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'[EE]:Why use a brushless motor?'
2006\08\15@104439 by L030010

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Hi all

can anyone please advice me why is it good to use a brushless motor? and if
i need a PID control of position is it a good choice?

Regards


2006\08\15@110533 by Tamas Rudnai

face picon face
In short: better performance, more precise controlling of the rotation speed
and of course longer lifetime. In a brushed motor you are rely on the
contacts of the brushes and the speed relies on the input voltage basically.
With brushless you control the whole with a proper electric device. With the
brushless motors you can even get an outrunner that gives much better
performance as the stator and the windings are standing still while only the
relatively light magnets are spinning.

Tamas


On 15/08/06, L030010 <spam_OUTl030010TakeThisOuTspamsingnet.com.sg> wrote:
>
> Hi all
>
> can anyone please advice me why is it good to use a brushless motor? and
> if
> i need a PID control of position is it a good choice?
>
> Regards
>
>
> -

2006\08\15@111615 by M. Adam Davis

face picon face
More precise speed control, longer life (fewer things to wear out),
and much lower electrical noise.

They aren't meant to hold a particular position, though, so it
probably isn't a good choice for position control.  You could use it
that way, but probably not very precisely.

-Adam

On 8/15/06, L030010 <.....l030010KILLspamspam@spam@singnet.com.sg> wrote:
> Hi all
>
> can anyone please advice me why is it good to use a brushless motor? and if
> i need a PID control of position is it a good choice?
>
> Regards
>
>
> -

2006\08\15@112419 by Tamas Rudnai

face picon face
I think for that purpose a stepper motor would be better, is not it?

Tamas


On 15/08/06, M. Adam Davis <stienmanspamKILLspamgmail.com> wrote:
{Quote hidden}

2006\08\15@133729 by Herbert Graf

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On Tue, 2006-08-15 at 22:44 +0800, L030010 wrote:
> Hi all
>
> can anyone please advice me why is it good to use a brushless motor? and if
> i need a PID control of position is it a good choice?

Efficiency, durability and precise control of speed, compared to normal
DC brushed motors.

What you give up of course is the most basic form of driving circuitry
is much more complicated. This is negated when precise speed control is
needed however.

For controlling position I believe you are thinking of what's called a
"stepper" motor. It is also "brushless", but has many more poles
allowing for relatively precise positioning.

TTYL

2006\08\15@215832 by Mike Singer

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Herbert Graf wrote:
> > can anyone please advice me why is it good to use a brushless motor? and if
> > i need a PID control of position is it a good choice?

> For controlling position I believe you are thinking of what's called a
> "stepper" motor. It is also "brushless", but has many more poles
> allowing for relatively precise positioning.

Servos in HDD are also "brushless", position quite precise under PID.

MS

2006\08\15@232403 by Herbert Graf

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face

On Wed, 2006-08-16 at 05:58 +0400, Mike Singer wrote:
> Herbert Graf wrote:
> > > can anyone please advice me why is it good to use a brushless motor? and if
> > > i need a PID control of position is it a good choice?
> …
> > For controlling position I believe you are thinking of what's called a
> > "stepper" motor. It is also "brushless", but has many more poles
> > allowing for relatively precise positioning.
>
> Servos in HDD are also "brushless", position quite precise under PID.

Very true, but I wouldn't consider them "motors" per se, which is what
the thread was about.

A brushless motor isn't very good for positioning since it doesn't have
a great deal of "holding power" at stall. I'm sure you could devise some
PID system to turn a brushless motor into a pseudo servo, but frankly a
stepper is just so much easier I don't really see the point.

TTYL

2006\08\16@004713 by Mike Singer

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Herbert Graf wrote:
> > Servos in HDD are also "brushless", position quite precisely under PID.
>
> Very true, but I wouldn't consider them "motors" per se, which is what
> the thread was about.

Yes, you may call them servo drive, that's very close in meaning  to
motor, at least for my level of English.


> A brushless motor isn't very good for positioning since it doesn't have
> a great deal of "holding power" at stall.

It depends on efficiency of close loop. (Encoders provide positioning
information back)


> I'm sure you could devise some PID system to
> turn a brushless motor into a pseudo servo,

Google says that brushless motors are often called servo motors. I
think, you meant that steppers are usually better for positioning than
brushless (servo) motors.


> but frankly a stepper is just so much easier I don't really see the point.

I'm sure (but I can't prove it) that the very brushless (servo) motors
are used in CDs and DVDs to precisely position head, not steppers, for
example.


Best regards,
MS.


PS
Herb, I remember I was somewhat wrong to you a couple of years ago, my
apologies.

2006\08\16@011800 by Denny Esterline

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> > > For controlling position I believe you are thinking of what's called a
> > > "stepper" motor. It is also "brushless", but has many more poles
> > > allowing for relatively precise positioning.
> >
> > Servos in HDD are also "brushless", position quite precise under PID.
>
> Very true, but I wouldn't consider them "motors" per se, which is what
> the thread was about.
>
> A brushless motor isn't very good for positioning since it doesn't have
> a great deal of "holding power" at stall. I'm sure you could devise some
> PID system to turn a brushless motor into a pseudo servo, but frankly a
> stepper is just so much easier I don't really see the point.
>
> TTYL

Sorry, I can't see your logic here. Most (all?) modern CNC machining centers use brushless motors to drive the ballscrews. Holding a 10,000 pound part against a 5k rpm cutting tool driven by a 30hp motor and maintaining 0.0002" accuracy is certainly one of the more demanding positioning applications I can think of.
http://www.haascnc.com/details_HMC_HS.asp?ID=37#HMCTreeModel

Of course they cost ~$0.5Million.

-Denny



2006\08\16@015439 by Tony Smith

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{Quote hidden}

Servos also don't lose torque at speed, the faster you drive a stepper, the
less torque it has.  

Steppers aren't all that accurate anyway, (just because it says 1.8 degrees
per step doesn't mean it moves exactly 1.8 degrees), but they're close
enough for most jobs since the error doesn't accumulate.  Microstepping
improves things a bit.

Steppers are much easier to control, a driver can be as little as 4
transistors and PIC 508.

Tony

2006\08\16@042447 by Tamas Rudnai

face picon face
Wait a minute! We are talking about two things: stepper motor can be
positioned without measuring its position while servo motor uses some
position measurements (in model servos for example a potentional meter that
rotates together with the shaft so that the servo receives a pulse with
modulated signal which converted to voltage and then it will compare with
the one accumulated by the pot meter. In that case the shaft moves between
120 degree while the motor inside makes several tans or several hundreds of
rotations (depending on the gear used for that servo housing). So yes, that
way you can use a brushless motor just as anything else to position. But
with stepper motor you can make a small rotation by its nature, so that 1
step is X degree which is known by your controller therefore do not have to
measure and could be calculated how many steps needed to achive the Y
degree. If you disassemble an old floppy disk you will even see a threaded
shaft that moves the head up and down. That shaft is powered with a stepper
motor and the controller knows how many staps needed to position to cylinder
Z -- the position of the head is not monitored except if data cannot be read
it went back to park position and tried to repositioning again, that caused
that strange noise with the damaged floppies.

Tamas


On 16/08/06, Mike Singer <EraseMEznatokspam_OUTspamTakeThisOuTgmail.com> wrote:
{Quote hidden}

> -

2006\08\16@043151 by Tamas Rudnai

face picon face
Do not forget that if you appy a very good gear then you can hold any power
you want. It is nothing about the torque of the motor. It is the same as you
have a lift fork and you can lift off 2 tonns while with your hand you can
make only 100Kg. Positioning is the same: the motor rotates lets' say 1
whole cycle to make 0.0001" movement or something like that and then it
measures the position and if not statisfied can be rotate one more this or
that way... it is everything about gears and measuring the position, pretty
much less depends on the motor.

Tamas


On 16/08/06, Denny Esterline <firmwarespamspam_OUTtds.net> wrote:
{Quote hidden}

> -

2006\08\16@074634 by Gerhard Fiedler

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Tony Smith wrote:

> Servos also don't lose torque at speed, the faster you drive a stepper, the
> less torque it has.  

What kind of electric motor doesn't lose torque with increasing speed?
Doesn't a typical electric motor have an almost linear drop of torque with
increasing speed? Highest torque (stall torque) at (almost) 0 speed, 0
torque at the shaft at max. speed (when all the power is used to overcome
internal friction and other losses)?

The characteristics of a complete servo are of course also determined by
the controller, but the controller only can reduce the max. torque the
motor can deliver at any given speed, it can't increase it.

Gerhard

2006\08\16@080220 by olin piclist

face picon face
Gerhard Fiedler wrote:
> 0 torque at the shaft at max. speed (when all the power is used
> to overcome internal friction and other losses)?

Only indirectly.  Even if the motor was ideal and had no friction and zero
loss, you would still have 0 torque at the unloaded shaft speed.  The
unloaded shaft speed is the limit where the voltage applied to the motor is
cancelled by the back EMF of the motor acting like a generator.  In reality
that back EMF is a little less than the applied voltage, with the remainder
times the current going to the various power losses such as friction and
winding resistance.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2006\08\16@090853 by Tony Smith

picon face
> > Servos also don't lose torque at speed, the faster you drive a
> > stepper, the less torque it has.
>
> What kind of electric motor doesn't lose torque with increasing speed?
> Doesn't a typical electric motor have an almost linear drop
> of torque with increasing speed? Highest torque (stall
> torque) at (almost) 0 speed, 0 torque at the shaft at max.
> speed (when all the power is used to overcome internal
> friction and other losses)?
>
> The characteristics of a complete servo are of course also
> determined by the controller, but the controller only can
> reduce the max. torque the motor can deliver at any given
> speed, it can't increase it.
>
> Gerhard


Yes, the torque of a servo goes down with speed, I meant that you can tune
the system so the torque is constant regardless of speed.  Servos have a
much higher RPM than steppers, so when geared down it gives them a flatter
torque curve.  A servo can do high speed & high torque, a stepper can't.
Given two similar spec motors, the servo will usually work better.

Servos have better acceleration/deceleration preformance than steppers too.
Nicer curves.

A bad habit of steppers is resonance.  All steppers have a particular speed
where this happens, and they stall or miss steps.  Varies with load, just to
liven thing up.

Steppers have an advantage in that when the controller goes nuts, they stop.
Servos just keep going, and crash your new $200 cutter into a clamp.

Regardless, steppers are cheap and easy to drive.  Yay for steppers.

Tony

2006\08\16@095035 by Howard Winter

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flavicon
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Gerhard,

On Wed, 16 Aug 2006 08:46:00 -0300, Gerhard Fiedler wrote:

>...
> What kind of electric motor doesn't lose torque with increasing speed?

A synchronous motor.

> Doesn't a typical electric motor have an almost linear drop of torque with
> increasing speed? Highest torque (stall torque) at (almost) 0 speed, 0
> torque at the shaft at max. speed (when all the power is used to overcome
> internal friction and other losses)?

I don't know what you mean by a typical motor (there are very many designs) but no, a synchronous motor has maximum torque at its design speed,
and it falls off either side of that.  They may have zero torque at zero RPM - I remember an electric clock when I was young which would not
self-start - you had to pull out and release a knob on the back, which gave it a kick and it would run from then on, but from a standing start it was
stalled.

"Capacitor start" motors again have insufficient torque to start in their running configuration, and a capacitor is switched into circuit to change the
characteristics until a certain speed is reached, and it's then switched out.

Cheers,


Howard Winter
St.Albans, England


2006\08\16@150501 by Gerhard Fiedler

picon face
Howard Winter wrote:

>> What kind of electric motor doesn't lose torque with increasing speed?
>
> A synchronous motor.

It shows that I'm not working a lot with AC :)  I implied (in the context
of this thread not too far off) DC motors.

Gerhard

2006\08\16@231659 by John Chung

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Any shops carry servos at a reasonable price?

John

--- Tony Smith <@spam@ajsmithKILLspamspamrivernet.com.au> wrote:

{Quote hidden}

> --

2006\08\16@233235 by William Chops Westfield

face picon face

On Aug 16, 2006, at 8:16 PM, John Chung wrote:

> Any shops carry servos at a reasonable price?
>
Are you talking brushless servo motors like most of the discussion,
or radio control "servo" complete units, which are usually based on
brushed motors?

I gather that the former "servo motor" term comes from the fact that
this sort of motor with it's fine-grained brushless controllability
is used in the INDUSTRIAL scale positioning market (combined with
external feedback and controller electronics.)  The self-contained
RC "servo" is a bit of a misnamed thing (although perhaps like
Xerox and Aspirin, the mis-used term is now more common.)

BillW

2006\08\17@005838 by John Chung

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Radio control servo.

John

--- William Chops Westfield <KILLspamwestfwKILLspamspammac.com> wrote:

{Quote hidden}

> --

2006\08\17@041818 by Tony Smith

picon face
Some people use treadmill motors.  Cheap & work well, apparently.  Probably
AC.

It's not the motor that drives up the price, it's the controller.  These
have a good reputation with the DIY crowd - http://www.geckodrive.com

Power supply gets pricey too, comparatively high volt & high amp.  I've got
a couple of microwave transformers I'm going to rewind one day.

Tony


> {Original Message removed}

2006\08\17@075050 by Gerhard Fiedler

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William ChopsWestfield wrote:

> The self-contained RC "servo" is a bit of a misnamed thing (although
> perhaps like Xerox and Aspirin, the mis-used term is now more common.)

I always thought that a servo is a combination of a motor (that would be
the "servo motor"), usually some kind of gear box (sometimes the
combination of motor and gear box seems to be called "servo motor"), a
position feedback sensor, a position controller and a motor driver.

In that sense, the self-contained RC servo is a typical servo (as it
usually contains all these elements), and I don't see any misnaming. The
misnaming seems to be when calling a servo motor a "servo". Am I wrong?

Gerhard

2006\08\17@080703 by Michael Rigby-Jones

picon face


{Quote hidden}

A "servo" is the generic name for a positional control system, which usualy employes negative feedback.  An RC Servo is pretty much a self contained positioning system. A "Servo Motor" is usualy a geared motor with some form of position/velocity feedback but no control system.  Calling such a motor a "servo" is technicaly incorrect.

Regards

Mike

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