Shepard v1.1 Dev Log (Thrust DAQ) Started: 05-20-13
The work done on Shepard v1.1 on 05-20-13.
The DAQ (Data AcQuisition) system for version 1.1 of the Shepard Test Stand is quickly getting left behind technology-wise. We believe that we've found a good source for small load cells, which makes the use of FSRs (Force Sensing Resistors) not worth pursuing any more. The sample rate due to the thermocouple amplifier is far too slow as well, and we're looking at switching to a non-contact temperature sensor to obtain faster sensor response during runs. However, we still have a partner organization that needs a version of Shepard to experiment with, pre-2.0, and I thought that I'd use a hybrid platform of the old and the new technologies to test out some of our ideas in version 1.1. Hopefully I'll end up with a setup that our partner can utilize for their testing that will also be a good test bed for our new components.
The two ICs in this test circuit are the INA122 Instrumentation Amplifier , and the ADS1118 amplifier/ADC , set up to amplify thermocouple inputs. The ADS1118 required a MSOP10 to DIP converter so that I could use it with a breadboard/protoboard. Thanks to Aaron Harper for going through the painstaking process of soldering/reflowing such a small package for me.
I'll start with the INA122, although both ICs will be on the breadboard so that I can plan the layout appropriately.
Using the following resources, I came up with the circuit below:
- http://www.robotshop.com/phidgetbridge-wheatstone-bridge-sensor-interface-2.html (Documentation link)
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It made sense to me that for testing purposes, the gain of the INA122 should be adjustable, so I swapped out the 1k RG fixed resistor for a 10k 15-turn potentiometer. I then experimented with the gains shown in the INA122's application information. I found that the gains of 1000 (200 ohms) to 5000 (40.2 ohms) gave a good amount of change with the force that I applied with my fingers. The next step was to clamp the load cell down to my desktop and hang some weight from it, using a hanging scale for reference.
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The max thrust we should ever see from an Estes motor (A through E) on Shepard is around 30 Newtons (6.74 lbs). If we want to calibrate to 1.4 times the max thrust we'll ever see (per good engineering practices), we'll need to calibrate to 42 Newtons (9.44 lbs). I thought that I'd go ahead and round that value up to set the gain so that 9.5 lbs was the max, or would "fill" the Arduino's ADC. I started to use jugs of water to reach this weight (simulated thrust). Unfortunately, I didn't have the right kind of clamps to hold one end of the load cell properly. I believe my best bet of getting the gain set and calibrating the load cell will be to make the minor modifications necessary to mount the load cell in the Shepard v1.1 structure used during Yuri's Night (starts at about 1:03:19). Then I can use the previous calibration mechanism (pulley and string) to set and calibrate the system.
Thoughts, questions, suggestions? Feel free to leave them as comments on this news feed.