Thursday, October 28, 2010

Computer interface to a low cost electronic kitchen scale (part 1)

On a few occasions I was looking for a low cost electronic weigh scales with a computer interface. It seems they don't exist. There are electronic kitchen scales for as little as €15, but as soon as you require a computer interface you're into specialized scientific/point-of-sale equipment and the starting price jumps to €400+.

I had a spare low cost kitchen scale lying around that was purchased in Maplin (model KS108) a few years ago. Opening it up one can see the weight/force (also known as a "load cell") sensor is a cantilever beam anchored to the base of the unit with strain gauges glued to the top and bottom of the beam. The strain gauges are covered in a white epoxy for environmental protection. A weight (force) applied to the business end of the beam causes the beam to bend slightly causing the strain gauges on top to stretch and those on the bottom to compress.


A commonly used configuration comprises 4 strain gauges wired in a "full bridge" Wheatstone bridge. All four resistors of the bridge (see schematic further down) are strain gauges. Two on the top (R1, R4) and two on the bottom (R2, R3). This configuration provides good sensitivity to bending, and automatically compensates for resistance variation due to temperature.


The interface to the sensor comprises 4 wires: the red and black wires are are the 'excite' (labeled E+ and E- on the PCB) and white and blue are the sense wires (labeled S+ and S-).

The changes in resistance of individual strain gauges are too small to measure accurately by direct measurement. However in a Wheatstone bridge configuration the tiny changes in resistance causes the bridge to unbalance and measurable potential difference is generated on on the sense wires.

I was hoping to piggy back my own sensor electronics without interfering with the operation of the scale. Unfortunately it seems the scale's electronics was interfering with my ability to measure anything. So I cut the sensor wires and attached a simple plug / socket to allow the sensor to be connected to my system but still have the option to revert back to the original configuration if required.

After disconnecting the sensor from the scale's PCB I was able to measure a small voltage difference on the sense wires. But only 1.4mV when 1kg is applied. This is far too small to be measured directly by the ADC (each step of a 0–5V 10 bit ADC is about 5mV).

The two sense wires are connected to an op amp in differential configuration (a ST Microelectronics TS954 dual rail-to-rail op amp) set at moderately high gain produces a signal useable by an ADC.
Normally the Rg resistor is set set to the same value as Rf. In this configuration Vo is 0V when no force is applied to the sensor and can swing up the the supply rail (5V).

If Rg is omitted, at rest (zero force) the output from the op amp (Vo) is about 2.5V. Depending on how the sense wires are connected to the op amp, Vo can swing from 2.5V to 5V or from 2.5V down to 0V (the ground rail). So it seems we lose half of the ADC's dynamic range. However we can put this situation to good use.

Let's call the voltage change due to the application of force on the sensor as ΔV. Instead of wiring the excite wires to the power/ground rail, two digital IO output lines (D0 and D1 in the schematic) are used to excite the sensor. With D0 set to logic high (5V) and D1 set to logic low (0V) a measurement is taken: a0 = 2.5V + ΔV. By toggling the IO lines the excitation polarity is reversed and another measurement is taken: a1 = 2.5V - ΔV. The difference (Δa = a0 - a1) is 2.5V + ΔV - 2.5V + ΔV = 2ΔV. The measuring resolution is doubled. Another advantage of this setup is that is helps eliminate common mode interference.

A word of caution: one must allow sufficient time for the signal to settle after changing the excitation polarity. As can be seen from the oscilloscope trace (taken at Vo where a grid square = 200µs x 1V) at least 500µs is required for the sense voltage to settle after a reverse in direction.



To illustrate this noise reduction in action, I deliberately introduced mains noise by pinching one of the sense wires with my fingers. I wrote a Arduino sketch to take 300 samples with alternating polarities (and allowing 750µs settling time). In the chart below, one can see a0 and a1 have 50Hz mains noise of amplitude ~ 15 ADC units, but the noise in Δa has an amplitude of only ~ 5 ADC units.


Calibration

Ideally a calibration weight kit (expensive!) is used for calibration. But this is not intended to be a super accurate scales. I am aiming for ±0.5g. For that resolution there is a calibration kit (literally) in your pocket: coins are minted to precise specifications. Freshly minted coins are best, but any clean coin in reasonable condition will suffice for this purpose.

The following calibration chart was obtained by mounting Euro 10c (4.10g), 20c (5.74g) and 50c (7.80g) coins on the scales.

Using Gnuplot to plot and curve fit the data to the equation of a line: Δa(x) = c + mx
where x is the mass in grams applied to the scale.

Gnuplot curve fit reports:
c               = 17.3111          +/- 0.09565      (0.5525%)
m               = 2.49864          +/- 0.0006569    (0.02629%)


As can be seen from the residuals plot there is excellent linearity (the residuals are random and ranging from -0.8 to +0.8 ADC units with a root-mean-square of just 0.376). The slope or sensitivity (m) has a value of 2.49864 (let's call that 2.5) which means that each gram added registers a Δa of 2.5 ADC units, or each ADC increment is 1/2.5 = 0.4g. Combined with the noise (the spread in residuals) and it seems we are safely in the ±0.5g area.

An Arduino was used for the initial prototyping. The following sketch (below) excites the sensor in both direction, takes a time averaged ADC of Δa and spits it out on the serial port.

In part 2 I will discuss the interfacing to a computer, the computer side software and possible applications.



/**
 * Take measurement from weigh scales sensor and output to serial
 * port. By reversing polarity of excite signal can reduce common
 * mode noise. Take 256 samples and get mean before outputing 
 * result -- this helps reduce noise and adds extra resolution.
 * Joe Desbonnet, 28 Oct 2010. jdesbonnet@gmail.com. 
 */
 
 // Define bridge excite digital IO lines
#define D0 3
#define D1 2

// Use ADC port 0
#define AN0 0
 
// Delain in microseconds to allow signal to settle 
// after excite polarity reversal
#define DELAY 800

void setup()   {
  
  Serial.begin(115200); 
  
  pinMode(D0, OUTPUT);
  pinMode(D1, OUTPUT);
}

void loop()                     
{
  
  // Get zero point ('tare') of scale
  int tare = measure ();
  
  while (true) {
     Serial.println ( measure() - tare );
     delay (1000);
  }
  
}

int measure () {
  int i,a0,a1;
  long s;
  for (i = 0; i < 256; i++) {
    digitalWrite (D0,HIGH);
    digitalWrite (D1,LOW);
    delayMicroseconds(DELAY);
    a0 = analogRead(AN0);
     
    // reverse polarity
    digitalWrite (D0,LOW);
    digitalWrite (D1,HIGH);
    delayMicroseconds(DELAY);
    a1 = analogRead(AN0);
    
    s += (long) (a0 - a1);
      
   } // next i
   
   // Don't care about polarity
   if ( s < 0) {
       s = -s;
   }
   
   // As there is a large number of samples being averaged we
   // can probably extract another bit or two from the ADC.
   // Going to be conservative and going for one extra bit.
   // So instead of dividing by 256, will divide by just 128
   // instead (ie right shift 7 bits).
   return s>>7;
}

8 comments:

Steve Romero said...

So what would you use the interface for? What data logging applications are available to make use of an interface like this?

Thanks

jdesbonnet said...

I've got a small Java mini application in the works. It does simple calibration, weight in kg or lbs, tare etc. As for application - perhaps small trader point-of-sale (POS scales start at about $400!). It wouldn't be too difficult to expand the software to multiply weight by a cost-per-kg of a selected product and send to a label printer.

It will also be useful for situations where you want a graph of weight vs time. I'd be curious to see how the weight a bee hive changes as honey reserves build up. Unfortunately you'd need something more substantial than a kitchen scales for that (perhaps a bathroom scales?).

jbucks said...

I have a use for such an interface! =) I help to run a small urban farm in Berlin, and I'm interested in having a scale which can transfer information to a computer. I was lucky and found your post. We would love to use a such a scale for weighing harvests. I know it's been a couple of years since this was posted, but has there been any new information about this?

David Aplin said...

hi, i am attempting to wire an inexpensive digital kitchen scale. it is powered with a 9v battery and i am trying to "hardwire" the unit by using the positive and negative wires from a discarded 9v wall wart. so far so good, my clumsy hands have managed to solder the wires from the wall wart to the battery receptacle in the correct configuration...it works. however, the unit has some sort of timer on its circuit board that automatically shut the scale off after approx 90 seconds, i guess to save the 9v battery from running down. i would like to locate the timer and bypass it so that i can plug the unit in and have it stay on until i unplug it. the on/off switch has a very tiny cap/resistor barrel shaped thing located within approx 3-5 mm of the switch. i thought that perhaps this thing is the functional timer. it's too small for me to see any detail on it other than it seems to have coloured stripes. any suggestions?
would be most thankful!
regards,
dave aplin
cliffsidehearth@gmail.com

jdesbonnet said...

David,

The simplest way to implement a timer is to do it in software in the microcontroller (MCU). If it's digital and made in the last 20 years then it almost certainly has a small MCU. So I doubt you can isolate any discrete components to disable that power-off behavior.

So how to defeat the power off? that's a good question. It's possible (although unlikely) that it can be done through some menus. If it has menus, you might want to check that or read the user manual if you still have it (try googling for it if you don't).

Next thought: you could implement a simple piece of electronics that emulated the power button being pressed every 30 seconds or so. Something like a 555 timer could do the job. But you'd have to understand how the switch is configured. Personally I'd use a small cheap PIC microcontroller (eg 12F675... but that involves getting skilled and tooled up in programming those devices.

If you post the make/model of the scales maybe and/or links to photos I can think about the problem some more.

stretchedbag said...
This comment has been removed by the author.
enoughwealth@yahoo.com said...

Nice article! I'll use this as the starting point for a project I have in mind (building a platform animal scale for use on our alpaca farm, based on resting a platform on four cheap 100kg bathroom digital scales and combining the readings and recording them on my laptop. It would be a worthwhile project as commercial vet scales cost $400+. We used to have a second-hand airport luggage scale that did the job, but it no longer works).

A couple of questions:
1. Is there a 'part 2'?
2. If a platform was resting on four such scales will the total weight reading be correct even if the platform isn't perfectly balanced/level on the scales ie. if one scale is getting more of the load, the others should get less load but the total load should be constant?

Joe Desbonnet said...

Sorry, I didn't get to part 2 (yet! :-)

Sounds like an interesting project. Yes, I believe the sum of the for bathroom scales is the total weight, and perfect balancing is not needed. Just as long as the force being exerted on the scales is vertical. Btw: many electronic scales them selves comprise 4 corner strain guage sensors that are summed.

Have you considered purchasing a new hanging scales? I brought one recently (0 - 200kg, 0.1kg accuracy) for about $50, but I've seen them even cheaper on Amazon.