>
>On Saturday 18 June 2005 23:12, Peter Lissenburg wrote:
> > Hi all,
> > and sorry about this off topic, but I need the collective wisdom of
> > the group. Is it OK, possible, wise, to use the noise spikes of the drive
> > voltage of a DC motor to measure it's speed?
> > Thanks for any guidance.
> > Peter L. >
>Yahoo! Groups Links >
>




Peter,

Some time ago I remember seeing a method using back EMF to measure motor
speed without a tachometer. I can't remember if it was Linear technology
or Maxim but a quick search on Google turned up:
http://www.acroname.com/robotics/info/articles/back-emf/back-emf.html

I'd be curious to know if that worked.

Jonathan. -----Original Message-----
From: 68HC12@68HC... [mailto:68HC12@68HC...] On Behalf
Of Peter Lissenburg
Sent: Sunday, 19 June 2005 6:06 PM
To: 68HC12@68HC...
Subject: Re: [68HC12] OT; brushed DC motor speed measurement

Thanks guys,
I'm looking at the CRO with a small motor running. I'm seeing
bursts of spikes which get more numerous and spread over a longer burst
as
I increase the speed. Also I think I'm seeing asynchronous spikes as
well.
Maybe carbon build up or other dirt under the brush. Sounding a bit hit
and
miss, also with the variation between motors and load. This might not be

worth following.
I think it might be best to follow a more conventional line for the
moment.
Thanks for your time.
Peter L. At 12:25 AM 19/06/2005 -0400, you wrote:
>Peter,
>
>Clearly, the spikes offer a way to very accurately measure the speed
(e.g.
>using an HC12 to measure the time between spikes). The key is where
and how
>does one pick up those noise spikes. The terminals of a DC motor
driven by a
>battery don't have much for spikes since the battery has such a low
internal
>impedance.
>
>The big voltage spike takes place on the armature winding that has just

>broken
>contact with the brushes. One might pick this up by stuffing a wire
into the
>motor to act as a capacitive pick up.
>
>The model airplanes that use electric motors usually have a by-pass cap
right
>at the motor terminals to reduce radio interference, so this suggests
rf
>pickup is another possibility.
>
>The current going into the motor does have a ripple. Some (actually
many)
>years ago I considered using this to measure the speed of the cars on a
>track-car set my son had (he moved on to other things before that
project got
>far).
>
>I would also expect a very large variation between makes of motors,
some
>being
>relatively "quiet," and motor load will also have a big affect.
>
>On a DC motor speed is often sensed by the back emf in a bridge circuit
>arrangement, however this is an analog solution, and more difficult to
get
>high accuracy.
>
>Regards,
>
>Donald E Haselwood >
>
>On Saturday 18 June 2005 23:12, Peter Lissenburg wrote:
> > Hi all,
> > and sorry about this off topic, but I need the collective
wisdom of
> > the group. Is it OK, possible, wise, to use the noise spikes of the
drive
> > voltage of a DC motor to measure it's speed?
> > Thanks for any guidance.
> > Peter L. >
>> Service.




Thanks Jonathan,
that's a very well written article. I always thought I knew about
the back EMF idea, but I did have it a bit wrong.
Unfortunately I should have been more explicit, as I need to monitor
displacement rather than speed, although the two are definitely related,
counting pulses seems to be simpler, than integrating speed over time. And
the HC12's timer module is well suited to counting pulses.
But I do have another project that I used the Back EMF idea, and now I know
how to improve it.
So thanks for that as well.
I have just managed to solder on a slotted disc onto the end of the motor
shaft. This will do for a few prototypes. After that we can probably find
one ready made for production.
Thanks again.
Peter L. At 07:50 PM 19/06/2005 +1000, you wrote:
>Peter,
>
>Some time ago I remember seeing a method using back EMF to measure motor
>speed without a tachometer. I can't remember if it was Linear technology
>or Maxim but a quick search on Google turned up:
>http://www.acroname.com/robotics/info/articles/back-emf/back-emf.html
>
>I'd be curious to know if that worked.
>
>Jonathan. >-----Original Message-----
>From: 68HC12@68HC... [mailto:68HC12@68HC...] On Behalf
>Of Peter Lissenburg
>Sent: Sunday, 19 June 2005 6:06 PM
>To: 68HC12@68HC...
>Subject: Re: [68HC12] OT; brushed DC motor speed measurement
>
>Thanks guys,
> I'm looking at the CRO with a small motor running. I'm seeing
>bursts of spikes which get more numerous and spread over a longer burst
>as
>I increase the speed. Also I think I'm seeing asynchronous spikes as
>well.
>Maybe carbon build up or other dirt under the brush. Sounding a bit hit
>and
>miss, also with the variation between motors and load. This might not be
>
>worth following.
>I think it might be best to follow a more conventional line for the
>moment.
>Thanks for your time.
>Peter L. >At 12:25 AM 19/06/2005 -0400, you wrote:
> >Peter,
> >
> >Clearly, the spikes offer a way to very accurately measure the speed
>(e.g.
> >using an HC12 to measure the time between spikes). The key is where
>and how
> >does one pick up those noise spikes. The terminals of a DC motor
>driven by a
> >battery don't have much for spikes since the battery has such a low
>internal
> >impedance.
> >
> >The big voltage spike takes place on the armature winding that has just
>
> >broken
> >contact with the brushes. One might pick this up by stuffing a wire
>into the
> >motor to act as a capacitive pick up.
> >
> >The model airplanes that use electric motors usually have a by-pass cap
>right
> >at the motor terminals to reduce radio interference, so this suggests
>rf
> >pickup is another possibility.
> >
> >The current going into the motor does have a ripple. Some (actually
>many)
> >years ago I considered using this to measure the speed of the cars on a
> >track-car set my son had (he moved on to other things before that
>project got
> >far).
> >
> >I would also expect a very large variation between makes of motors,
>some
> >being
> >relatively "quiet," and motor load will also have a big affect.
> >
> >On a DC motor speed is often sensed by the back emf in a bridge circuit
> >arrangement, however this is an analog solution, and more difficult to
>get
> >high accuracy.
> >
> >Regards,
> >
> >Donald E Haselwood
> >
> >
> >
> >
> >On Saturday 18 June 2005 23:12, Peter Lissenburg wrote:
> > > Hi all,
> > > and sorry about this off topic, but I need the collective
>wisdom of
> > > the group. Is it OK, possible, wise, to use the noise spikes of the
>drive
> > > voltage of a DC motor to measure it's speed?
> > > Thanks for any guidance.
> > > Peter L.
> >
> >
> >
> >> Service. >
>
>Yahoo! Groups Links >
>




You did not say whether this was to be a generic method, or tied to a
specific motor. The voltage spikes you may have seen are characteristic of
a low cost motor with poor quality brushes. Larger, industrial motors use
anisotropic graphite brushes, that bridge adjacent commutator segments
without shorting them together. Thus there is a minimum of current
disruption and voltage spiking. Likewise, the signal level will likely
change significantly with brush wear. A series inductor may be required to
enhance the voltage variation, with the consequence of also enhancing brush
wear. At 11:12 PM 6/18/2005, you wrote:
>Hi all,
> and sorry about this off topic, but I need the collective wisdom
> of the group.
>Is it OK, possible, wise, to use the noise spikes of the drive voltage of a
>DC motor to measure it's speed?
>Thanks for any guidance.
>Peter L. >
>
>Yahoo! Groups Links >
>




--- In 68HC12@68HC..., Peter Lissenburg <peter@s...> wrote:
> Thanks guys,
> I'm looking at the CRO with a small motor running. I'm seeing
> bursts of spikes which get more numerous and spread over a longer burst as
> I increase the speed. Also I think I'm seeing asynchronous spikes as well.
> Maybe carbon build up or other dirt under the brush. Sounding a bit hit and
> miss, also with the variation between motors and load. This might not be
> worth following.
> I think it might be best to follow a more conventional line for the moment.

Excellent idea, most people go to great lenghts to suppress these for RFI reasons, apart from that the better the motor the lower these spikes and they are hard to separate from mains noise.

Measuring the back EMF is the normal cheap way, I do understand you want to do it on the cheap:-)

I used a switching method once and measured the EMF during the period the power was off (pulsed drive), that avoids getting errors from the drive current so you don't need to compensate, CMOS gate and a cap although in that particular case it was mainly to compensate for the very long cable runs.

Lastly if you use pancake motors drill a hole in the case where the wires bond on the edge, makes an excellent cheap tacho by counting these go past...

Don't forget a spinning motor packs a wallop of energy, do use lots of little resistors before getting too close to the CPU board:-)

Cheers

Theo

> At 12:25 AM 19/06/2005 -0400, you wrote:
> >Peter,
> >
> >Clearly, the spikes offer a way to very accurately measure the speed (e.g.
> >using an HC12 to measure the time between spikes). The key is where and how
> >does one pick up those noise spikes. The terminals of a DC motor driven by a
> >battery don't have much for spikes since the battery has such a low internal
> >impedance.
> >
> >The big voltage spike takes place on the armature winding that has just
> >broken
> >contact with the brushes. One might pick this up by stuffing a wire into the
> >motor to act as a capacitive pick up.
> >
> >The model airplanes that use electric motors usually have a by-pass cap right
> >at the motor terminals to reduce radio interference, so this suggests rf
> >pickup is another possibility.
> >
> >The current going into the motor does have a ripple. Some (actually many)
> >years ago I considered using this to measure the speed of the cars on a
> >track-car set my son had (he moved on to other things before that project got
> >far).
> >
> >I would also expect a very large variation between makes of motors, some
> >being
> >relatively "quiet," and motor load will also have a big affect.
> >
> >On a DC motor speed is often sensed by the back emf in a bridge circuit
> >arrangement, however this is an analog solution, and more difficult to get
> >high accuracy.
> >
> >Regards,
> >
> >Donald E Haselwood
> >
> >
> >
> >
> >On Saturday 18 June 2005 23:12, Peter Lissenburg wrote:
> > > Hi all,
> > > and sorry about this off topic, but I need the collective wisdom of
> > > the group. Is it OK, possible, wise, to use the noise spikes of the drive
> > > voltage of a DC motor to measure it's speed?
> > > Thanks for any guidance.
> > > Peter L.
> >
> >
> >
> >Yahoo! Groups Links
> >
> >
> >
> >


> --- In 68HC12@68HC..., Peter Lissenburg <peter@s...> wrote:
> > Thanks guys,
> > I'm looking at the CRO with a small motor running. I'm seeing

PS, Had a look a the pulse method described.

A few comments, firstly, using pulse methods will reduce brushlife and make lots of noise, look at the surpressor coils and snubbers used in SCR DC drives:-)

If you are using very small motors with pulse or tacho feedback, pure DC drive is best, only takes a bit of integrating of the pulse output from the CPU.
Bit more heat in the drive circuit.

Without such feedback you can still use the back EMF even when the motor is continously driven by means of IR feedback, this compensates for the voltage drop across the windngs etc.
Pretty well established techniques.

The CRO pictures show the drop quite well, he must have used a motor made with resistance wire... Cheers,

Theo