OK, done some soldering and breadboarding.

Running the motor gives a graph on the screen. I've adjusted it to max 6v and min 6v. Do I have a current reading?

 

OK. The current gets just slightly more complex. Not difficult though. Since the DAQ measures voltage we need to convert a current to a voltage. There are a number of ways to do this. Your motor is really a pretty low current device, less that 2 Amps even in a stall. The method I like would be to use one of these little units. Similar units are available globally from a host of manufacturers. I like that particular unit because it includes an operational amplifier and two small adjustment pots all making for very easy setup and use. A Google of "current to voltage" will bring up dozens of hits. Just focus on current to voltage as sometimes Google ends up including Voltage to Current, not what you want. You also want something that will cover the range of your motor and a little extra. What you end up with is a voltage that is proportional to the current. "Current Sensors" is another good Google term. For your needs I like using a low current Hall Effect type sensor and what I linked to uses the ACS 712 and includes the mounting board and amplifier.

Ron

 

Why don't you just put a few 1 ohm resistors together in parallel and then mount that group of 1 ohm resistors in series with your motor. Them use your DAQ to measure the voltage drop across the 1 ohm resistor pack. If you have 10, that is a 0.1 ohm equiv and you will measure 10 millivolts per milliamp. It should be a fairly low error for your motor current measurement. Use more resistors or smaller resistors if better accuracy is needed. If the ADC in the DAQ is not good at low voltage measurements, then add an op amp to amplify by 50 for a 10 volt full scale.

 

Why don't you just put a few 1 ohm resistors together in parallel and then mount that group of 1 ohm resistors in series with your motor. Them use your DAQ to measure the voltage drop across the 1 ohm resistor pack. If you have 10, that is a 0.1 ohm equiv and you will measure 10 millivolts per milliamp. It should be a fairly low error for your motor current measurement. Use more resistors or smaller resistors if better accuracy is needed. If the ADC in the DAQ is not good at low voltage measurements, then add an op amp to amplify by 50 for a 10 volt full scale.

I thought about using a shunt but didn't suggest it because his motor specifications look like this:
12 Volt Motor
No Load = .070 Amp
Max Efficiency = .320 Amp
Stall = 1.7 Amp

Using 10 EA. 1.0 Ohm resistors in parallel to make a shunt would give a 0.1 Ohm shunt. Looking at for example the maximum efficiency current under a normal full load on the motor I get .320 Amp * .1 Ohm = .032 Volt or 32 mV. The DAQ has a differential input of +/- 10 Volts which is a fixed gain, no choice. That 32 mV would be a hard read on the chart. I agree 10 mV per mA would be sweet but a ,1 Ohm shunt won't yield that as I can see? 0.001 A * 0.1 Ohm = 100 uV per mA. Am I missing something in this? Thus rather than a shunt I suggested the Hall Effect Sensor which includes amplification.

Ron

 

I thought about using a shunt but didn't suggest it because his motor specifications look like this:
12 Volt Motor
No Load = .070 Amp
Max Efficiency = .320 Amp
Stall = 1.7 Amp

Using 10 EA. 1.0 Ohm resistors in parallel to make a shunt would give a 0.1 Ohm shunt. Looking at for example the maximum efficiency current under a normal full load on the motor I get .320 Amp * .1 Ohm = .032 Volt or 32 mV. The DAQ has a differential input of +/- 10 Volts which is a fixed gain, no choice. That 32 mV would be a hard read on the chart. I agree 10 mV per mA would be sweet but a ,1 Ohm shunt won't yield that as I can see? 0.001 A * 0.1 Ohm = 100 uV per mA. Am I missing something in this? Thus rather than a shunt I suggested the Hall Effect Sensor which includes amplification.

Ron

I was just hoping the guy could get up and running in the middle of Norway without ordering something else. I wasn't sure if the Daq could handle low voltages so I suggested an op amp with a gain of 50 to account for the low signal if needed. Not perfect but at lest he can get walking until his additional parts come in and he can run.

 

I was just hoping the guy could get up and running in the middle of Norway without ordering something else. I wasn't sure if the Daq could handle low voltages so I suggested an op amp with a gain of 50 to account for the low signal if needed. Not perfect but at lest he can get walking until his additional parts come in and he can run.

I agree, not knowing how parts accessibility is on that side of the pond makes it difficult. The DAQ he is using while nice with good charting software is not great for low level signals. Using a .1 Ohm shunt he will see about 32 mV under normal conditions. A gain of 100 would be sweet using an opamp. Like any port in a storm I guess.

Ron

 

Why don't you just put a few 1 ohm resistors together in parallel and then mount that group of 1 ohm resistors in series with your motor. Them use your DAQ to measure the voltage drop across the 1 ohm resistor pack. If you have 10, that is a 0.1 ohm equiv and you will measure 10 millivolts per milliamp. It should be a fairly low error for your motor current measurement. Use more resistors or smaller resistors if better accuracy is needed. If the ADC in the DAQ is not good at low voltage measurements, then add an op amp to amplify by 50 for a 10 volt full scale.

Which Op Amp would be suitable?
I have LM358, LM386, UA741CP, LTC1050, LM324AN, TL074CN, TL084CN, MC1458P and CA3130.

I was just hoping the guy could get up and running in the middle of Norway without ordering something else. I wasn't sure if the Daq could handle low voltages so I suggested an op amp with a gain of 50 to account for the low signal if needed. Not perfect but at lest he can get walking until his additional parts come in and he can run.

So was I.

The accessibility of components is fairly good, I've used Mouser, DigiKey and Jameco. One thing that is usually killing the fun is the shipping cost. Jameco ships to Norway for $32, and then it takes 14 days (Last time I used them). Mouser use FedEx, or something similar. Shipping time appx 3-4 days. Mouser has free shipping if I order more than a certain amount. I don't remember DigiKey right now. I've not used Sparkfun yet, but I'll guess they ship to Norway too.

So I can parallel connect 10 pcs 1 Ohm resistor, connect that to a 50 gain Op Amp, and then connect that output to the Daq + pin, and then I have a current measure device? I'll guess the Daq - pin connects to the motor.

 

@nerdegutta

I would do it like the image below. Your op amps are not good all the way down to -power supply so you need to add a small battery for -V power of the op amp. If you will always have at least 0.04 amps to the motor, then you can just use your 12v supply without the battery since you will always have two volts at the output (minimum for most of your op amps. I didn't check the 358 chip). Maybe someone else can comment - I have to run.

The "100m" is 0.1 ohm from your 10 x 1 ohm bundle. This circuit essentially converts current x resistance = voltage. Ultimately you are measuring voltage with your ADC on the DaQ but converting to voltage based on the 0.1 ohm resistor with ohms law.

 

Yes, you can give this a try. Get 10 EA of 1.0 Ohm resistors and place all 10 in parallel. This will give you about a 0.1 Ohm resistor. That will act as your current shunt. The below images are a quick and dirty solution which should work and get you a display of current.





In the schematic R4 is your current shunt. The drawing shows a LM158 which represents 1/4 of the LM358 you mentioned having. I1 in the schematic simulates a current of 0 to 500 mA (0 to .5 Amp). All I did was ramp the current up over 2.5 seconds and down over 2.5 seconds to simulate your motor or any current load out there. The Vout (red trace) is what the voltage out looks like proportional to the current in. So here is what is going on. The current through R4 generates a proportional voltage drop of (in this case) 0 to 50mV. The opamp has a gain of 100 so the 0 to 50 mV becomes a 0 to 5 volt signal. That would be passed on to your DAC input channel.

Your overall circuit will look like this:



Does all this make sense to you? Any questions just ask. This should get you up and running. Just remember with this setup we only look at Low Side current so when you do any motor reversals the shunt needs to be upstream.

<EDIT> Well I see Gopher was up and about ahead of me taking forever on this. </EDIT>

Ron

 

Thanks guys!

Now I know what to do this evening.

Sweet!!

 

@nerdegutta
If you don't have the 1 ohm resistors, you can use 30 cm of 0.255 mm copper wire (30 awg) to get 0.1 ohm.

If you want to get more accurate, you can use 3 cm for 0.01 ohm resistance and increase gain on the op amp to 500.


Let us know if it works out.

 

Let us know if it works out.

I'll report back, but it might take some time. There's an emergency at work, so I have to put in extra hours, and from Monday and two weeks ahead, I'm in the Homeguard. But I'll keep you updated on any progress.

 

So...

After a few busy weeks, I have some time to work on this again. Have I been able to duplicate the schematic? I'm not sure about the connections on the Op Amp.


Please advice.

 

Make it like my post #29.Your amp gain looks to be only about 5.

Ron

 

Make it like my post #29.Your amp gain looks to be only about 5.

Ron

Have I replicated correctly? Since the motor is mounted, I have to use screw terminals and some hook-up wire to connect to the motor and battery.

 

That should work.

Ron

 

That should work.

Ron

Great!

I'll design a PCB, and etch a board. Any layout concerns?

 

Great!

I'll design a PCB, and etch a board. Any layout concerns?

Before making a PCB I would breadboard the circuit. Also include decoupling caps at the opamp input pins.

Ron

 

If you want to have the fun of designing a circuit and writing a bit of software, The PSoC evaluation boards from Cypress are pretty good. PSoC combines an analog front end, the ADC, a processor and some eeprom and serial ports to download.

http://uk.farnell.com/webapp/wcs/st...tegoryId=700000005178&langId=44&storeId=10151

 

Hey..How much was shipping to you Nerdegutta.
Looks interesting thing to have around if you know what I mean.