TOPIC: Jetex type thrust rig.

Our cousins in the model rocket fraternity have been measuring the thrust of model rocket motors for quite some time. After seeing a rocket motor thrust rig recently I decided to have a go at putting one together that was more suited to our longer duration burn times.

This uses an Arduino micro-controller, an HX711 strain gauge and some software culled from the net and modified by me.

The motor uses a strain gauge, which is based on a resistor which varies in value as the metal bar it is bonded to is pushed against. Strain gauges are used in electronic scales. In this case the resistor is connected to an analogue to digital converter which sends a number corresponding to the changed resistance to the Arduino. The Arduino checks this number every second and writes it back to my laptop which displays it on a logging screen.

I can copy/past the logging screen results into a spreed sheet to make a graph just like in proper engineering!

Note the vertical strain gauge for measuring static thrust and below the analogue to digital converters for two strain gauges. (I'll add the second one later). To the right is the Arduino micro-controller that samples the thrust and sends it to my laptop. The sampled thrust is displayed on a logging window on the laptop and can be copy/pasted into a spreadsheet.

When the system starts up it first measures the off load value of the strain gauge and then idles until the thrust starts. When the thrust starts it enters a logging loop and re-samples the strain gauge every second for 30 seconds. (At each sample it multiplies the number coming back from the strain gauge by a constant that turns it into grams.)

I'm still playing with the software here and I may sample the thrust at a higher rate and then average it for every second.

I'll shortly post the code and a parts list for anyone who wants to make such a rig.

(I'm writing several posts, BTW, because the system keeps timing me out.)

Here's the strain gauge and it's electronics. (cost €6.80)
www.amazon.de/gp/product/B07L82QXVD/ref=...04_s00?ie=UTF8&psc=1
The Arduino, (cost €22.80)
www.amazon.de/Arduino-Uno-Rev-3-Mikrocon...ustrial%2C188&sr=1-4

And here's the sight you can download the Arduino development environment.

Once you've got the Arduino you can connect it to your laptop with the supplied cable. (This also powers the board) and start getting accustomed to it.

For the strain gauges there's a bit of soldering and the requirement to download some library files to support the strain gauge A2D hardware. HX711-multi.

Here's the current version of the code. (largely derived from the example code given for the HX711-multi.) You'll need to add the library files, HX711-multi, to the Arduino IDE.


Start of code...
#include "HX711-multi.h"

// Pins to the load cell amp
#define CLK A0 // clock pin to the load cell amp
#define DOUT1 A1 // data pin to the first lca
#define DOUT2 A2 // data pin to the second lca
#define DOUT3 A3 // data pin to the third lca
#define BOOT_MESSAGE "Jetex/Rapier thrust guage "
#define TARE_TIMEOUT_SECONDS 14
#define SCALE_FACTOR 1850
#define MAX_LOG_COUNT 50
#define FIRING 10
#define CHANNEL_COUNT sizeof(DOUTS)/sizeof(byte)
byte DOUTS[3] = {DOUT1, DOUT2, DOUT3};
long int results[CHANNEL_COUNT];
int start_log, log_count;

HX711MULTI scales(CHANNEL_COUNT, DOUTS, CLK);

void setup() {
Serial.begin(115200);
Serial.println(BOOT_MESSAGE);
Serial.flush();
pinMode(11,OUTPUT);
tare();
start_log = 0;
log_count = 0;
}

void tare() {
bool tareSuccessful = false;
unsigned long tareStartTime = millis();
while (!tareSuccessful && millis()<(tareStartTime+TARE_TIMEOUT_SECONDS*1000)) {
tareSuccessful = scales.tare(20,10000); //reject 'tare' if still ringing
}
}

void sendRawData() {
if(log_count != MAX_LOG_COUNT)
{
scales.read(results);
//Serial.println(results[1]);
//Started firing ?
if (-results[1]/SCALE_FACTOR > FIRING && start_log == 0)
{
start_log = 1;
Serial.print("Thrust started...");
}
if(start_log != 0)
{
// Serial.println(": \t T:");
Serial.print("T: ");
Serial.print(log_count);
Serial.print(", ");
Serial.print(-results[1]/SCALE_FACTOR);
Serial.println(",gms");
log_count++;
}
}
delay(1000);
}

void loop() {

sendRawData(); //this is for sending raw data, for where everything else is done in processing

//on serial data (any data) re-tare
if (Serial.available()>0) {
while (Serial.available()) {
Serial.read();
}
tare();
}
}
...end of code
This is still under development and I may sample at a higher rate and then average each second's thrust. The results may be being distorted by noise.

Here's some initial results:
Secs gram mN
T: 0 32 313.92
T: 1 24 235.44
T: 2 49 480.69
T: 3 28 274.68
T: 4 35 343.35
T: 5 24 235.44
T: 6 25 245.25
T: 7 34 333.54
T: 8 23 225.63
T: 9 32 313.92
T: 10 25 245.25
T: 11 25 245.25
T: 12 34 333.54
T: 13 25 245.25
T: 14 29 284.49
T: 15 27 264.87
T: 16 25 245.25
T: 17 36 353.16
T: 18 25 245.25
T: 19 9 88.29
T:20 0 0