12Vdc Solenoid Driver Circuit requ'd.

[Rich Grise]

I suggest a spring and dashpot. The solenoid can snap in and pull on the
spring. It is easier to get springs that repeat well.

I think 4 seconds is too long of a time.

Hardly "sophisticated"...

If you make the air leak on the hash pot just right, you might be able to
get the Startrek door opening "pshhhht" sound out of it.
--[/QUOTE]
Then you'd need something for that little "toink" at the end. ;-)

Cheers!
Rich

 

[Tom Seim]

Well- that post is just pseudo-OnT #10,000 indicating you don't know
your butt from a hole in the ground about anything electronic. Give it
up, non-technical maggot swivel chair operator senior scientist IV. Your
only skill set is making a power point presentation about your
half-assed organization that serves no purpose other than preserving itself.

I guess that's why I'm employed and you're not.

 

[Tom Seim]

Plunger position varies the inductance so there is some chance of a closed
loop solution. If you servo the inductance to follow a ramp, the punger
would have to move slowly. The tricky bit is measuring the inductance,
applying the power and closing the loop all at the same time.


--

It may be possible to take an COTS solenoid and modify it with a
custom plunger/pole face to be more linear. This might be easier than
a nonlinear feedback controller.

 

[N. Thornton]

Well- that post is just pseudo-OnT #10,000 indicating you don't know
your butt from a hole in the ground about anything electronic. Give it
up, non-technical maggot swivel chair operator senior scientist IV. Your
only skill set is making a power point presentation about your
half-assed organization that serves no purpose other than preserving itself.

if you check out a diagram for an 'inductor dynamic loudspeaker'
you'll find just such a mechanism, position linearly proportional to
coil current. A modern moving coil speaker mecha would also give you
that. Even the old balanced armature movement would give you, if not
linearity, close enough for the job.

I cant imagine the OP finding those old movements though, would have
to be made to order, and tronic control would presumably be cheaper
and easier.

If youre driving the coil pwm, during the off half cycle the rate of
decay of current tells you the inductance which tells you position, so
I should think it would be quite doable.


NT

 

[John Fields]

It may be possible to take an COTS solenoid and modify it with a
custom plunger/pole face to be more linear. This might be easier than
a nonlinear feedback controller.

---
Arguably the easiest way would be to run it open-ended. To get the
ramping function, connect it to a power supply, crank up the voltage
until it (the plunger) moved to the starting position, then measure
the voltage at however many intervals you wanted to consider until it
got to where it was supposed to end up, then take that table and
interpolate between the intervals to get the granularity you wanted,
burn that data into ROM, hook a DAC to the data outputs of the ROM and
a counter to the address inputs and count through them so that the
time from start to finish is as long as you want the plunger to be
moving, and you're done. Oh... the output of the DAC needs to feed a
voltage follower which can output enough current to drive the
solenoid.

 

[Fred Bloggs]

I guess that's why I'm employed and you're not.

You just don't 'sound' like someone who is employed. That's why you
became a 'body' for Battelle.

 

[petrus bitbyter]

It forms part of a product development. Solenoid orientation is
horizontal. Load is sprung which restores the plunger after a "on"
period of about 4 seconds. The need for slow motion is to add
sophistication.

Maybe it's too late and I'm too sleepy but let me think aloud. As far as I
understand from the discussion so far, the solenoid is a typical on/off
device like I mentioned before. To induce it to show some analog behavior,
you'll have to control the power that is driving it: The current. Although
the solenoid is inductive by its nature, it is the magnetical "force" that
is wanted. So the coil will have not too many turns of relative thick wire
to get the maximum current as fast as possible. One way to slow down the
movement is reducing the current as tried by PMW. Even then the current is
build up so fast that the result is stuttery. One way to slow down the
current is a coil in series with the solenoid. In contrary with the solenoid
it will need as many turns as possible, a core of a magnetic material that
enlarges induction and wire that should be thick enough to withstand the
maximum current through the solenoid. (The pure DC resistance has also be
taken into account.) I can only guess a coil on its own would become very
fat but combined with PMW you may have something to start with. If load and
other circumstances are the same during every full stroke, you can use a
micro and some heuristically build tables to become a more or less lineair
movement like some others mentioned already. Should be enough.

But.. thoughts keep coming. Should a solenoid driven by high enough a
frequency PMW become stuttery? And if it does will a serial coil be enough
to prevent it? Maybe the plunger has to overcome some friction? If that is
the situation, the load is variable and highly unpredictable. As long as the
load moves, it requires much less power to keep it moving then when it has
stopped and has to start moving again. Remember we are controlling the power
by controlling the current. (For myself I remember some older drill speed
controllers. Rotation speed was also controlled by controlling the power and
became very dependable of the load.) Think you will need a position
determining device that reports to a controller which decides what current
is required to obtain the movement you wish to be made.

Also the dashpot comes to my mind. Will a stroke of 10mm be enough to build
up enough pressure to reduce the speed adequately? What about cost and
maintenance? That mechanical things tend to wear out much more rapidly then
electronics. At this point I've only questions. Leave the answers to
mechanics.

petrus bitbyter

 

[Ken Smith]

[... me ..]

If you make the air leak on the hash pot just right, you might be able to
get the Startrek door opening "pshhhht" sound out of it.
--

Then you'd need something for that little "toink" at the end. ;-)

Something has to stop the travel.

or: Add a PIC and a little speaker and you can make it close "with a self
satisfied hum".

 

[Ken Smith]

It may be possible to take an COTS solenoid and modify it with a
custom plunger/pole face to be more linear. This might be easier than
a nonlinear feedback controller.

Yes but easier may not be worth it. You have to make the modifications on
each unit or solve the messy control problem once. If it is mass
produced, the second may be the best way to go. I suspect that a PIC,
a power tranisistor a few diodes and resistors would about solve it.

 

[Ken Smith]

Arguably the easiest way would be to run it open-ended.

I disagree. Getting inductive elements that are accurate to 10% is hard.
Getting 1% resistors is easy. I strongly suspect that using feedback
would allow a wider range of mechanical parts.

To get the
ramping function, connect it to a power supply, crank up the voltage
until it (the plunger) moved to the starting position, then measure
the voltage

Yes but .... "crank up the current" while monitoring the voltage. The
inward motion of the plunger causes the voltage to rise abruptly. If you
digitized that curve, and applied that voltage profile, your idea may work
well enough. We are just talking about getting a "quality feel" out of
some "cheap junk" parts here. We are not launching a space probe.

 

[Rich Grise]

[... me ..]

If you make the air leak on the hash pot just right, you might be able
to get the Startrek door opening "pshhhht" sound out of it.
--

Then you'd need something for that little "toink" at the end. ;-)

Something has to stop the travel.

or: Add a PIC and a little speaker and you can make it close "with a self
satisfied hum".

Well, here's the files:
http://www.old-hippie.com/sound_files/pg/pgw/startrek.html

( I really like that domain name! ;-) )

Cheers!
Rich

 

[Rich Grise]

Also the dashpot comes to my mind. Will a stroke of 10mm be enough to
build up enough pressure to reduce the speed adequately? What about cost
and maintenance? That mechanical things tend to wear out much more rapidly
then
electronics. At this point I've only questions. Leave the answers to
mechanics.

Just use one of these, and build a LIM:
http://rich_grise.tripod.com/images/Solenoid.gif

Cheers!
Rich

 

[John Fields]

I disagree. Getting inductive elements that are accurate to 10% is hard.
Getting 1% resistors is easy. I strongly suspect that using feedback
would allow a wider range of mechanical parts.

---
Hmmm... You're arguing, so I guess I was right. Anyway, take a look
at the last two sentences in your post. Unless you're using actual
physical position as as feedback, I don't think using 1% resistors
will make much difference if the characteristics of the magnetic
structure are as variable as you seem to think they are.

How about it? Have you got any data to supprt your claim?
---

Yes but .... "crank up the current" while monitoring the voltage. The
inward motion of the plunger causes the voltage to rise abruptly.

---
So what? Whether you crank either the current or the voltage the
other will follow and the plunger will move. The voltage will settle
in a few milliseconds or so, and when it does and the solenoid plunger
is where you want it to be, _that's_ when you measure the voltage.
There's really no reason to monitor it while the plunger is being
moved, since what's important is what the voltage across the solenoid
happens to be when the plunger is positioned properly, not what it's
doing while it's getting there.
---

If you digitized that curve, and applied that voltage profile, your idea may work
well enough.

---
"If"? Did you miss this part?:

"burn that data into ROM, hook a DAC to the data outputs of the ROM
and a counter to the address inputs and count through them so that the
time from start to finish is as long as you want the plunger to be
moving, and you're done."
---

We are just talking about getting a "quality feel" out of
some "cheap junk" parts here. We are not launching a space probe.

---
Seems like, for some reason, you always have to follow grudging
acceptance with a reprimand. NIH envy or something like that,
perhaps?

In any case, continuing along the open-ended digital path, it's a
small jump from

[COUNTER]--[ROM LUT]--[DAC]--[DC AMP]--[SOLENOID]

to

[µC]--[R-2R]--[DC AMP]--[SOLENOID]

Which would certainly be cheap, (even for a one-off) but hardly junk
or rocket science.

BTW, perhaps a way to even out the differences between the
electrical/mechanical transfer functions in the solenoids would be to
implement a mechanical 'spring stretcher' which would allow the center
position of the solenoid to be adjusted mechanically with a given,
empirically derived current flowing through the winding.

Or perhaps (even simpler) a spring of fixed length and spring rate,
and offset and span adjustments on the amp?

 

[Ken Smith]

Unless you're using actual
physical position as as feedback, I don't think using 1% resistors
will make much difference if the characteristics of the magnetic
structure are as variable as you seem to think they are.

How about it? Have you got any data to supprt your claim?

For 1% resistors making a difference when you end up having to make the
thing self-calibrate no.

For inductors better than 10% being hard to get, consult any makers data
sheets.

For electromechanical things not repeating well I only have my hands on
experience with this.

---


---
So what? Whether you crank either the current or the voltage the
other will follow and the plunger will move. The voltage will settle
in a few milliseconds or so, and when it does and the solenoid plunger
is where you want it to be, _that's_ when you measure the voltage.

That won't work for 2 reasons.

(1) For a constant current, the force on a plunger increases as it move
into the coil. This means that when the plunger starts to move will
usually be when it snaps completely in.

(2) The moving plunger either (a) changes the current or (b) changes the
voltage, depending on which of the two you are controlling. The current
does not settle quickly at all.

 

[John Fields]

For 1% resistors making a difference when you end up having to make the
thing self-calibrate no.

For inductors better than 10% being hard to get, consult any makers data
sheets.

---
We're not talking inductors here as much as we are electromagnets and
the change in force which occurs as the spring-loaded plunger
traverses the bore and, for the purpose of this discussion, I'm going
to assume that 10% repeatability from unit to unit is easily
achievable unless I'm apprised otherwise.
---

For electromechanical things not repeating well I only have my hands on
experience with this.

---
Errmmm... O.K., but hard drive head motors are pretty good, and folks
have been using magnetic compasses to get un-lost for a long time, no?
---

That won't work for 2 reasons.

(1) For a constant current, the force on a plunger increases as it move
into the coil. This means that when the plunger starts to move will
usually be when it snaps completely in.

---
You forget that:

1. There's a spring connected to the plunger which will be opposing
the force exerted by the magnetic field.

2. Strictly speaking, the current will only be constant when it
settles down between the discrete voltage steps used to drive the
plunger. But once it does, and the plunger stops moving, the current
in the coil will be simply I = E/R and the position of the plunger
will be the equilibrium point between the force generated by the
magnetic field and the opposing force generated by the spring.
---

 

[John Fields]

We're not talking inductors here as much as we are electromagnets and
the change in force which occurs as the spring-loaded plunger
traverses the bore and, for the purpose of this discussion, I'm going
to assume that 10% repeatability from unit to unit is easily
achievable unless I'm apprised otherwise.

 

[Ken Smith]

We're not talking inductors here as much as we are electromagnets and
the change in force which occurs as the spring-loaded plunger
traverses the bore and, for the purpose of this discussion, I'm going
to assume that 10% repeatability from unit to unit is easily
achievable unless I'm apprised otherwise.

Assume away and lets see if an open loop design comes out as useful.

Errmmm... O.K., but hard drive head motors are pretty good,

Harddrive "voice coil" motors are quite a different matter than the
solenoids commonly used in consumer or industrial products.

and folks
have been using magnetic compasses to get un-lost for a long time, no?

You can have a 100:1 difference in the size and shape and magnetization of
a compass and it still point north quite well.

You forget that:

1. There's a spring connected to the plunger which will be opposing
the force exerted by the magnetic field.

Using the www.digikey.com/WebLib/Pontiac coil/Web Data/L-82 Solenoid as
the example.

Taking points from the lower curve, about as well as I can and also taking
the the normal spring equation:

F = KX

We can say that the system will be stable if the spring's force will grow
faster than the plunger's. We can assume that we are operating in the
linear range of the magnetics so that increasing the current by 10%
increases the force by 10%.

Position force
0.1 25
0.2 11
0.3 6
0.4 4
0.5 2

You can see the the solenoid doesn't follow a linear curve. If we assume
that the sping is zero force at 0.6 inches, the spring constant must be,
lets use, 5 to make the 0.1 point equal the value from the curve.

Based on this examine the X=0.2 position and notice that a higher current
is needed to match the spring at this point than at the X=0.6. This means
that the system is not stable at this position without some external
feedback.

2. Strictly speaking, the current will only be constant when it
settles down between the discrete voltage steps used to drive the
plunger. But once it does, and the plunger stops moving, the current
in the coil will be simply I = E/R and the position of the plunger
will be the equilibrium point between the force generated by the
magnetic field and the opposing force generated by the spring.

You are assuming that a sping can have the needed curve shape to make the
position stable. If that could be done then yes, the system will settle
to a current with the plunger at the desired position. If not, the
plunger will always either snap in or pop out before things really have
settled.

How quickly it settles depends on the time constant of the coil and
driver and on the inertia and damping of the plunger. What do you
mean by "not quickly"? That is, do you have a number in mind?

The main thing the settling time depends on is the mechanical time
constants. For practical designs they are the bigger value. The
mechanical time constant will be 10s of mS up to 100s of mS.

 

[John Fields]

Assume away and lets see if an open loop design comes out as useful.


Harddrive "voice coil" motors are quite a different matter than the
solenoids commonly used in consumer or industrial products.



You can have a 100:1 difference in the size and shape and magnetization of
a compass and it still point north quite well.



Using the www.digikey.com/WebLib/Pontiac coil/Web Data/L-82 Solenoid as
the example.

Taking points from the lower curve, about as well as I can and also taking
the the normal spring equation:

F = KX

We can say that the system will be stable if the spring's force will grow
faster than the plunger's. We can assume that we are operating in the
linear range of the magnetics so that increasing the current by 10%
increases the force by 10%.

Position force
0.1 25
0.2 11
0.3 6
0.4 4
0.5 2

You can see the the solenoid doesn't follow a linear curve. If we assume
that the sping is zero force at 0.6 inches, the spring constant must be,
lets use, 5 to make the 0.1 point equal the value from the curve.

Based on this examine the X=0.2 position and notice that a higher current
is needed to match the spring at this point than at the X=0.6. This means
that the system is not stable at this position without some external
feedback.



You are assuming that a sping can have the needed curve shape to make the
position stable. If that could be done then yes, the system will settle
to a current with the plunger at the desired position. If not, the
plunger will always either snap in or pop out before things really have
settled.



The main thing the settling time depends on is the mechanical time
constants. For practical designs they are the bigger value. The
mechanical time constant will be 10s of mS up to 100s of mS.

---
O.K.

You're probably right, but I don't have the time to really get into it
to find out, so I'll just concede this time. :-/

I've just taken on a contract which is going to keep me busy pretty
much full time until the end of the month, plus I'm finishing up
editing a book for Sams Technical Publishing, plus I've got an
interesting instrumentation application to quote, so I'm going to be
outta here for a month or so...

See ya,

 

[Tom Seim]

You just don't 'sound' like someone who is employed. That's why you
became a 'body' for Battelle.

Like I said, I guess that's why I'm employed and you're not. Nobody
willing to pay for your services, fred? Might it be your attitude? Or
maybe your personality.

 

[Ken Smith]

You're probably right, but I don't have the time to really get into it
to find out, so I'll just concede this time. :-/

Thats a very unsatisfying way to win an argument, but I'm glad to hear
that you've got something more interesting to do.