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Issue
99, October 1998
X-Y Graphing Data
Logger
by
Alberto Ricci Bitti
Start
• Machine
Muscle
• Using
Componentware
• Low-Power
Guys
• Flash
Risc Glue
• The Graphic
Engine
• Casio
Protocol
• Main
Code
• Inside
The Box
• Setting
up the Casio
• First
Graph
• Modeling
The Data
• Broader
Vision
• Software,
References & Sources
FIRST GRAPH
Look at the real-world example
in the sidebar "Straightening curves—Linearizing
a Tank-Level Sensor." Here, a float drives a potentiometer
sensing the liquid level in a 25-l, irregularly shaped
tank.
I want to figure out the
relationship between the sensor output and the quantity
of liquid left in the tank and to gather enough data to
build a model for the control processor.
The complex relationship
between angle, height, and volume makes an analytical
approach impractical. It would be better to measure the
sensor output at known liquid quantities.
The potentiometer output
is brought to input x. It’s powered at 4 V
to avoid damaging the inputs.
In Listing 1, you see the
simple control program that’s required. List is the
equivalent of an array in the Casio world. Seq allocates
the memory space for a list and initializes it. Here,
two lists—one for the liquid quantity and one for
the sensor output—are created and filled with data
in a simple for-next loop.
Seq(N,N,0,25,1) --> List1
Seq(0,N,0,25,1) --> List2
For 1--> N To 26
Receive(X)
X --> List2[N]
Locate 1,1, N
Locate 10,1, X
While (Getkey): WhileEnd
Next
Listing 1—This
simple program makes 25 samples and places them
in List2. Another list is filled with ascending
numbers used as x-axis values on an x-y
graph. List is the equivalent of an array in
the Casio world.
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At each cycle, 1 l of liquid
is added and a measurement is taken. This process continues
until the tank is full. When the program ends, the two
lists hold the quantity of liquid as well as the sensor’s
output level.
Even if data visualization
commands could be included in the program, it is convenient
to look at nonrepetitive tasks manually.
The STAT menu lets you examine
the data tables. If you select the GRPH submenu and GPH1,
Graph1 is then displayed as an x-y scatter
graph of List1 (liters) over List2 (sensor output).
Although nonlinear, it’s
immediately clear that the sensor output is suitable for
measurements. An anomaly at about 5 l, due to the presence
of a pump niche that reduces the available volume, is
equally evident.
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