Back-EMF speed detection in motors

How calculated from PWM?

Flyback diode? Or full H-bridge? ...Jim Thompson

-- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

I assume you are using a PWM drive scheme? Can you arrange to sample the back EMF during the PWM off-time? That would get rid of the R factor to a large extent, depending on filtering.

Jon

What about the inductance? ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

As JL alluded to, if you PWM slowly enough to allow that to work, the current in the motor has to be discontinuous, which means high RMS current ( => large I^2*R losses).

Best regards, Spehro Pefhany

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"it's the network..."                          "The Journey is the reward" 
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In our 'heavy current' lectures we were taught to allow 2v drop per brush, regardless of current. No explanation was given for the rule-of-thumb.

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Le Fri, 22 Mar 2013 12:51:29 -0500, Tim Wescott a écrit:

How do you supply the motor? Is that a MOSFET bridge or do you have some free wheeling diode somewhere?

With the latter you have Vavg = D Vs - (1-D) Vd = D(Vs-Vd) - Vd which indeed has about a 1V offset per diode.

--
Thanks, 
Fred.

Le Sat, 23 Mar 2013 15:37:54 +0000, Fred Bartoli a écrit:

Oops, obviously make that Vavg = D Vs - (1-D) Vd = D(Vs+Vd) - Vd

--
Thanks, 
Fred.

[snip]

Works just as well the other way around...

This is based on a half-H-bridge drive, thus no diode forward drop to confuse the issue.

But I suspect the diode drop could be likewise easily subtracted out. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

Pondering my navel, I don't think it matters, half-H-bridge or simply a flyback diode... the average of the voltage at the upper motor terminal is still equal to Vm + 2*Im*Rm ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

Tim, Suggestion... Instal a fudge factor to take out the offset or gain error or whatever you think you are seeing and re test the system. Use this as a troubleshooting step, not as a fix. It is possible that the fudge factor will not behave as you expect and this will give you additional insight to the true problem. For example you may install a linear gain fudge factor term and discover that the error is actually second order.... Etc. Be sure to come back and let us know the answer. Mark

Full H-bridge. There'd be no way of knowing with flyback diodes -- you'd have to measure.

--
Tim Wescott 
Control system and signal processing consulting 
www.wescottdesign.com

I'm using an external resistor. It's stored in a variable named MotorR.

--
Tim Wescott 
Control system and signal processing consulting 
www.wescottdesign.com

It's H bridge, with very little dead time.

--
Tim Wescott 
Control system and signal processing consulting 
www.wescottdesign.com

On a sunny day (Sat, 23 Mar 2013 21:32:50 +0100) it happened Fabio_78 wrote in :

mm, maybe I should order a lipo-safety bag... :-) But with all those modifications I think Boeing made, may as well use lipo! Would claim back at least some weight !

Le Sat, 23 Mar 2013 16:39:14 -0500, Tim Wescott a écrit:

Yes, but which techno, and how are the switches driven? Is it MOSFET, or BJT/IGBT, in which case you have to have free wheeling diodes across the transistors. Do you have some "clever" low current ripple mode, or just plain diagonal switching?

You say the offset is circa 20/30RPM. What are your typical figures? (Vrail, motor constant or nominal speed at Vrail, typical current)

--
Thanks, 
Fred.

The motor needs to reverse?

See my other posts, I'm no longer certain that a flyback diode hurts you calculation in the slightest. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

;-) ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

Ignoring my added compensating R for the moment, what does the average terminal voltage (at the motor) have to be?

Take Vm = speed voltage, Im = motor current, Rm = winding resistance

The _average_ voltage has to be Vavg = Vm + Im*Rm

Irrespective of how it's driven, flyback diode or not.

Why is that? Return to fundamentals to see how easy the solution is

...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

Many (most?) DC motors have brushes that span more than one commutator segment.

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"For a successful technology, reality must take precedence  
over public relations, for nature cannot be fooled." 
                                       (Richard Feynman)