What Dr Joe Wants for Christmas...
 

...if it arrives by Kick off, I'll be happy.

Reading this thread about how to control an Omni Driver robot, I read about this paper ($1.50 for a copy from Circuit Cellar -- worth it!) written by Rich LeGrand who works for Charmed Labs (who, as it turns out make some pretty cool robot things -- check out their site -- really) .

The paper make reference to the idea of using Back EMF to control a motor. The great thing about it is that you don't need to put any extra stuff on the motor to get PID control. At the Charmed Labs site I found this video that dropped my jaw.

If you want to learn more about how it works, look at this link from another cool robot website, Acroname.com. I think they really do a pretty good job of making the concepts understandable.

Anyway, I think that some smart person at a company that makes stuff for FIRST robots (IFI, AndyMark, Radio Shack, etc.) could make a mint and, more importantly could do all of us FIRST robot builders a huge favor, if they made an Victor-like product that had PID motor control via Back EMF sensing built in. Actually, Acroname.com is part of the way there already in that they offer an H-bridge with velocity feedback built in (Yes, it is too wimpy for FIRST it still shows the idea).

Think about it. Right out of the box, you could have robots driving the way robots aught to be driven: in closed loop not open loop mode.

I am not sure I'll be able to sleep at all on Dec. 24th...

...visions of well behaved robots danced in their heads...

Thoughts?

Joe J.

Re: What Dr Joe Wants for Christmas...
 

Wowh. man i realy want that. imagine a robot that goes straight exactly how far you would like it to go. and arms. Dont even get me started

Re: What Dr Joe Wants for Christmas...
 

this is crazy...put me down for it all too...i want but of course i can't have so sad...

robot going straight :ahh: that's crazy cool...

Re: What Dr Joe Wants for Christmas...
 

WOW
Tiff. and Tytus are thinking motion aspects...what about the reverse...
Automic holonomic/mecanum parking brake anyone?

Re: What Dr Joe Wants for Christmas...
 

why not instead of using an expensive piece of equiptment, use a simple brake . and save all that money?

Re: What Dr Joe Wants for Christmas...
 

I'm thinking the whole point of Back EMF is you don't need to use any expensive equipment, such as encoders.

Re: What Dr Joe Wants for Christmas...
 

If a robot using a brake does in-fact get moved, it stays on brake, in its new moved to spot.
If a robot with that control system gets moved, its tries to get back to where it was before
Also, I think the circuitry would weigh less then a mechanical brake.


EDIT//
Joe, I am confused, I understand that it can maintain constant motor speed and motor position, but can it determine distance? Or is that just run through a formula through the constant motor speed function?

Re: What Dr Joe Wants for Christmas...
 

I wonder if this technology is or can be used in CNC machines.

Re: What Dr Joe Wants for Christmas...
 

You might be able to set up your own Back EMF controller using the current sensors we got a few years back. If I read the article right (I skimmed), the readings are made by setting up an interval in which to drive the motors, and to kill voltage and let them free spin. If 50 hertz is the magic number, a nice function to drive the motors in a Back EMF compliant way could be created with the onboard PIC timers.

Now I'm not an electrical guy, but if the current sensors are wired in series with the motor in question, would they not measure current whether being taken from the power source (battery) or produced by motors while floating the windings?

If so, we may be able to set something like this up..

1) Create a timer driven interrupt at 50hz which checks a state flag and does either of the following...
- If in "Drive" State, Drives motor with whatever speed setting needed (either manual or autonomous control)
- If in "Read" state, kills power to the motor, and reads the current being produced.

2) Compare current produced to expected current generated.

3) Drive motors accordingly in next "Drive" state. (ie. Motors are set to neutral, but X amount of positive current is being produced. We would then know the direction and speed the motor is being back driven, and know how to compensate for it by incorporating that number into a PID control loop)

This all may seem very naive on my part, but it might work :confused:

Re: What Dr Joe Wants for Christmas...
 

This seems potentially useful, but (if implemented like the Acroname device) it's quite likely illegal by FIRST's 2005 rules. ("Custom circuits may not...[d]irectly affect any output devices on the robot, such as by providing power directly to a motor, supplying a PWM signal to a speed controller or supplying a control signal to a relay module.") Maybe, depending on the exact implementation, you could convince the RC and Victor to simulate this functionality without contravening the rules, but you'd still need a custom circuit for sensing, and you'd have to hope that the standard controls could respond quickly enough—which may be a sticking point.

So if people want it, they'd better ask FIRST for the appropriate rule change at the earliest possible opportunity, so that <R53> doesn't get grandfathered into 2006—or instead, get IFI build it, and add to add it to the controls portion of the kit.

Re: What Dr Joe Wants for Christmas...
 

Sounds like an interesting idea. Perhaps someone might make that suggestion over here where it might be seen by some of the right folks. :)

-dave

Re: What Dr Joe Wants for Christmas...
 

Whoa,
Don't go getting all excited yet. The technology behind the this discussion still has a lot of variables in place that would make this impractical for our use. There is too much production variation in the motors such that any two motors will not produce the same voltage when turned at the same speed. Add to that the resistance of the power leads and their length, temperature, connector resistance, etc. and you will see that precise speed control and distance is a going to be inaccurate. It may be possible for a theoretical controller to "learn" the circuit charachteristics, but at varying speeds and the short distances traveled I think there is still a lot of room for error. I would need a little more convincing before I would jump in on this.

Re: What Dr Joe Wants for Christmas...
 

There are parameters that get tuned but I don't think this would prevent this from being a very useful feature.

Note that the guys at Charm Labs are selling robot systems right now that do just what we are talking about. Somehow they are dealing with motors variation, temperature variations, connector resistance, etc.

Also, talking to Jim Zondag of the Killer Bees (#33) about this, the auto industry has gone to this type of a system for the motors that drive the vent doors inside of HVAC units (replacing pots and/or switches for position feedback). I think this is very encouraging news.

From my point of view the most disturbing thing is that the process requires that the PWM be less than 100% In most cases that would be just fine BUT there are a lot of cases where I would want to have the motors get full power.

I am extremely hopeful.

Joe J.

Re: What Dr Joe Wants for Christmas...
 

This thread got me thinking, ( I know, sometimes that can be dangerous) " How could I simply implement something like this with the hardware and software we already have?" Here is what I came up with.

In a nutshell, the EMF system is taking feedback directly from the motor during a period of "coasting" without any drive signal.

Why not add a small motor or generator, like an Micro RC car motor, to the driven axis and get feedback from it? The drive motor will turn the driven motor causing the driven motor to create a voltage/current source proportional to the speed of the drive motor. With gearing, and a little programming, this could be used in a closed loop feedback system similar to the EMF systems. This would negate the need to interrupt the PWM signal, sample the EMF value and then re-apply the PWM in our systems. We could do the sample and drive signal modifications in real time.

BTW, I know there are already motors available with tachometer circuitry built in. Many of these use optical encoders and put pulse trains out and some are strictly analog. Either design should be fairly "easy" to implement.

Thoughts???

Re: What Dr Joe Wants for Christmas...
 

I am all for other sensors, especially if they are baked into the motors in the kit. I suppose that Chiaphua Motors (the makers of the CIM motor) would be willing to design in a cost effective quadrature encoder on the motor they sell FIRST if FIRST asked them. When I was with Delphi, I was AMAZED at how cheaply these guys could make a motor with very cool features (it was my research into finding a better motor supplier for Delphi's Power Sliding Door applications that got Chiaphua involved in FIRST).

One thing I don't like about this approach is that we are loading down the CPU. Eventually we will get to the point where interrupt load makes keeping track of dozen or so motors brings the MicroChip processor to its knees.

The Victors already have a brain onboard (as would most modern speed controllers). Given that, it would be nice to fob off some of the sensing and thinking required to implement closed loop motor control onto that remote brain.

Before folks object, Yes, I know that this would still require tuning and passing of parameters to get the PID loop to behave, but it would be a trade-off worth the cost.

I really think that FIRST could benefit from hiding the implementation details of feedback loops while making them standard operating proceedure of FIRST robots. We want FIRST robots to act like robots but we don't give folks the tools that robot designers have. Ask Copioli and other pros that design robots for a living: Closed loop motor control is a given.



Joe J.