MECH3660 MECH9660 Manufacturing Engineering
Assignment 1 (15%) – Part C (5%)
Data Acquisition and LabVIEW – Digital Bathroom Scales Case Study
Due: Friday 15th April 2016 (week 6), 4:30pm
Background:
The way a digital scale works is based on a component called a strain gauge. A strain gauge can be used to measure the force applied on an object.
When a material is pushed or pulled with a force, it stretches or compresses. A strain gauge is made from wire that is adhered to the surface on which strain needs to be measured. When the material deforms, the strain gauge wire consequently contracts or stretches with the material. The wire acts like a resistor and the longer the wire the higher the resistance. Thus, the more force applied to the material, the higher the wires resistance. This, in essence, is how a strain gauge operates.
Strain gauges are used in sensors called load cells, which usually consist of four strain gauges in a Wheatstone bridge configuration (feel free to look this up, but the technical details of a Wheatstone bridge is beyond the scope of this assignment). Four strain gauges in a load cell is called “full bridge”. Load cells of one strain gauge (quarter bridge) or two strain gauges (half bridge) are also available.
Load cells require excitation (input) from a known voltage source. They output a proportional voltage signal in mV/V of the excitation value, that ramps up to a specified value at full load. The voltage output is typically in the order of a few millivolts and therefore requires amplification. The output of the load cell can be scaled to calculate the force applied to the transducer.
Digital bathroom scales are often made from a number of load cells, each measuring the downward force exerted on the scale when a person stands on it. The measured force is expressed in newtons, but can be scaled easily to “kilogram-force” (kgf), which is the force exerted by a mass of one kilogram under the standard acceleration due to gravity on earth (9.8ms-2). Since bathroom scales do not have to be extremely accurate, and are generally used on earth, kgf is a perfectly good approximation of a person’s weight in kg. The outputs of all load cells in kgf are simply added together to give an overall body weight.
The aim of this assignment is to go through some of the design consideration, hardware simulation and programming exercises that would be undertaken when designing a digital bathroom scale product.
Questions – PART i:
Many things must be considered when choosing appropriate data acquisition hardware for use in a product, including but not limited to sample rate, resolution, number of channels, and cost.
We have the following specifications that we want to meet with our digital bathroom scale product design:
• Our bathroom scales should include 4 load cell sensors
• Our bathroom scales should measure a body weight range of 40 kg to 160 kg
• Our bathroom scales should take no longer than 5 seconds to take a measurement, in which at least 10 raw values should be averaged.
1. With this in mind, select the best load cell model to use based on the following three options. Provide reasoning for your choice of load cell, and assume that the physical size of all load cells presented below are appropriate for our design.
a. Model 41 DX Precision Low Profile Load Cell
Excitation: 10 VDC
Bridge Resistance: 350 ?
Bridge Configuration: Full Bridge
Measurement Range: 0 to 25 kg
Operating Temperature: -54 to 121 °C
Linear Output Scale: 0 mV/V = 0 kgf, 4 mV/V = 2 kgf Cost: $745 AUD
b. Model 13 SB Subminiature Load Cell Excitation: 10 VDC
Bridge Resistance: 350 ?
Bridge Configuration: Full Bridge
Measurement Range: 0 to 42 kg
Operating Temperature: -54 to 121 °C
Linear Output Scale: 0 mV/V = 0 kgf, 4 mV/V = 7 kgf
Cost: $1,255 AUD
c. Model 41 PC Low Profile Load Cell
Excitation: 2.5 VDC
Bridge Resistance: 350 ?
Bridge Configuration: Full Bridge
Measurement Range: 0 to 42 kg
Operating Temperature: -54 to 121 °C
Linear Output Scale: 0 mV/V = 0 kgf, 4 mV/V = 7 kgf
Cost: $815 AUD
2. Which of the following National Instruments Data Acquisition cards would be the best choice for this application, considering the inputs and outputs required for our selected load cells (note: you will have to look up the specifications for these DAQ cards on ni.com). Provide reasoning for your choice of hardware.
a. NI 9219
b. NI 9237
c. NI 9235
d. NI 9211
Questions – PART ii:
a. Simulate your selected data acquisition module from Part i in an “NI cDAQ-9178” CompactDAQ Chassis in MAX.
b. Create a LabVIEW VI that acquires 100 samples from all four load cell channels on the DAQ device, at a rate of 100 Hz.
i. You will need to set up the sensor characteristics, measurement range and kgf scaling factor for EACH of the four channels in DAQ Assistant.
ii. Display the raw data on a graph
iii. Calculate and display the mean of each load cell’s 100 samples on the front panel iv. Calculate and display the total mean weight of the person (add the four load cell outputs) on the front panel.
(Note: we cannot change the simulated data that is generated by our simulated device, so our weight measurement will not change on subsequent runs)
c. Calculate the patients BMI
i. Create a “Height” control on the front panel where the user can enter their height in cm
ii. Use the formula: BMI = Weight in Kilograms / (Height in Meters)2 to calculate the patients
BMI.
iii. Display the BMI result on the front panel.
iv. Display the BMI category on the front panel, given the following data from the Australian Government:
• Under 18.5: Underweight
• 18.5 to 24.9: Healthy Weight Range
• 25.0 to 29.9: Overweight
• Over 30.0: Obese
Submission Instructions and Marking Criteria
When you have completed your written answers and LabVIEW program, include all SIDs in each filename as per the example below and upload your files through Blackboard.
310275433_307111228_308456399.docx
310275433_307111228_308456399.vi
If these rules are not adhered to, your assignment will not be marked!
Part i: (1.5 Marks):
1. Correct Load Cell Selection and Reasoning (0.5 Marks)
2. Correct DAQ device selection and Reasoning (1.0 Marks)
Submit your responses and reasoning in a word document.
Part ii: (3.5 Marks)
Submit your LabVIEW file(s).
As well as receiving marks for the correct Data acquisition setup and BMI calculations, your LabVIEW code will be used to analyse your methodology and understanding of the software package. It will be used to provide partial marks where relevant. Consider the following:
- Logical function/vi selection and ordering
- Meaningful labels on graphs and indicators
- Intuitive Front Panel design
- Block Diagram Comments
- Neat, readable code