12Vdc Solenoid Driver Circuit requ'd.

[Ken Smith]

In article <43ejm0ha07qkv8urjlp46nu0ns7mce0s6m@4ax.com>,
John Fields <jfields@austininstruments.com> wrote:
[...]

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.


--
--
kensmith@rahul.net forging knowledge

 

[petrus bitbyter]

"PT" <PT@lisome.com> schreef in bericht
news:Mzr9d.18019$5O5.7011@news-server.bigpond.net.au...

"John Fields" <jfields@austininstruments.com> wrote in message
news:57gam01homm7c6ejui0obser0nhs8670la@4ax.com...
On Thu, 07 Oct 2004 11:31:13 GMT, "PT" <PT@lisome.com> wrote:


All replies sofar have provided me with lateral thought and I thank each
and
everyone for that. My current thinking now is to continue with PWM
and/or
some form of damping such as springs and a cap. Spehro mentioned
"dashpot"
which sounds interesting.

What say?

As well, I have a maximum space consideration of 30mm in height, which
is
why I have chosen this path. However I am still open to any
suggestions.
The load is approx. 500g.

---
Could you say something about the physical orientation of the solenoid
and the load? That is, are they coaxial and vertical with gravity
supplying the restoring force for the load, or are they arranged in
some other way?

--
John Fields


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


---
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[John Fields]

On Sun, 10 Oct 2004 16:27:54 +0000 (UTC), kensmith@green.rahul.net
(Ken Smith) wrote:

In article <ejfim0d7uk2rg8lra8eug2msctvvatl655@4ax.com>,
John Fields <jfields@austininstruments.com> wrote:
[...]
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.

---
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?
---

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.

---
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.

---
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.
---

(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.

---
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?

--
John Fields

 

[John Fields]

On Sat, 9 Oct 2004 23:46:41 +0000 (UTC), kensmith@green.rahul.net (Ken
Smith) wrote:

In article <6hdgm0hagnu67sjrifcopnel377ssuejem@4ax.com>,
John Fields <jfields@austininstruments.com> wrote:
[...]
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.

---
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?
---

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.

---
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

[ľC]--[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?

--
John Fields