How to control voltage when powering two DCmotors (not simultaneously)

[davidd314152wind4]

Hi,

I am using three outputs to control two DC motors. The motors do not run simultaneously. I'd like to use the remote-sensing of a power supply to control the voltage close to the motor terminals.

There are five possible states, controlled by three digital output lines:

000 - M1 off, M2 off
100 - M1 fwd, M2 off
011 - M1 rev, M2 off
001 - M1 off, M2 fwd
110 - M1 off, M2 rev

The middle bit switches a connection common to both motors between 12V and ground. The first bit switches the remaining connection on motor 1 between 12V and ground, while the last bit does so for the motor 2.

This is accomplished with an H-bridge consisting of four relays.

Is there a common method of remotely controlling power supply voltages in a situation like this? I find it simple to control the voltage across a single motor using a second H-bridge but I'm not clear on to do so for both of them.

I was thinking of using logic gates to switch between two different H bridges when in one of the four active states and the power supply terminals when both motors are off.

Any thoughts?

 

[itsmoked]

I am not clear on your scheme yet.

1) You have a single relay H-bridge?

2) You want to run these two motors with one H-Bridge but with two power supplies?

You could use one power supply and add a single extra relay in each motor lead to disconnect the individual motors. Or the same relay and just use its NO and NC contacts to direct the HB output to one or the other motor depending on if the 'steering' relay is energized or not.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidd31415]

itsmoked,

1. The H-bridge is composed of four relays.

2. I am running one of two motors at a time with one power supply and one H-bridge. I'd like to sense voltage in one of five ways depending on which of the five modes is active...

000 - M1 off, M2 off - sense at power supply output
100 - M1 fwd, M2 off - sense with +sense on M1+, -sense on common
011 - M1 rev, M2 off - sense with +sense on common, -sense on M1+
001 - M1 off, M2 fwd - sense with +sense on M2+, -sense on common
110 - M1 off, M2 rev - sense with +sense on common, - sense on M2+

 

[itsmoked]

Oh.. You just want to sense motor voltage.

What a hassle. First you should review why, since no one else would. Voltage across a DC motor is far less useful than current. Also your relays and wiring should not be providing much loss so just measuring the voltage at the the supply should be enough.

But if you want to persist then you essentially need something like a DMM that reads both polarities. Perhaps you can use an Of The Shelf DMM with serial communications. Otherwise you need a bipolar reading voltage circuit since your polarity switches with direction.


If for some reason I needed the motor voltage as in your case, I'd set up the system to measure only the voltage at the supply so it could be just a single ended measurement. Then I'd run the system and measure the full load voltage at the motor terminals with a DMM. I'd do the math to find the losses and then let your controller do the math to correct out the relays and wiring, giving you the motor terminal voltage via the power supply terminal readings. If you need to see all the variations then monitor the current coming out of the supply and you could then equate the motor terminal voltages at any loading.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidd31415]

Thanks Keith,

Reasoning is to perform tests per specifications. Losses vary as ambient temperature surrounding devices under test change.

Standard procedure has been to control the voltage at the power supply output terminals and offset it slightly to ensure the voltage at DUT terminals is within spec tolerance. When environments change this tolerance is not always maintained.

 

[itsmoked]

If it is just for testing why not go with a Keithly or HP DMM with USB comm or something similar? Then the DMM can be easily calibrated by any standards lab.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Switching sense leads worries me. If you have a failled open event, the sense terminals may see zero volts and drive the supply to maximum voltage attempting to compensate.
The difference between supply voltage and motor voltage is mainly due to voltage drop. Use larger conductors to lower the voltage drop. You may consider a voltage/current alarm to detect and inform you when ambient or other influences have put you outside your specification tolerance.
Or am I missing something here?


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidd31415]

Yes, the failed open failure mode is worrisome. I've planned to limit the maximum output voltage at the power supply outputs to protect against this case.

The problem with larger conductors is that anything larger than 18 AWG become impractical due to space available for wire routing, wire flexibility that is required (at low cost), and sheer number of wires.

The alarm approach makes sense; not automated but better than the current situation.

Thanks,

David