parts
 

I sort of understand what's going on here. Since schools are closed today due to the northeastern snow storm, I will run to radio shack today and pick up the electronics to build a couple of these circuits. If I take this schematic to radio shack, will they be able to help me find everything that I need? You mentioned getting the right op-amp. Which specific kind will I need? ie, what specifications should it have?

One last thing, I am planning on calculating the current for 2 motors, not just 1. Will I have to build 2 seperate/identical circuits and hook them up completely independently? Or can I use this single circuit for measuring 2 currents from different motors?

Thanks for all your help! :)

Re: parts
 

No, I don't believe that Radio Shack will have the particular OP-AMP I used. I bought it at DigiKey. (LM6134)

Note that each of these OP-AMPs packages have 4 OP-AMPS in them...so you can measure motor currents on four motors with in of these packages, (Radio Shack may sell an OP-AMP that has similar characteristics.....but I am not sure. They tend to seel pretty "jelly-bean" devices.)

-Quentin

trip to Radioshack......
 

I went to Radio Shack, and bought 90% of the stuff on the schematics. I bought a simple Op-Amp ($.99), but on the back of the box schematics, it only has 2 inputs, and an output. Your schematics have a ground coming out, as well as another wire (#4 on sheet) that appears to be +12v. I guess I'll have to take another trip out to R.S. and pick one up that looks similar.

Also, it dawned on me just now that i will have to put many resisters in series to create a net resistance of 12.1kOhm's and 2.2kOhms. I did not see these resisters at R.S, so i didnt buy them, only the nice round 10X mulitples R.S. had. Looks like 1 more trip out to Radio Shack today........ but probably not the last..... :eek:

Re: trip to Radioshack......
 

Well, the resistor values used can be changed....all you end up with is a different range of output. (I chose the values to optimize the output for both 30A and 20A breakers)

If you went with 10K and 100K, you would have a gain of 10 vs 8.26 which will give you a lower full scale MAX current measuring ability. This is actually BETTER for the 40A breaker since it will have a lower resistance.

But I can't over-state the importance of the OP-AMP. Something like a 741 or a 324 will just NOT do the job at all. (If I remember, the 324 is good at sensing close to the lower supply, not the upper supply)

BTW: The 6134 is pin compatible with the LM324, so if you built it for the 324, (in a socket) you can simply switch in the 6134 when they come in.

-Quentin

what exactly?
 

i bought a LM324 when i was out, but your saying it ISNT a good choice? If i need to go buy another one, exactly which should I buy? I have a 724 and LM324 here now, will the LM324 be good enough?

339
 

I have a 339 Quad Comparator here also, will this work? from the schematics on the package, it looks like the same type of OP AMP.

Re: 339
 

Sorry....this is a comparitor, which while similar, is not the same.

Besides, I'm not sure the common mode input voltage range of that device includes the voltage rail.

The 339 and 324 are older devices, and though they are single supply devices, I think they sense down at GND better than Vcc. (At least that is the case for the 324)

Sorry,
-Quentin

We are planning to put a simple comparator circuit across the ground lead of each motor. By comparing the voltage drop across this wire to a reference produced by a trim pot, the computer can determine when the motor is stalled, or at least drawing sufficient current to trip the breaker. The program will then automatically reduce power to the motors.
This will be especially useful during the autonomous period, when we might try to drive through a wall at full speed.

We also plan to use this circuit to detect that the window motor has reached the end of its turning point, rather than use limit switches. Limit switches have a habit of breaking at inopportune times.

By using the ground lead instead of the +12V lead you can use a garden variety comparator that senses down to ground, like the LM339. The open-collector output can be connected directly to a digital input on the robot controller, with no need for zener diode over-voltage protection. You would want to put a low-pass filter (series resistor with a capacitor to ground) on the input to remove the pulse-width-modulation pulses, as it is the average current you are interested in measuring.

I could post a schematic if anyone is interested.

Sorry I'm late

So did the controller get a analog or digital input?

Please!!

I don't understand why you say the 324 wouldn't work. It's working for me, the circuit I built measures current and thanks ot a creative selection of resistors, I have 1amp to 0.1vdc ratio, allowing measurement up to 51amps where the zener diode clamps prevent it from going higher as a safety precaution, so far it appears to have been working perfectly.

Schematic for current sensor
 

A relatively simple current sensor is depicted in the following schematic.
It uses a quad LM339 comparator, and all parts should be available at Radio Shack.

The positive input of the comparator is connected to the ground pin on the Victor 884, and the negative input is connected to the common ground bolt on power distribution block. The voltage to be measured is then the voltage drop across the ground wire connecting each Victor to the power distribution block.

The potentiometer (R2) is adjusted to provide a very low voltage to the negative input of the comparator (U1 - LM339). When the positive input rises above this voltage, which will happen when sufficient current is drawn by the motor, the comparator will change state. The output of the comparator is connected to a digital input on the robot controller. When the controller senses that the comparator output has changed state, it can reduce power to the motor, or, if in autonomous mode, it might stop the robot completely, as it may be jammed up against a railing.

The positive input is filtered with a low pass filter (R3, C1) to remove the rapid voltage fluctuations caused by pulse width modulation of the motor. The values here may need to be adjusted, as I have not calculated the optimal values for this circuit, but have just stolen them from bigqueue's drawing of a different circuit

.

To Steve,
It is a multiplexed analog signal to one input on the RC.
To jskene,
Nice, simple design. How many motors do you monitor and does your software cutback to a predetermined value or does it step down until there is no trip sensed?

Al;
We are just building the circuit today. We plan to put it on all 4 drive motors, and also on an arm motor to allow us to determine when it has reached the end of its stop.

Regarding your question on what the robot does when it senses an over-current condition - in autonomous mode we are probably going to back up a bit then go forward, one time. If the condition repeats, then we'll just shut everything down and wait until manual control starts. This will save us from being E-stopped.

In manual mode, we will back off power, and maybe flash a warning lamp for the operator.

Due to the high starting current of the drive motors, we may have the robot controller ignore the overcurrent condition for 1/2 second when the operator has requested a rapid acceleration or change in direction. This should prevent us from tripping out uneccesarily. The 40 amp breakers will take momentary high current spikes.

Note:
The original version of the schematic had the +5V and GND leads of the LM399 reversed. The current version (Ver. 2) is correct.

Jerry

Jerry,
Sounds good. Let me know how it works. As of last week, we are planning on putting our Stangsense current monitoring on this years robot. We also are monitoring five motors, but at present use the data to highlight high risk operations to the drive team and to have data to analyze post-match.
We are hoping to have a portable version that we can help teams analyze problems on their robot. If we can get the time after ship, we hope to be able to substitute our current monitors for breakers in suspect robots and give teams an idea of what is actually taking place.

Here is a picture of a sense wire. Note that there is a two conductor wire used to feed to the custom circuit board. One wire is attached at the push on terminal and the other is attached to the wire, one foot away. The only thing remaining is to cover the terminal and the solder joint with heatshrink and cut the extra length to fit. This wire will be installed between the circuit breaker panel and the speed controller.