Automate turning of manual rotary switch?

Got a little problem. I've been asked to come up with something that turns a
manual 3 way rotary switch from one position to another and back again under
software control. The switch is one of those bulky things with a pointer
style knob. They basically want to control a piece of equipment without
modifying it in any way.

Does anyone know of a switch actuator manufacturer? I'm looking for
something I can place over the switch with a motor or solenoid to turn the
switch (about 45 degrees per switch position).

Thanks!

Mark.

 

Thanks for that. That's something like what I'm looking for, except I don't
need the wafers. However, the switch has mechanical stop positions on it
that the motor would have to overcome, so may need a bit more gearing.

Mark.

 

I don't.

But you reminded me of a remove control TV I used to have. It
used a few metal bars that were "hammered" when you pushed a
mechanical button. (Basically, tuning forks.) These emitted
a precise audible tone which was decoded and used by the TV
set to cause the tuner to move left or right (you had one
control for each.) The TV could be set while sitting in a
rocking chair from across the room! The knob was _also_
designed as a standard manual tuner so the remote wasn't
required, at all. You could, of course, use the TV as a
"normal" TV of the time where you had to get up and switch
channels by hand.

Of course, manufacturers soon got rid of the manual tuner
assemblies and digitized these and then began using buttons
and remote controls which, today, seem more necessary than
mere convenience, anymore.

Interesting question.

Jon

 

Indeed, but this is a one-off so a ahnd crafted solution would be OK (as
long as it's reliable!)

Ideally leave the knob on, but could remove it and replace it. I'm not sure
whether the knob is press-fitted or screwed.

No, there will be a little control box with a manual override so all done in
software.

Good question. There are some existing fixings I could attach to, I'm sure
they could accept a few holes drilled in the panel that has the switch.

This might be quite a tough mechanical challenge, as you say it sounds easy
but the devil is in the details. The switch has click positions, I may need
a way to sense it has seated properly in a particular position. Also I don't
want this to ever sit just off centre of a position in case the switch
contacts arc.

Mark.

 

Tuning forks to control tellies! Nice. I shan't be using that method though

 

I think the stepper may have to be geared to get the torque needed to move
the switch into the click positions. Technically with a stepper you
shouldn't need a position ecoder as long as it is calibrated for position,
however lose a step or two and you're in trouble, so I think you're right
that an absolute position sensor would be needed.

What might work is stepping to one of the nominal click positions then
releasing (I assume this allows the shaft to freely rotate?). Assuming the
gearing isn't too large the switch may then self align, similar to your
springy coupling suggestion.

Another interesting possibility is having some kind of torque sensor on the
shaft itself. By measuring the torque as the switch clicks into place it
might be possible to reliably detect it. Can you get torque sensors like
that?

Mark.

 

I would use a large RC servo and couple that to the shaft or knob.

 

Yup, here's one for $119 that has 250 oz-in of torque at 4.8V

http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXTHR3&P=ML

 

Thanks Phil. Wasn't aware that servos like this had good torque. I'll look
into that.

Mark.

 

Nice. Thanks!

 

Replace the knob with a gear. Drive that gear with a gear attached
to a motor's shaft. A cam on the driven gear can activate a micro
switch for the stop positions. You'll need a gear motor already
reduced in speed to keep the driven gear relatively small - maybe
something like cat # DCM-276 ($9.00) or DCM-245 ($13.75) or
DCM-110 ($3.50) at Allelectronics. http://www.allelectronics.com/
The first two mentioned might provide enough torque to turn the
switch with direct coupling and no additional gearing advantage.

It's a kludge, but a kludge is what they're asking for, whether
they are aware of it or not. The response will be slow at 5 or
6 rpm with the first two motors and 1:1 gearing or direct coupling,
in the neighborhood of 1/2 second. With the DCM-110, it depends on
the gearing you use. Without knowing the torque requirements
it's a guess whether any of the motors will work for you.

Ed

 

If this is a one off project, you can get some old 8-track drives and
remove the ratchet, paw and solenoid.

Once you look it over, you'll understand how it works.

hamilton

 

Perhaps proportional ball valve actuator, the kind you can put on a valve
and control with 0-10V
-ek

 

hehe. No batteries to replace. Can get tuning forks from
the store if you need them laying about the house. Might
even be able to learn to whistle it out without one, which
means you are never looking around for the darned remote any
more. It does have its advantages.

Jon

 

For this application, I'd suggest an R/C servo intended for boat
rudders. Those are geared down, slow, and not too expensive.
Here's an example, for $37:

http://www.hobbypeople.net/gallery/474142.asp

There are plenty of resources on the Web on how to drive an R/C
servo from a computer, so I'll let you find that

John Nagle

 

Thanks John. I actually haven't seem the real switch yet, but I know
sometimes these things can require quite a bit of torque to overcome the
click position. I think I'll need to measure it first so I can choose an
appropriate R/C servo.

Mark.

 

Sure, but what you measure is torque reaction on the motor housing.

 

Just about as ugly as it needs to be, this thing is out of site in a control
room and there's plent of room as it's an open rack. What they don't want is
to modify any of the control logic or wiring as they have a maitenance
contract with a third party. I want to come up with a solution that's
completely non-intrusive that simply automates the otherwise manually
controlled on/off switch.

Mark.

 

If all you want to do is switch it on and off, why not just leave
it on and switch the power to it with a relay?


T

 

How far apart are the positions ?

90 Degrees, 30 Degrees, 120 Degrees

This looks like its going to be a Real Rube Goldberg type of operation.

hamilton