How to sense motor current to actuate brake?

Greetings All,
I'm using a servo motor to drive a slide and I would like to sense the
current it draws in order to actuate an electromagnetic brake. So the
more current the motor is drawing the more drag the brake puts on
another shaft. The motor is a DC permanent magnet type and will be
drawing 20 amps max at 80 volts max. I'm trying to accomplish a force
feedback system. The amount of drag needs to be easily adjustable
because I don't yet know just how much drag I'll need. One idea I have
is to use a shunt in series with the motor and sense the voltage
across the shunt just like a DC ammeter. Then amplify that voltage and
use it to power the brake. But I don't know how to do it. Maybe use
the voltage to control a transistor which in turn controls the power
to the brake? Can anyone here point me in the right direction? Maybe
tell me what kind of devices to use?
Thank You,
Eric

 

[Hammy]

On Sun, 05 Oct 2008 07:01:53 GMT, etpm@whidbey.com wrote:

Greetings All,
I'm using a servo motor to drive a slide and I would like to sense the
current it draws in order to actuate an electromagnetic brake. So the
more current the motor is drawing the more drag the brake puts on
another shaft. The motor is a DC permanent magnet type and will be
drawing 20 amps max at 80 volts max. I'm trying to accomplish a force
feedback system. The amount of drag needs to be easily adjustable
because I don't yet know just how much drag I'll need. One idea I have
is to use a shunt in series with the motor and sense the voltage
across the shunt just like a DC ammeter. Then amplify that voltage and
use it to power the brake. But I don't know how to do it. Maybe use
the voltage to control a transistor which in turn controls the power
to the brake? Can anyone here point me in the right direction? Maybe
tell me what kind of devices to use?
Thank You,
Eric

Here is the "Current Measurement Applications Handbook" from Zetex.

http://www.zetex.com/3.0/appnotes/apps/an39.pdf

Heres Zetex's site."sensing for high voltage motor control"

http://www.zetex.com/3.0/application.asp?app=58&top=5&curr=40

Current Limiter for the Motor Control ICs from nxp

http://www.nxp.com/acrobat_download/applicationnotes/AN93008_EIE.pdf

If you search Digikey or other distrubutors for current sense you will
find a couple of hundred integrated high side current sensors of
varying prices and accuaracies.

 

[Stephen J. Rush]

On Sun, 05 Oct 2008 07:01:53 +0000, etpm wrote:

Greetings All,
I'm using a servo motor to drive a slide and I would like to sense the
current it draws in order to actuate an electromagnetic brake. So the
more current the motor is drawing the more drag the brake puts on
another shaft. The motor is a DC permanent magnet type and will be
drawing 20 amps max at 80 volts max. I'm trying to accomplish a force
feedback system. The amount of drag needs to be easily adjustable
because I don't yet know just how much drag I'll need. One idea I have
is to use a shunt in series with the motor and sense the voltage across
the shunt just like a DC ammeter. Then amplify that voltage and use it
to power the brake. But I don't know how to do it. Maybe use the voltage
to control a transistor which in turn controls the power to the brake?
Can anyone here point me in the right direction? Maybe tell me what kind
of devices to use?

The current increases with the load on the motor. If you use that to
apply a brake, you have *positive* feedback, which will quickly lock the
shaft. What about measuring the actual force on the load?

 

[Jamie]

Stephen J. Rush wrote:

On Sun, 05 Oct 2008 07:01:53 +0000, etpm wrote:


Greetings All,
I'm using a servo motor to drive a slide and I would like to sense the
current it draws in order to actuate an electromagnetic brake. So the
more current the motor is drawing the more drag the brake puts on
another shaft. The motor is a DC permanent magnet type and will be
drawing 20 amps max at 80 volts max. I'm trying to accomplish a force
feedback system. The amount of drag needs to be easily adjustable
because I don't yet know just how much drag I'll need. One idea I have
is to use a shunt in series with the motor and sense the voltage across
the shunt just like a DC ammeter. Then amplify that voltage and use it
to power the brake. But I don't know how to do it. Maybe use the voltage
to control a transistor which in turn controls the power to the brake?
Can anyone here point me in the right direction? Maybe tell me what kind
of devices to use?


The current increases with the load on the motor. If you use that to
apply a brake, you have *positive* feedback, which will quickly lock the
shaft. What about measuring the actual force on the load?
Unless he's using the motor in regen state to slow down a load? We do

that our self's at work if the motor exhibits to much current in regen
state for over a set time..
The brake has an DC amplifier/Drive with slow accel set.


http://webpages.charter.net/jamie_5"

 

[Jasen Betts]

On 2008-10-05, etpm@whidbey.com <etpm@whidbey.com> wrote:

Greetings All,
I'm using a servo motor to drive a slide and I would like to sense the
current it draws in order to actuate an electromagnetic brake. So the

Is this for user feedback or some sort ot automatic control loop?

Bye.
Jasen

 

[z]

On Oct 5, 2:30 pm, "Stephen J. Rush" <sjr...@comcast.net> wrote:

On Sun, 05 Oct 2008 07:01:53 +0000, etpm wrote:
Greetings All,
I'm using a servo motor to drive a slide and I would like to sense the
current it draws in order to actuate an electromagnetic brake. So the
more current the motor is drawing the more drag the brake puts on
another shaft. The motor is a DC permanent magnet type and will be
drawing 20 amps max at 80 volts max. I'm trying to accomplish a force
feedback system. The amount of drag needs to be easily adjustable
because I don't yet know just how much drag I'll need. One idea I have
is to use a shunt in series with the motor and sense the voltage across
the shunt just like a DC ammeter. Then amplify that voltage and use it
to power the brake. But I don't know how to do it. Maybe use the voltage
to control a transistor which in turn controls the power to the brake?
Can anyone here point me in the right direction? Maybe tell me what kind
of devices to use?

The current increases with the load on the motor.  If you use that to
apply a brake, you have *positive* feedback, which will quickly lock the
shaft.  What about measuring the actual force on the load?

yeah, that's what I was thinking; shouldn't it be the less current,
the more the brake? which also gives you the opportunity of applying
the brake more or less proportional to the voltage across the motor.

 

[whit3rd]

On Oct 5, 12:01 am, e...@whidbey.com wrote:

I'm using a servo motor to drive a slide and I would like to sense the
current it draws in order to actuate an electromagnetic brake. So the
more current the motor is drawing the more drag the brake puts on
another shaft.

If the brake exerts force on a shaft, it's just another servo-motor,
isn't
it? That suggests you use two motors, two amplifiers, and
some proportional inputs to the amplifiers.

Or, do you mean 'drag' in the sense of viscous drag, a force
that is proportional to the square of velocity?

You mention 'force feedback', is that because there is some unknown
third quantity (variation of the driven mass, or friction, or
inelastic
deformation) in the system, and you want to compensate in some
way for that quantity, in real time, rather than prepare/design for
a fixed value?

 

On Sun, 05 Oct 2008 07:05:42 -0400, Hammy <spamme@hotmail.com> wrote:

On Sun, 05 Oct 2008 07:01:53 GMT, etpm@whidbey.com wrote:

Greetings All,
I'm using a servo motor to drive a slide and I would like to sense the
current it draws in order to actuate an electromagnetic brake. So the
more current the motor is drawing the more drag the brake puts on
another shaft. The motor is a DC permanent magnet type and will be
drawing 20 amps max at 80 volts max. I'm trying to accomplish a force
feedback system. The amount of drag needs to be easily adjustable
because I don't yet know just how much drag I'll need. One idea I have
is to use a shunt in series with the motor and sense the voltage
across the shunt just like a DC ammeter. Then amplify that voltage and
use it to power the brake. But I don't know how to do it. Maybe use
the voltage to control a transistor which in turn controls the power
to the brake? Can anyone here point me in the right direction? Maybe
tell me what kind of devices to use?
Thank You,
Eric

Here is the "Current Measurement Applications Handbook" from Zetex.

http://www.zetex.com/3.0/appnotes/apps/an39.pdf

Heres Zetex's site."sensing for high voltage motor control"

http://www.zetex.com/3.0/application.asp?app=58&top=5&curr=40

Current Limiter for the Motor Control ICs from nxp

http://www.nxp.com/acrobat_download/applicationnotes/AN93008_EIE.pdf

If you search Digikey or other distrubutors for current sense you will
find a couple of hundred integrated high side current sensors of
varying prices and accuaracies.
Thanks for the info Hammy. I'll check out the links.

Eric

 

On Mon, 6 Oct 2008 13:47:04 -0700 (PDT), whit3rd <whit3rd@gmail.com>
wrote:

On Oct 5, 12:01=A0am, e...@whidbey.com wrote:

I'm using a servo motor to drive a slide and I would like to sense the
current it draws in order to actuate an electromagnetic brake. So the
more current the motor is drawing the more drag the brake puts on
another shaft.

If the brake exerts force on a shaft, it's just another servo-motor,
isn't
it? That suggests you use two motors, two amplifiers, and
some proportional inputs to the amplifiers.

Or, do you mean 'drag' in the sense of viscous drag, a force
that is proportional to the square of velocity?

You mention 'force feedback', is that because there is some unknown
third quantity (variation of the driven mass, or friction, or
inelastic
deformation) in the system, and you want to compensate in some
way for that quantity, in real time, rather than prepare/design for
a fixed value?
I wasn't clear in my first post. Re-reading it I see that I imply that

the brake goes on the motor shaft. The brake actually is for another
shaft that is turned by hand. This shaft has an encoder and a dial on
it. I have already made the setup and turning the shaft with the dial
causes the servo motor to follow. The next step is to load the servo
and for me to sense the load by a drag put on the shaft I'm turning.
Thanks for all the replies so far.
Eric

 

[John Fields]

On Wed, 08 Oct 2008 17:48:05 GMT, etpm@whidbey.com wrote:

I wasn't clear in my first post. Re-reading it I see that I imply that
the brake goes on the motor shaft. The brake actually is for another
shaft that is turned by hand. This shaft has an encoder and a dial on
it. I have already made the setup and turning the shaft with the dial
causes the servo motor to follow. The next step is to load the servo
and for me to sense the load by a drag put on the shaft I'm turning.
Thanks for all the replies so far.

---
Nicely stated.

You want force feedback so that as the load on the servo motor varies,
you want that load to be reflected back to the crank you're turning by
making it harder to turn when the servo's heavily loaded and easier to
turn when it's less heavily loaded, yes?

What I'd do would be to make a dashpot by mechanically connecting a
permanent-magnet DC motor to the shaft you turn by hand, and then use a
MOSFET's drain and source terminals connected across the input terminals
of the motor in order to change its [the MOSFET's] resistance as a
function of the load across the servomotor, such that:

Low load = High resistance, and

High load = Low resistance

That way, the higher the current in the servo motor the lower the
resistance of the MOSFET and the more difficult it would be turn your
shaft, according to Lenz's law.


JF

 

On Wed, 08 Oct 2008 18:24:47 -0500, John Fields
<jfields@austininstruments.com> wrote:

On Wed, 08 Oct 2008 17:48:05 GMT, etpm@whidbey.com wrote:


I wasn't clear in my first post. Re-reading it I see that I imply that
the brake goes on the motor shaft. The brake actually is for another
shaft that is turned by hand. This shaft has an encoder and a dial on
it. I have already made the setup and turning the shaft with the dial
causes the servo motor to follow. The next step is to load the servo
and for me to sense the load by a drag put on the shaft I'm turning.
Thanks for all the replies so far.

---
Nicely stated.

You want force feedback so that as the load on the servo motor varies,
you want that load to be reflected back to the crank you're turning by
making it harder to turn when the servo's heavily loaded and easier to
turn when it's less heavily loaded, yes?

What I'd do would be to make a dashpot by mechanically connecting a
permanent-magnet DC motor to the shaft you turn by hand, and then use a
MOSFET's drain and source terminals connected across the input terminals
of the motor in order to change its [the MOSFET's] resistance as a
function of the load across the servomotor, such that:

Low load = High resistance, and

High load = Low resistance

That way, the higher the current in the servo motor the lower the
resistance of the MOSFET and the more difficult it would be turn your
shaft, according to Lenz's law.


JF
Greetings John,

If I understand your post the voltage across a shunt placed in one of
the servo motor power leads could be used, by connecting to the gate
of a MOSFET, as a variable resistor shorting the power leads of a PMDC
motor in order to make it hard to turn. Is that correct? I have
shorted the leads on several PMDC motors that I have and none of them
have enough drag. A couple servo motors I have come close but they are
way too big, and too valuable, to use. My idea is to use a magnetic
brake. I already have one and a mock up shows that it will work fine.
This is good because it's a stock item and I might need lots of them.
Thanks,
Eric

 

[John Fields]

On Fri, 10 Oct 2008 01:00:38 GMT, etpm@whidbey.com wrote:

On Wed, 08 Oct 2008 18:24:47 -0500, John Fields
jfields@austininstruments.com> wrote:

On Wed, 08 Oct 2008 17:48:05 GMT, etpm@whidbey.com wrote:


I wasn't clear in my first post. Re-reading it I see that I imply that
the brake goes on the motor shaft. The brake actually is for another
shaft that is turned by hand. This shaft has an encoder and a dial on
it. I have already made the setup and turning the shaft with the dial
causes the servo motor to follow. The next step is to load the servo
and for me to sense the load by a drag put on the shaft I'm turning.
Thanks for all the replies so far.

---
Nicely stated.

You want force feedback so that as the load on the servo motor varies,
you want that load to be reflected back to the crank you're turning by
making it harder to turn when the servo's heavily loaded and easier to
turn when it's less heavily loaded, yes?

What I'd do would be to make a dashpot by mechanically connecting a
permanent-magnet DC motor to the shaft you turn by hand, and then use a
MOSFET's drain and source terminals connected across the input terminals
of the motor in order to change its [the MOSFET's] resistance as a
function of the load across the servomotor, such that:

Low load = High resistance, and

High load = Low resistance

That way, the higher the current in the servo motor the lower the
resistance of the MOSFET and the more difficult it would be turn your
shaft, according to Lenz's law.


JF
Greetings John,
If I understand your post the voltage across a shunt placed in one of
the servo motor power leads could be used, by connecting to the gate
of a MOSFET, as a variable resistor shorting the power leads of a PMDC
motor in order to make it hard to turn. Is that correct? I have
shorted the leads on several PMDC motors that I have and none of them
have enough drag. A couple servo motors I have come close but they are
way too big, and too valuable, to use. My idea is to use a magnetic
brake. I already have one and a mock up shows that it will work fine.
This is good because it's a stock item and I might need lots of them.

---
If you already have a solution, then why you posted for help in finding
a solution eludes me.


JF

 

On Thu, 09 Oct 2008 23:20:52 -0500, John Fields
<jfields@austininstruments.com> wrote:

On Fri, 10 Oct 2008 01:00:38 GMT, etpm@whidbey.com wrote:

On Wed, 08 Oct 2008 18:24:47 -0500, John Fields
jfields@austininstruments.com> wrote:

On Wed, 08 Oct 2008 17:48:05 GMT, etpm@whidbey.com wrote:


I wasn't clear in my first post. Re-reading it I see that I imply that
the brake goes on the motor shaft. The brake actually is for another
shaft that is turned by hand. This shaft has an encoder and a dial on
it. I have already made the setup and turning the shaft with the dial
causes the servo motor to follow. The next step is to load the servo
and for me to sense the load by a drag put on the shaft I'm turning.
Thanks for all the replies so far.

---
Nicely stated.

You want force feedback so that as the load on the servo motor varies,
you want that load to be reflected back to the crank you're turning by
making it harder to turn when the servo's heavily loaded and easier to
turn when it's less heavily loaded, yes?

What I'd do would be to make a dashpot by mechanically connecting a
permanent-magnet DC motor to the shaft you turn by hand, and then use a
MOSFET's drain and source terminals connected across the input terminals
of the motor in order to change its [the MOSFET's] resistance as a
function of the load across the servomotor, such that:

Low load = High resistance, and

High load = Low resistance

That way, the higher the current in the servo motor the lower the
resistance of the MOSFET and the more difficult it would be turn your
shaft, according to Lenz's law.


JF
Greetings John,
If I understand your post the voltage across a shunt placed in one of
the servo motor power leads could be used, by connecting to the gate
of a MOSFET, as a variable resistor shorting the power leads of a PMDC
motor in order to make it hard to turn. Is that correct? I have
shorted the leads on several PMDC motors that I have and none of them
have enough drag. A couple servo motors I have come close but they are
way too big, and too valuable, to use. My idea is to use a magnetic
brake. I already have one and a mock up shows that it will work fine.
This is good because it's a stock item and I might need lots of them.

---
If you already have a solution, then why you posted for help in finding
a solution eludes me.


JF
Greetings John,

Thanks for your replies so far. I don't have a complete solution yet
because I don't really know how to do the elecronics part. I should
have been even more clear and stated that I had the brake already. It
is a 24 volt device and it draws only about 3 amps. And even though I
kind of understand in theory about how a MOSFET works I don't know
enough yet how to decide which one to use. On top of that, I had a
hard time believing it would be so simple, just a shunt and a MOSFET.
Since the MOSFET will be acting as a variable reisitor will it need to
dissipate as many watts as a variable resistor would? And in order to
make the brake adjustable electrically wouldn't it be best to vary the
voltage supply to the brake instead of using a variable resistor in
series with the power supply to the brake?
Thanks,
Eric

 

[whit3rd]

On Oct 10, 12:58 pm, e...@whidbey.com wrote:

... I had the brake already. It
is a 24 volt device and it draws only about 3 amps.

And the goal is to make the brake respond proportional to
the (very large) current in the servo motor, assuming
current is proportional to the motor force.

If you rewound the brake so it was (what was it? 80 amps?)
instead of 3 amps, you could just wire it in series with
the servo motor. What you want is proportional current
in the brake and the motor.

If the servo amplifier is a switching type, you can probably
program a switchmode power supply with a copy of the
servo internal switch signal, to make a slave current
source to drive the brake.

 

On Fri, 10 Oct 2008 14:29:35 -0700 (PDT), whit3rd <whit3rd@gmail.com>
wrote:

On Oct 10, 12:58=A0pm, e...@whidbey.com wrote:
... I had the brake already. It
is a 24 volt device and it draws only about 3 amps.

And the goal is to make the brake respond proportional to
the (very large) current in the servo motor, assuming
current is proportional to the motor force.

If you rewound the brake so it was (what was it? 80 amps?)
instead of 3 amps, you could just wire it in series with
the servo motor. What you want is proportional current
in the brake and the motor.

If the servo amplifier is a switching type, you can probably
program a switchmode power supply with a copy of the
servo internal switch signal, to make a slave current
source to drive the brake.
Thanks for the ideas. But what I need is something adjustable and as

simple and as inexpensive as possible. The amount of force feedback
will be different for different setups and people. So the ability to
use stock components and easy adjustability are important.
Thanks again,
Eric

 

[John Fields]

On Fri, 10 Oct 2008 19:58:03 GMT, etpm@whidbey.com wrote:

On Thu, 09 Oct 2008 23:20:52 -0500, John Fields
jfields@austininstruments.com> wrote:

On Fri, 10 Oct 2008 01:00:38 GMT, etpm@whidbey.com wrote:

On Wed, 08 Oct 2008 18:24:47 -0500, John Fields
jfields@austininstruments.com> wrote:

On Wed, 08 Oct 2008 17:48:05 GMT, etpm@whidbey.com wrote:


I wasn't clear in my first post. Re-reading it I see that I imply that
the brake goes on the motor shaft. The brake actually is for another
shaft that is turned by hand. This shaft has an encoder and a dial on
it. I have already made the setup and turning the shaft with the dial
causes the servo motor to follow. The next step is to load the servo
and for me to sense the load by a drag put on the shaft I'm turning.
Thanks for all the replies so far.

---
Nicely stated.

You want force feedback so that as the load on the servo motor varies,
you want that load to be reflected back to the crank you're turning by
making it harder to turn when the servo's heavily loaded and easier to
turn when it's less heavily loaded, yes?

What I'd do would be to make a dashpot by mechanically connecting a
permanent-magnet DC motor to the shaft you turn by hand, and then use a
MOSFET's drain and source terminals connected across the input terminals
of the motor in order to change its [the MOSFET's] resistance as a
function of the load across the servomotor, such that:

Low load = High resistance, and

High load = Low resistance

That way, the higher the current in the servo motor the lower the
resistance of the MOSFET and the more difficult it would be turn your
shaft, according to Lenz's law.


JF
Greetings John,
If I understand your post the voltage across a shunt placed in one of
the servo motor power leads could be used, by connecting to the gate
of a MOSFET, as a variable resistor shorting the power leads of a PMDC
motor in order to make it hard to turn. Is that correct? I have
shorted the leads on several PMDC motors that I have and none of them
have enough drag. A couple servo motors I have come close but they are
way too big, and too valuable, to use. My idea is to use a magnetic
brake. I already have one and a mock up shows that it will work fine.
This is good because it's a stock item and I might need lots of them.

---
If you already have a solution, then why you posted for help in finding
a solution eludes me.


JF
Greetings John,
Thanks for your replies so far. I don't have a complete solution yet
because I don't really know how to do the elecronics part. I should
have been even more clear and stated that I had the brake already. It
is a 24 volt device and it draws only about 3 amps. And even though I
kind of understand in theory about how a MOSFET works I don't know
enough yet how to decide which one to use. On top of that, I had a
hard time believing it would be so simple, just a shunt and a MOSFET.
Since the MOSFET will be acting as a variable reisitor will it need to
dissipate as many watts as a variable resistor would? And in order to
make the brake adjustable electrically wouldn't it be best to vary the
voltage supply to the brake instead of using a variable resistor in
series with the power supply to the brake?

---
What I had in mind was something like this: (View in Courier)

RATE<----------------------------------+
|
Vs>--+ |
| |
[LOAD] +----+ |
| | | | |H|===
+--------|+\ D | | |A|
| | >--+---G | | |N|
| +--|-/ | S | | |D|
| | | | [MOTOR]--[ENC]--|C|
[SHUNT] +--[R1]--+ | | |R|
| | | | |A|
| [R2] | | |N|
| | | | |K|
GND>-+-----+--------------+----+

where, as the current through the load increased,the voltage across the
shunt would also increase,and then with some gain from the opamp the
voltage on the gate of the MOSFET would rise, lowering the resistance of
its drain-to-source channel.

Then, since the PM motor is being driven, mechanically, by the crank
attached to its shaft, it's being used as a generator and the lower the
resistance connecting its terminals the harder it'll be to turn.

From your description, though, the magnetic brake you have on hand also
seems to be a DC motor, but one which is excited externally in that the
voltage impressed across its terminals will generate a torque which will
be felt at the hand crank.

If that's the case, then you could try this:

RATE<-----------------------------+
|
| |H|===
+24>---------------------+ | |A|
| | |N|
Vs>--+ | | |D|
| [BRAKE]---[ENC]----|C|
[LOAD] | |R|
| | |A|
+--------|+\ D |N|
| | >--+--G |K|
| +--|-/ | S
| | | |
[SHUNT] +--[R1]--+ |
| | |
| [R2] |
| | |
GND>-+-----+-------------+

or, if the response seems to be too 'quick', this:

RATE<-----------------------------+
|
|
+24>---------------------+ |
| |
Vs>--+ | |
| | |
[LOAD] | |
| | |
+--------|+\ D |
| | >--+--G | |H|===
| +--|-/ | S | |A|
| | | | | |N|
[SHUNT] +--[R1]--+ | | |D|
| | [BRAKE]---[ENC]---|C]
| [R2] | |R|
| | | |A|
GND>-+-----+-------------+ |N|
|K|


JF

 

On Sat, 11 Oct 2008 07:38:29 -0500, John Fields
<jfields@austininstruments.com> wrote:

On Fri, 10 Oct 2008 19:58:03 GMT, etpm@whidbey.com wrote:

On Thu, 09 Oct 2008 23:20:52 -0500, John Fields
jfields@austininstruments.com> wrote:

On Fri, 10 Oct 2008 01:00:38 GMT, etpm@whidbey.com wrote:

On Wed, 08 Oct 2008 18:24:47 -0500, John Fields
jfields@austininstruments.com> wrote:

On Wed, 08 Oct 2008 17:48:05 GMT, etpm@whidbey.com wrote:


I wasn't clear in my first post. Re-reading it I see that I imply that
the brake goes on the motor shaft. The brake actually is for another
shaft that is turned by hand. This shaft has an encoder and a dial on
it. I have already made the setup and turning the shaft with the dial
causes the servo motor to follow. The next step is to load the servo
and for me to sense the load by a drag put on the shaft I'm turning.
Thanks for all the replies so far.

---
Nicely stated.

You want force feedback so that as the load on the servo motor varies,
you want that load to be reflected back to the crank you're turning by
making it harder to turn when the servo's heavily loaded and easier to
turn when it's less heavily loaded, yes?

What I'd do would be to make a dashpot by mechanically connecting a
permanent-magnet DC motor to the shaft you turn by hand, and then use a
MOSFET's drain and source terminals connected across the input terminals
of the motor in order to change its [the MOSFET's] resistance as a
function of the load across the servomotor, such that:

Low load = High resistance, and

High load = Low resistance

That way, the higher the current in the servo motor the lower the
resistance of the MOSFET and the more difficult it would be turn your
shaft, according to Lenz's law.


JF
Greetings John,
If I understand your post the voltage across a shunt placed in one of
the servo motor power leads could be used, by connecting to the gate
of a MOSFET, as a variable resistor shorting the power leads of a PMDC
motor in order to make it hard to turn. Is that correct? I have
shorted the leads on several PMDC motors that I have and none of them
have enough drag. A couple servo motors I have come close but they are
way too big, and too valuable, to use. My idea is to use a magnetic
brake. I already have one and a mock up shows that it will work fine.
This is good because it's a stock item and I might need lots of them.

---
If you already have a solution, then why you posted for help in finding
a solution eludes me.


JF
Greetings John,
Thanks for your replies so far. I don't have a complete solution yet
because I don't really know how to do the elecronics part. I should
have been even more clear and stated that I had the brake already. It
is a 24 volt device and it draws only about 3 amps. And even though I
kind of understand in theory about how a MOSFET works I don't know
enough yet how to decide which one to use. On top of that, I had a
hard time believing it would be so simple, just a shunt and a MOSFET.
Since the MOSFET will be acting as a variable reisitor will it need to
dissipate as many watts as a variable resistor would? And in order to
make the brake adjustable electrically wouldn't it be best to vary the
voltage supply to the brake instead of using a variable resistor in
series with the power supply to the brake?

---
What I had in mind was something like this: (View in Courier)

RATE<----------------------------------+
|
Vs>--+ |
| |
[LOAD] +----+ |
| | | | |H|===
+--------|+\ D | | |A|
| | >--+---G | | |N|
| +--|-/ | S | | |D|
| | | | [MOTOR]--[ENC]--|C|
[SHUNT] +--[R1]--+ | | |R|
| | | | |A|
| [R2] | | |N|
| | | | |K|
GND>-+-----+--------------+----+

where, as the current through the load increased,the voltage across the
shunt would also increase,and then with some gain from the opamp the
voltage on the gate of the MOSFET would rise, lowering the resistance of
its drain-to-source channel.

Then, since the PM motor is being driven, mechanically, by the crank
attached to its shaft, it's being used as a generator and the lower the
resistance connecting its terminals the harder it'll be to turn.

From your description, though, the magnetic brake you have on hand also
seems to be a DC motor, but one which is excited externally in that the
voltage impressed across its terminals will generate a torque which will
be felt at the hand crank.

If that's the case, then you could try this:

RATE<-----------------------------+
|
| |H|===
+24>---------------------+ | |A|
| | |N|
Vs>--+ | | |D|
| [BRAKE]---[ENC]----|C|
[LOAD] | |R|
| | |A|
+--------|+\ D |N|
| | >--+--G |K|
| +--|-/ | S
| | | |
[SHUNT] +--[R1]--+ |
| | |
| [R2] |
| | |
GND>-+-----+-------------+

or, if the response seems to be too 'quick', this:

RATE<-----------------------------+
|
|
+24>---------------------+ |
| |
Vs>--+ | |
| | |
[LOAD] | |
| | |
+--------|+\ D |
| | >--+--G | |H|===
| +--|-/ | S | |A|
| | | | | |N|
[SHUNT] +--[R1]--+ | | |D|
| | [BRAKE]---[ENC]---|C]
| [R2] | |R|
| | | |A|
GND>-+-----+-------------+ |N|
|K|


JF
Greetings John,

The schematics help some. I thought I might need an op-amp. Now I'll
look in the books to figure out how to choose which op-amp and what R1
and R2 do. I'll let you know how things work out.
Thanks,
Eric