Heidenhain 11(micro)App sinusoidal linear encoders and USB 6009

[feanorou]

Hello, I am not familiar with electronics, yes I have been using one similar to these models http://www.tek.com/oscilloscope/tds1000  to inspect thatmeasuring machine, but it was 9 years ago. I remember using this adapter (JPG attached) to get the 2 signals from the Heidenhain card socket ( pin 1-green, pin 2-yellow, pin 5-shield -----> to two BNC connectors).

In that case it was quite easy,  I just plugged the BNC connectors in CH1 and CH2 and saw the circle on the screen, now I am looking for instructions about using USB 6009.

Connecting green-AI0, yellow-AI1, shield-GND seems logical, but I am doublechecking before risking to damage the unit.

I feeI I should control the max.current entering the USB 6009, (11.5 microampere seem a low value to me) but I do not know whether I should use and how

to connect a shunt resistor. 

Thank you for your support

[feanorou]

I think I should use this configuration with a 450kOhm resistor, in that case I would read a value around 4.95 V, for 11 microampere. Correct?

[feanorou]

is this correct? Using a 450 kOhm resistor I transform the sinusoidal current 11uA into a 5V sinusoidal tension to be measured between AI0 and GND?

Thank you

[REDS · Accepted solution]

Hi feanorou,

Thanks for using our Discussion Forums. You are on the right track, but I would like to give you some information. If you connect a shunt resistor in parallel to a current source, you will get a voltage source in series with the shunt resistor you connected. So you will get a 4.95V voltage source if you connect a 450K resistor in parallel to a 11uA sine wave current source.

Please notice that the datasheet refers to uVpp (peak to peak, number 2 in the image). Normally (when you don't have a DC level involved) you would like to stick with Vp (Peak voltage) which is the amplitude (number 1 on the image). So 11 uApp is 5.5 uA peak.

That being said, you also need to take into account the internal resistance of the 6009, which is 144K. This is a problem in this case since you will not have a big voltage drop in the internal resistance of the device, since the 450K resistor is bigger than the Input Impedance, it will take most of the voltage drop. What I recommend here is for you to put an OpAmp (Operational Amplifier) in between the encoder and the 6009, like the LM358. This is an IC that will not only amplify the voltage signal you are getting from the connection of shunt resistor with the encoder, but you are also going to have a low impedance at the OpAmp's output, which is desirable so you will have most of the voltage drop in the 6009.

Please reference Page 7, the model where it says non-inverting is a configuration you could use. The gain is determined by (1+ R2/R1) in the figure.

National Semiconductors LM358 Datasheet

[feanorou]

Thank you for the detailed explanation, is it not sufficient to convert to an appropriate voltage level close to 5V within USB 6009 specs?

The measurement does not need to be accurate, I just have to check form and stability of the two combined signals on the XY graph, when the measuring head is diplaced along the glass scale. 

My only worry is to damage the hardware when making inappropriate connections. 

Starting to use additional components is a challenge and I am willing to learn, it takes time.

Would it be safer to work with a myDAQ (instead of USB 6009) which has a built-in DMM instead?

[REDS]

Hi feanorou,

You are very welcome. If you have a myDAQ, that would be a better option since it has a 16 bit resolution (vs 14 bits) and a much higher input impedance, > 10 G ohm (vs 144K ohm) as you can see on the myDAQ's Specifications, comparing it to the 6009. I do understand that getting started on new hardware takes time, and depending on how well you read the small signal will determine if more circuitry will be needed or not.

Page 39 of the encoder actually shows a recommended circuit to amplify the signal and make it more readable for other measuring devices. But if you don't need that much accuracy, you can expect better results using the myDAQ then you would use the 6009.

Just please be sure that you are not exceeding the maximum power ratings that your encoder can provide. This is in part why I suggested using an OpAmp, since you will have a layer of amplification between the encoder and the DAQ device, so you will not be demanding power to the device, just to the amplifier.