Need ideas for strain sensor and type of DAQ


Recommended Posts

Any suggestions for a simple wheatstone bridge strain guage pressure transducer for use with a simple robotic arm? I would like to control CLOSE with digital I/O and then generate a REVERSE command using an IF vPressure =/> x (milli)volts.

I'm guessing I can use DAQX software to control...looking for inexpensive strain guage ideas. (I see now that DAQX is the Active X version of DAQ), I do own VBasic but would rather do this as easily and as inexpensively as possible as it is for a college project on Haptics.

I have found a somwhat cheap arm that I can operate with 2 I/O (open/close) and assume I need 1 A/D channel to recognize the voltage bias developed from the strain guage. Am I heading in the right direction?

Any thoughts mucho appreciated.

Link to comment
Share on other sites

If you are looking for a pressure sensor (most of which are bridge based) to measure the pressure of a gas, check out the "integrated" pressure sensors from Motorola:

You can buy these from digikey.

If you are looking for a strain gauge (or gage) to measure force (or similar), then you need to get an actual strain gauge and glue it to the surface of interest. Since you are looking for an inexpensive solution, I suggest the following:

- Get one strain gauge. has many choices, but it looks like you have to spend $50 to buy quantity 10.

- Use 3 resistors and perhaps a potentiometer to complete your bridge.

- Use the EI-1040 to signal condition the differential output from the bridge.

Link to comment
Share on other sites

Thanks for both replies - I'm investigating both possibilities - I found that was a weath of data just from a learning point of view...what I am calling pressure feedback is actually force feedback not to be confused with gas or vacuum measurements and they definitely have the sensor, but it is $69 before added electronics. The second suggestion with the "naked" amplifier was more in line with what I was originally thinking since I had previously repaired some restauraunt scales and was struck by the simplicity of the strain gage involved.

My next question is what is the cheapest version of DAQ or does anyone know of a similar product (l'm familiar with LabView) that can:

1. interface with the LabJack - since it seems like the easiest and best value for both A/D and I/O on the market; and

2. perform control functions - it seems that even with DAQ with control, it's going to run several hundred dollars to read 1 A/D, make a IF/THEN control, and fire up 1 I/O line....I know, it seems that "rolling your own" A/D with an I/O port would be the way to go but this is a fall semester project and I'm not THAT good...

Link to comment
Share on other sites

As far as DAQfactory, I believe the special LabJack version for $125 will do what you want. If you have not bought your LabJack U12 yet, you can get a bundle from azeotech for $219.

Our free sample application LJlogger has some simple trigger capability that might do what you need. Check out Section 3.4 of the LabJack U12 User's Guide.

Beyond these options, I think you will have to program, in which case there are various free options.

Link to comment
Share on other sites

There are two general types of strain gages. Metal (usually foil) and semiconductor.

Metal gages have a "Gage factor" of about 2 to 2.1, which is the ratio of change in resistance per unit of strain. For example, a strain of 0.001 inches per inch (1000 microinches per inch) will result in a gage resistance change of 0.002 Ohms per Ohm for a gage factor of 2.0.

Semiconductor gages usually have a gage factor in the range of 90 to 120. That is like having an amplifier with a gain of 50. Semiconductor gages allow you to use lower strain values so that there is greater margin of safety against overloading the transducer.

You want the gage to measure the load, but temperature also causes output that can be significant compared to the measurement. Even the resistance change of the wire can cause a change in output.

Several things can be done to minimize the errors in strain measurements.

1. Get strain gages that are compensated for the thermal coefficient of expansion of the metal that you are using. Carbon steel, stainless, and aluminum are all different. Nonmetals are difficult to use as transducers and are generally avoided.

2. Even if you get compensated gages, it is good practice to set up your circuit with two identical gages, one mounted in the direction of the principal stress and the other transverse to the direction of pricipal stress, connected in adjacent legs of the Wheatstone bridge. Alternatively, if your transducer is a beam in bending, you can get the same temperature compensation effect if you install gages on opposite sides of the beam, to measure compression and tension, and connect them in adjacent legs of the bridge. Many suppliers sell biaxial gages; with one of the gages mounted to measure the strain in the direction of principal stress, and the other mounted to measure the strain in a direction transverse to the direction of the principal stress. If connected properly, that arrangement will also eliminate the errors due to temperature changes in the wire. You should cure the gages at a temperature above what they will see in service.

3. When using two gages, they are usually connected adjacent to each other so that they are in series between the +/- power terminals of the bridge. The resistors used to complete the bridge should have low temperature coefficients and should be identical as near as possible. It is common, but not necessary, to use completion resistors of the same nominal resistance as the gages.

4. You will need to provide a nulling circuit to balance the Wheatstone bridge. With 120 Ohm gages, it is common to use a 5kohm, 10 turn precision potentiometer connected across the power terminals of the bridge, with a 10k resistor between the wiper and the signal terminal that comes off the common point of the pair of completion resistors. You can try different values and what you need will depend on how well the gages and completion resistors are matched.

5. It is difficult to provide a real strain calibration with strain gages. It is customary to use a shunt resistor to calibrate the circuit. You can connect a resistor in parallel with one of the bridge completion resistors, with a switch to switch it into the circuit. Switch contact resistance is not critical. Depending on the strain that you are trying to measure, and the gage factor, the calibration resistor will be on the order of 100 to 1000 times the resistance of the bridge completion resistor. The accuracy of the calibration will be only as good as the accuracy with which the resistance ratio is known. This type of calibration removes the uncertainty caused by uncertainty in the supply voltage. You will get a lot of practice calculating the resistance of a pair of parallel resistors. This type of calibration corresponds to compression strain in the gage opposite the completion resistor being shunted.

6. Connections within the Wheatstone bridge should be soldered if at all possible. Power and output connections, and the balancing circuit, are not so critical.

That enough for now. Send me an EMail if you need any more.

Link to comment
Share on other sites

Thanks again all for the very educational posts. I am going ahead and getting the Labjack bundle. I had considered trying to do the whole thing with a PIC or an HC11 microcontroller since really all I'm trying to prove with my hpatics project is some comparision of sensory tactile feedback, which seems to be in the form of either IR, strain gage, or capacitive. I wanted to prove the extra sensitivity/specificity of using "smart" sensors vs. binary (light or dark). The semiconductor gage as described by Bob with a null pot fed into an A/D port seems to be the easiest solution in concert with the LJ logger which would equate a sensor voltage with x grams of pressure = turn grabber actuator motors off and remain quiescent.

Bob - do you know of any cheaper vendors than Omega for sensors. I'd love to find a manufacturers evaluation board with the null pot, etc ready to go rather than having to breadboard.

Again mucho thanks to all - I'm learning as much here as I am actually doing the project....randy

Link to comment
Share on other sites

It has been a few years since I bought strain gages and I no longer have catalogs. There have been a lot of developments and some of my suppliers have been merged. BLH now seems to be a German company and here is a link that will get you to a catalog without prices. Some of it is in German but a lot is in English. They describe an "inexpensive uncalibrated gage" which would probably work for you. I'm sure they sell in the USA.

You should try to use high resistance gages (perhaps 1000 ohms) so you can use high exictation voltage and therefore get good output. You should use at least two gages (one compression and one tension) for your transducer (which can be part of your machine). Then calibrate it for output versus load.

I doubt that you will find a manufacturers board, but you can mount the resistors and pot on a perforated board, or even solder the resistors to the terminals of the poetntiometer. I have mounted pots in a little handy-box and done all of the wiring off the terminals.

Link to comment
Share on other sites

  • 1 year later...
Guest Obsidian

I played around with this type of thing a while ago, on tactile sensors. There is a simple way to make a surface strain/pressure/tactile sensor if you don't need precise calibration. It uses on some of the anti-static foam that computer hardware typically ships in. The resistance of that foam drops when it is compressed. To make a tactile sensor based on that, just glue parallel strips of a conductive foil going in one direction along one flat face, and strips going in a perpendicular direction on the other face. The resistance between any strip on one face and any strip on the other face gives you a measure of the compression of the area where these two strips cross. For a position sensor, just use a bigger chunk of foam with only two attached conductors. This can also work as a strain sensor for small forces. Alternately, if you really need a stronger strain sensor, use an appropriately stiff spring and a slide potentiometer (get a linear pot, not log) - just attach the slide to one end of the spring (with a bit of wire in between, perhaps) and the body to the other end.

Link to comment
Share on other sites


This topic is now archived and is closed to further replies.