strain-rig

New version notes

Boards:

• Arduino MEGA 2560 - controls stepper, reads force input.
• HX711 - 24-bit ADC used to read force sensor. Uses Queuetue HX711 library.
• EasyDriver stepper driver. Max frequency 500kHz (2 us/cycle, far faster than motor can do).

Parts:

• Stepper Motor: 200 Steps/Rev, 1.2A/Phase. 9.09kHz (110 us/cycle) seems to be good max frequency.
• Linear screw: 4mm lead. See ball screw guide.

Interaction over serial:

• Input m12.3456789: move up by input number in cm. Negative (down) allowed.
• Input z: Zero the force sensor.

To compile:

1. Install the Queuetue HX711 library (Library Manager > Queuetue HX711 library).
2. Install the TimerThree library (Library Manager > TimerThree).

Before uploading to the board, you'll need to remove the capacitor between GND and RESET. That prevents accidental resets. You can put it back once you've uploaded.

Linux usage instructions

To use on Linux, you can install the Arduino IDE, or just use built-in terminal stuff:

1. Plug in the Arduino, and figure out which port it's on: $ dmesg | grep tty

   Example output:

   gabe@dormserver:~$ dmesg | grep tty
   [    0.000000] console [tty0] enabled
   [    1.341091] tty tty2: hash matches
   [1249326.491407] cdc_acm 3-2:1.0: ttyACM0: USB ACM device
   

   (Here, the Arduino will be /dev/ttyACM0.)

2. Make sure your user is in the dialout group, which lets you use serial devices. For example:

   sudo usermod -a -G dialout gabe

   You may need to log out and log in again for this to take effect. You only need to do this once.

3. Now, set the tty parameters with stty (replacing /dev/ttyACM0 with whatever your device is called):

   sudo stty -F /dev/ttyACM0 cs8 115200 ignbrk -brkint -icrnl -imaxbel -opost -onlcr -isig -icanon -iexten -echo -echoe -echok -echoctl -echoke noflsh -ixon -crtscts -hupcl min 1
   

4. Now that the Arduino is attached, start saving the output in the background. You can read from /dev/ttyACM0 (or whatever the device is) like a normal file. For example:

   cat /dev/ttyACM0 > output.txt &

5. You can check the latest output with tail, or view a continuous feed with tail -f:

   tail output.txt

6. You can write to the Arduino like you would any file or device. For example, to move up by 3cm:

   echo "m3" > /dev/ttyACM0

   As the rig starts moving, you should see the green LED on the front turn on.

7. To exit, run fg to bring the cat task to the foreground, and kill it with Control+C. The output from this session will be saved in the file you specified in step 4.

Here's an example session in full:

gabe@dormserver:~$ dmesg | grep tty
[    0.000000] console [tty0] enabled
[459805.013211] cdc_acm 3-2:1.0: ttyACM0: USB ACM device
gabe@dormserver:~$ sudo usermod -a -G dialout gabe
gabe@dormserver:~$ sudo stty -F /dev/ttyACM0 cs8 115200 ignbrk -brkint -icrnl -imaxbel -opost -onlcr -isig -icanon -iexten -echo -echoe -echok -echoctl -echoke noflsh -ixon -crtscts -hupcl min 1
gabe@dormserver:~$ cat /dev/ttyACM0 > output.txt &
[1] 30271
gabe@dormserver:~$ echo "z" > /dev/ttyACM0
gabe@dormserver:~$ echo "m1" > /dev/ttyACM0
gabe@dormserver:~$ tail testoutput.txtgabe@dormserver:~$ tail testoutput.txt
Time (micros): 2863788 Position (cm): 1.00 Force (g): 0.17
Time (micros): 2956144 Position (cm): 1.00 Force (g): 0.14
[...]
Time (micros): 3695104 Position (cm): 1.00 Force (g): 0.19
gabe@dormserver:~$ echo "m2" > /dev/ttyACM0
gabe@dormserver:~$ echo "m.5" > /dev/ttyACM0
gabe@dormserver:~$ echo "m-3.5" > /dev/ttyACM0
gabe@dormserver:~$ fg
cat /dev/ttyACM0 > output.txt
^C
gabe@dormserver:~$ nano output.txt

Notes on calibration:

The HX711 is a 24-bit ADC, which, in our application, measures voltage from the load cell up to ±20mV. The largest signed 24-bit int is 8388607. Thus, a change of 1 from hx711.read() corresponds to (1/8388607) volts. For convenience, this unit is called "ticks".