KIT4007 User Guide - Revision history

Mparadis at 16:31, 1 June 2023

2023-06-01T16:31:14Z

← Older revision		Revision as of 16:31, 1 June 2023
Line 102:		Line 102:

	== Further Reading ==		== Further Reading ==
	For more information about how load cells work, see our [[Load Cell ~~Primer~~]].		For more information about how load cells work, see our [[Load Cell Guide]].

Lmpacent at 20:51, 24 April 2023

2023-04-24T20:51:24Z

← Older revision		Revision as of 20:51, 24 April 2023
Line 12:		Line 12:
	== Calibration ==		== Calibration ==

	The load cell inside the scale produces a small voltage differential based on how much strain its metal body is experiencing~~. You can learn more about the sensors in the load cell on our [[Strain Gauge Primer]]~~. How do we convert this small voltage into a useful unit of weight or force? The load cell needs to be calibrated.		The load cell inside the scale produces a small voltage differential based on how much strain its metal body is experiencing. How do we convert this small voltage into a useful unit of weight or force? The load cell needs to be calibrated.

	The simplest way to calibrate the load cell is by using linear interpolation using two known weights and their corresponding voltage measurements. The output of a load cell is linear, so once we find the line that connects those two points, we can use it to convert any voltage from the load cell into a weight or force in the same units as our known weights.		The simplest way to calibrate the load cell is by using linear interpolation using two known weights and their corresponding voltage measurements. The output of a load cell is linear, so once we find the line that connects those two points, we can use it to convert any voltage from the load cell into a weight or force in the same units as our known weights.

Mparadis at 18:41, 2 September 2021

2021-09-02T18:41:37Z

← Older revision		Revision as of 18:41, 2 September 2021
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	voltageRatioInput0 = VoltageRatioInput()		voltageRatioInput0 = VoltageRatioInput()
	voltageRatioInput0.setOnVoltageRatioChangeHandler(onVoltageRatioChange)		voltageRatioInput0.setOnVoltageRatioChangeHandler(onVoltageRatioChange)
			voltageRatioInput0.setChannel(0)
	voltageRatioInput0.openWaitForAttachment(5000)		voltageRatioInput0.openWaitForAttachment(5000)

Line 67:		Line 68:
	v2 = voltageRatioInput0.getVoltageRatio()		v2 = voltageRatioInput0.getVoltageRatio()

	# Calculate slope (m~~) and y-offset (b)~~		# Calculate slope 'm'
	m = (w2 - 0) / (v2 - v1)		m = (float(w2) - 0) / (v2 - v1)
	b = v1		# solve for b using zero point : b = y-mx
			b = 0 - ( m * v1)

	print("Calibration Complete: y = " + str(m) + "x + " + str(b))		print("Calibration Complete: y = " + str(m) + "x + " + str(b))

Mparadis at 20:06, 28 July 2021

2021-07-28T20:06:06Z

← Older revision		Revision as of 20:06, 28 July 2021
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	__NOTOC__		__NOTOC__
			[[Category:UserGuide]]
	== Assembly ==		== Assembly ==

Mparadis: Created page with "NOTOC == Assembly == right # Select a load cell (780g, 5kg or 25kg) and bolt it into the bottom plate with the arrow on the..."

2021-07-23T20:16:35Z

Created page with "__NOTOC__ == Assembly == link=|300px|right # Select a load cell (780g, 5kg or 25kg) and bolt it into the bottom plate with the arrow on the..."

New page

__NOTOC__

== Assembly ==

[[Image:KIT4007_Assembly.jpg|link=|300px|right]]

# Select a load cell (780g, 5kg or 25kg) and bolt it into the bottom plate with the arrow on the end of the cell pointing down toward the plate
# Guide the load cell cable out of the notch in the back of the plate
# Bolt the top plate onto the load cell

<div style="clear:both"></div>
== Calibration ==

The load cell inside the scale produces a small voltage differential based on how much strain its metal body is experiencing. You can learn more about the sensors in the load cell on our [[Strain Gauge Primer]]. How do we convert this small voltage into a useful unit of weight or force? The load cell needs to be calibrated.

The simplest way to calibrate the load cell is by using linear interpolation using two known weights and their corresponding voltage measurements. The output of a load cell is linear, so once we find the line that connects those two points, we can use it to convert any voltage from the load cell into a weight or force in the same units as our known weights.

Here's the process:

* First, clear the scale and take a voltage measurement. This will be our first data point.
* Next, find an object of known weight. You'll either need to measure the weight of something with another scale, or use a weight from a lab weight set. If possible, try to get a weight that will be close to the maximum weight you expect to be measuring for your project.
* Take a second voltage measurement with this object.
* Now we have two points, so we can calculate the slope of the line:

* And we can use the slope and our first point to solve for the y-offset:

* Now, we have a formula that we can use to convert any voltage from the load cell into a useful value!

Click below for a code sample in python that shows how calibration could be done at the start of your program:

{{UGC-Entry|Code Sample||
<syntaxhighlight lang=python>
from Phidget22.Phidget import *
from Phidget22.Devices.VoltageRatioInput import *
import time

calibrated = False
m = 0
b = 0

def onVoltageRatioChange(self, voltageRatio):
if calibrated:
# Calculate calibrated weight with y = mx + b
sys.stdout.write("\rWeight: " + str(round((m*voltageRatio)+b,2)) + "g ")

def main():
global calibrated
global m
global b

voltageRatioInput0 = VoltageRatioInput()
voltageRatioInput0.setOnVoltageRatioChangeHandler(onVoltageRatioChange)
voltageRatioInput0.openWaitForAttachment(5000)

try:
input("Clear the scale and press Enter\n")
except (Exception, KeyboardInterrupt):
pass

v1 = voltageRatioInput0.getVoltageRatio()

try:
w2 = input("Place a known weight on the scale, type the weight in grams, and press Enter:\n")
except (Exception, KeyboardInterrupt):
pass

v2 = voltageRatioInput0.getVoltageRatio()

# Calculate slope (m) and y-offset (b)
m = (w2 - 0) / (v2 - v1)
b = v1

print("Calibration Complete: y = " + str(m) + "x + " + str(b))
calibrated = True

try:
input("Press Enter to Stop\n")
except (Exception, KeyboardInterrupt):
pass

voltageRatioInput0.close()

main()
</syntaxhighlight>
}}

== Measurement Considerations ==

The measurement from load cells are susceptible to drift due to temperature changes in the wires. You may find that even when measuring the same object with no movement, the weight measurement may drift significantly. There are two ways to avoid this kind of error:

* For a long session of constant measurements, allow a 30-minute warm-up after the voltageRatioInput channel is enabled. The drift, due to self-heating of the wires, will eventually stop. Once stable, you can calibrate and measure.

* For brief, occasional measurements, you should enable the channel, immediately take the measurement, and then disable the channel. By doing this, there is so little current moving through the wires that the device does not get a chance to heat up. You should also follow this process when calibrating.

== Overloading ==

Usually load cells can safely be loaded at 150% of their maximum weight. For example, you could put a 7.5kg object on the scale when the 5kg load cell is inside. The measurement would saturate (see the '''Saturation''' event in the {{Phidget22 API}} for your Bridge Phidget for details), so you wouldn't be able to tell how much heavier than 5kg the object is, but you don't need to worry about damaging the load cell.

However, you need to be careful with the 780g load cell- its high sensitivity makes it vulnerable to damage if loaded beyond 780g. If there's a chance your objects will go above this maximum, we recommend using the 5kg load cell instead.

== Further Reading ==
For more information about how load cells work, see our [[Load Cell Primer]].

KIT4007 User Guide - Revision history

Mparadis at 16:31, 1 June 2023

Lmpacent at 20:51, 24 April 2023

Mparadis at 18:41, 2 September 2021

Mparadis at 20:06, 28 July 2021

Mparadis: Created page with "__NOTOC__ == Assembly == right # Select a load cell (780g, 5kg or 25kg) and bolt it into the bottom plate with the arrow on the..."

Mparadis: Created page with "NOTOC == Assembly == right # Select a load cell (780g, 5kg or 25kg) and bolt it into the bottom plate with the arrow on the..."