# Robot Controlled Theremin

Posted: 4/17/2012 3:25:30 PM

Joined: 7/20/2007

For my graduate robot control project, I'm having two robot arms controlling a Paia Theremax I just built. Yesterday I got everything set up in the lab and tuned, etc. The problem that I am facing is that I can only get about an octave in range with the range of motion of the robot arm. How can I increase the sensitivity?

The robot is all metal and is sitting on the same metal stand that the theremin is grounded to. As the end of the robot arm moves from about 18" (silent, low pitch) to nearly touching the antenna, I get about an octave range. Using my hand over the same range I can get about 4 octaves of pitch. Putting balls of stuff on the end like aluminium foil or static foam seemed to change the effect, but not in any dramatic way. What is the principle behind this?

My best theory is that since the metal cart and robot hardware are all acting as one plate of the capacitor, the moving robot arm isn't affecting the overal change in capacitance with the antenna. My body has a totally different charge and moving my arm has a greater relative affect on the capacitance, therefore I can get a wider range.

If my theory is correct, couldn't I connect a battery with one pole at the antenna and the other pole floating on the end of my robot arm and get a greater effect?

My project deadline is very rapidly approaching so I hope someone has an answer for me.

Posted: 4/17/2012 4:28:18 PM

From: 60 mi. N of San Diego CA

Joined: 2/15/2005

kballing said:

"My best theory is that since the metal cart and robot hardware are all acting as one plate of the capacitor"

I enjoy your project and this topic. My view is different than most about the capacitor plate theory though capacitance is involved. Think more on the idea that the pitch field has near instantaneous energy transfer from the antenna through your hand and that RF needs somewhere to go.

You have seen those Neon Gas Globes with all the energy fingers moving about. Earth ground directly your theremin and your robot arm; you will be amazed as it will then work just as your arm and large body mass.

You can’t have a deadline with a theremin, she has attitude!

Christopher

Posted: 4/17/2012 4:39:00 PM

Joined: 7/20/2007

The theremin and robot arm are both grounded.  However, I am not grounded. I've got my shoe soles and carpet separating me from any conductors in the floor, and I'm not touching anything else, but my hand still performs better than the robot arm.

Posted: 4/17/2012 4:56:10 PM

From: 60 mi. N of San Diego CA

Joined: 2/15/2005

Ok, I just did an experiment with a piece of 10" alum foil wrapped around the end of a wood dowel. The results are as you described ungrounded but perfect when a ground wire was connected. Theremin and foil connect to the same earth ground.

IMHO your theremin needs to be raise up at least 6" above and away from the metal cart or what you call ground is not really earth ground. Earth ground can be achieved using the ground side of your amplifier if it uses a three prong power plug.

How are you connecting to ground, does your theremin have a rich low end sound? 200hz

I am starting to panic with you!!!

Edit: Are you moving the robot out of the way when you test with your hand? Every part of the robot structure should be at least 18" from the theremin antenna with just the arm reaching into the pitch field. The robot metal structure could be grounding out the pitch field.

Posted: 4/17/2012 5:21:30 PM

Joined: 7/20/2007

My theremin sounds fine. Using my hands I can get a range from maybe 50Hz to about 5KHz.

Earth ground shouldn't matter, the point is that my theremin chasis and robot arm are connected to a common ground potential. (In this case, this happens to be earth ground as well).

Lifting the theremin off the metal cart won't have any effect since the power cable is still connected to the same outlet as the robot arm.

I still think that it is a matter of relative capacitance. Suppose I have two plates of a capacitor. Given a ground plate (antenna) that is small and the opposite plate that is large (body). Suppose I wiggle my finger one inch and get a change of 1 semitone of pitch. Now if I lean my whole body in one inch, at the same distance from the antenna, I would expect to get a greater change in pitch.

I think what is going on is that the robot arm is very small in comparison to the metal cart, computer, current amplifiers and other connected hardware. So even though my robot arm is moving 18 inches or so, it is like wiggling a baby finger instead of moving the whole body.

Posted: 4/17/2012 5:35:37 PM

From: 60 mi. N of San Diego CA

Joined: 2/15/2005

I am calling my shots in the dark and you have developed some good theories as to what is going on. The theremin is definitely working fine from your comment.

A Photo of your setup would be gold!

You did not answer the question: Are you moving the robot out of the way when you test with your hand. If so and you have to significantly re-tune the theremin when you put it back in place then the robot structure is grounding out the  pitch field!

What University are you at?

Posted: 4/17/2012 5:42:24 PM

Joined: 7/20/2007

Yes, I am retuning between robot control and human control. (This is actually very easy when you have things connected to an oscilloscope).

University of Utah. However, I'm at work right now, or I'd be in the lab testing things out.

Posted: 4/17/2012 5:52:33 PM

From: 60 mi. N of San Diego CA

Joined: 2/15/2005

I once canoed 5 days down the San Juan River from Mexican Hat into Lake Powell. It was an adventure just like Burt Reynolds in "Deliverance". The theremin will make you squeal...

When you get home, move the robot out of the way, tune for hand control and do the experiment I did with the alum foil on a broom handle or wooden dowel. The answer will be revealed.