master voltage control for motors

Hello,
I'm trying to design a circuit to control the speeds of several DC motors. I can control the speeds of each motor individually, but I'm having trouble coming up with a master control that can adjust the speeds of all motors, while keeping the individual controls relative.

Each motor is controlled by IC's with an adjustment pin (the IC is LAG 668). Speed can be adjusted by sending between 0-3v DC to the adjustment pin. 0 volts slows the motor down to a stop, and 3v speeds it way up.

Using potentiometers as a voltage dividers, I can control the speeds of each motor individually just fine. But I'm trying to make one master control that can adjust the speeds of all the motors while maintaining their individual adjustments.

I've tried adding two sub voltage dividers on the + and - feeding the individual dividers, but I'm not having luck with that.

Would a summing amplifier for each motor be the answer?

Thanks, I'm stumped!

From what you state I assume the high potential of the indivdual potentiometers (left me call the slave pots, you will see later why) is fixed at ~3V?
If so, remove that connection. Add another pot (master pot), which is connected between 0V and 3V. Connect the tap of this master-pot to the high side of the slave pots.
By changing the adjustment of the master pot, the control voltage to all slave pots will be changed, thus changing motor speed. Any slave pot can still change the speed of the individual motor within the limits defined by the master pot.
To reduce the influence of the load the salve pots are for the master pot, make the master pot have at least 10 times less resistance (e.g.: Master: 10 kOhm, Slaves: 100 kOhm).

Harald

Thanks Harald! I believe I tried every configuration, except this one. I'll let you know what happens. Thanks again!

Okay... I wired it up, but I'm still having problems. One thing, if the master pot is set towards zero, all the slave pots then only adjust the motors from slow to slow rather than slow to fast.

Is there a way to wire the master in parallel with the slave pots? I'm afraid the settings of the slave pots influence the master pot in a non-linear way when wired in series.

Here's a little diagram of how I have it wired now.

Thanks for the help!

You wired exactly as I meant it to be. That's how I understood the requirement.
From your last post I take it that you want to control the motors via the slave pots indenpendently of the master pot and vice versa. That is something different. It can be done by adding the voltage of the slave pot and the master pot for each motor. To avoid interference between the slave pots, you will need an adder circuit like e.g. here: http://homepages.which.net/~paul.hil...der/Adder.html

This will have the effect that if the master pot is set to full speed, the position of the slave pots is irrelevant. On the other hand, if the master pot is set to slow or even stop, the speed will depend only on the position off the slave pot.

You probably have to rethink your requirements: What exactly is it the relationship between the control exerted by the master and slave pots? What do you expect when either pot is at minimum or maximum?

Harald

Thanks for the reply. Yes, I would like to control each motor independently, but then have a master potentiometer to influence all the speeds uniformly.

I guess an analogy would be if I replaced each motor with a VCO where each slave pot controls its frequency, the master pot would tune all the VCO's, while keeping the slave pot tunings relative to each other. So if I had 12 vcos tuned to scale, the master pot could tune everything up or down a step, while keeping their relative spreads the same.

I'm attaching a simplified version of my dream circuit...

But,
isnt that, what the circuit I proposed does? Of course, when you turn down the master pot, the output of the slave pots goes down, that's what it is supposed to do.

Could it be that our misunderstanding here is that you want to be able to increase the speed by a slave pot even if the master pot is turned down (e.g. 50%)?
The circuit I proposed doesn't allow this because the master pot sets the max. voltage for all slaves. Only this way is it possible to control all slaves simultaneously.

As I understand now, what you want to do is to be able to set the master pot at e.g. 50% but you still want to be able to set the slave pot to 100% and get full speed?

That is a contradiction, because then a slave set at 100% would always overrule any setting of the master. Think about it. It could help for our common understanding if you set up a table where the position of (just one) slave pot marks the rows and the position of the master pot marks the columns (see attachment). Then fill in the desired speed setting. Hopefully this way we can find a solution.

Harald