Pet Food Supply Monitor Using Load Cells and Phidgets - Part 2

Step 1: Modify SBC Code

We need to make the following modifications to our C program running on the Phidget SBC:

• Utilize another channel from the PhidgetBridge 4-Input to interface with the load cells from our new scale.
• Create a function that will allow us to accurately weigh the cat based on the data from the load cells.
• After weighing, store weight information to a file on the Phidget SBC.

Adding Another VoltageRatioInput Channel

 
#define WEIGHT_BRIDGE_CHANNEL 	1 
#define BRIDGE_SERIALNUM		141009
#define WEIGHT_DATAINT			500
...
int main(void)
{
	...
	PhidgetVoltageRatioInputHandle weight;
	...
	//create weight VoltageRatioInput
	result = PhidgetVoltageRatioInput_create(&weight);
	if (result != EPHIDGET_OK) {
		Phidget_log(PHIDGET_LOG_ERROR, "failed to create voltage ratio object");
		return 1;
	}
	result = Phidget_setDeviceSerialNumber((PhidgetHandle)weight, BRIDGE_SERIALNUM);
	if (result != EPHIDGET_OK) {
		Phidget_log(PHIDGET_LOG_ERROR, "failed to set device serial number");
		return 1;
	}
	result = Phidget_setChannel((PhidgetHandle)weight, WEIGHT_BRIDGE_CHANNEL);
	if (result != EPHIDGET_OK) {
		Phidget_log(PHIDGET_LOG_ERROR, "failed to set channel");
		return 1;
	}
	result = PhidgetVoltageRatioInput_setOnVoltageRatioChangeHandler(weight, onVoltageRatioChangeHandler, NULL);
	if (result != EPHIDGET_OK) {
		Phidget_log(PHIDGET_LOG_ERROR, "failed to set on voltage ratio change handler");
		return 1;
	}
	result = Phidget_openWaitForAttachment((PhidgetHandle)weight, 2000);
	if (result != EPHIDGET_OK) {
		Phidget_log(PHIDGET_LOG_ERROR, "failed to open food channel");
		return 1;
	}
	result = PhidgetVoltageRatioInput_setBridgeEnabled(weight, 1);
	if (result != EPHIDGET_OK) {
		Phidget_log(PHIDGET_LOG_ERROR, "failed to enable bridge");
		return 1;
	}
	result = PhidgetVoltageRatioInput_setBridgeGain(weight, BRIDGE_GAIN_1);
	if (result != EPHIDGET_OK) {
		Phidget_log(PHIDGET_LOG_ERROR, "failed to enable bridge");
		return 1;
	}
	result = PhidgetVoltageRatioInput_setDataInterval(weight, WEIGHT_DATAINT);
	if (result != EPHIDGET_OK) {
		Phidget_log(PHIDGET_LOG_ERROR, "failed to set data interval");
		return 1;
	}
	...
}

You can see the code for adding another channel above. Note the following lines of code:

• result = PhidgetVoltageRatioInput_setOnVoltageRatioChangeHandler(weight, onVoltageRatioChangeHandler, NULL);
• result = PhidgetVoltageRatioInput_setDataInterval(weight, WEIGHT_DATAINT);

We will subscribe to the change event instead of using a get function like we did for the water and food scales. By subscribing to the change event we can receive updates about the state of the scale at a constant rate that is determined by WEIGHT_DATAINT.

Weighing The Cat

The onVoltageRatioChangeHandler implementation is shown below.

 
#define WEIGH_TIME 5000
...
double m[3] = {your calibration value, your calibration value, your calibration value};
double b[3] = {your calibration value, your calibration value, your calibration value};
double animalsWeight = 0;
int numSamples = 0;
int weighing = 0;
void
onVoltageRatioChangeHandler(PhidgetVoltageRatioInputHandle ch, void *ctx, double ratio) {	
	double weight = m[2]*ratio + b[2]; //convert to weight
	if(weight > 5){ //if weight > 5 lbs, assume cat is on scale
		if(weighing == 0){
			Phidget_log(PHIDGET_LOG_INFO,"Weighing started");
			weighing = 1;
		}
		numSamples++;
		animalsWeight += weight;
	}else{
		if(weighing){
			Phidget_log(PHIDGET_LOG_INFO,"Weighing ended, logging averaged weight");
			if(numSamples*WEIGHT_DATAINT >= WEIGH_TIME){ //only log if we have at least 5 seconds worth of data 
				animalsWeight /= numSamples;
				logData(animalsWeight);
			}
			//reset all variables
			numSamples = 0;
			animalsWeight = 0;
			weighing = 0;
		}
	}
}

Here is a breakdown of the code above:

• Convert voltage ratio value to a weight value (pounds).
• If there is enough weight on the scale, assume the cat has stepped on and begin taking samples.
• After the cat has stepped off the scale and the measured weight drops, determine if we have taken enough samples for a viable measurement. Currently we need at least 5 seconds worth of data before we will log.
• Log data to file and reset variables.

Those are the only changes we need for our C program. The next step will be modifying our JavaScript code in order to graph the data that is stored in the weightdata.txt file.

Step 2: Graph using vis.js

For my program, the weight data was logged to /usr/userapps/PetMonitor/data/weightdata.txt, therefore, in order for my webpage to access the file, I created a link to the data folder at my webservers docroot location. Note, I labeled the folder 'petdata' in order to differentiate between different data links.

Graphing was done using vis.js

 
function loadDataGraph() {
//grab data stored by C program, there is a link to weightdata.txt in the docroot location
var url = '/petdata/weightdata.txt';
var options = {
		legend: true,
		width: '75%',
		interpolation: false
	};

	var jq = $.get(url);
	jq.fail(function(jq,textStatus,errorThrown){
		console.log('loadDataGraph() failed');
	});
	jq.done(function(rawdata){
		console.log('success getting raw data');
		var groups = new vis.DataSet();
		groups.add({
			id:0,
			content: "Pet weight"
		});
		var container = document.getElementById('visualization');
		var data = rawdata.split('\n');
		var wdata = [];
		for (var d in data) {
			var l = data[d].split(',');
			if (l.length != 2)
			continue;
			wdata.push({ x: new Date(l[1]), y: parseFloat(l[0]), group: 0});
		}
		var dataset = new vis.DataSet();
		dataset.add(wdata);
		graph = new vis.Graph2d(container, dataset, groups, options);
});
}