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Sensors are wired using Category 5 or 6 Ethernet cable using our own #analog wiring standard. Two wires are used to carry the sensor output voltage designated as S- and S+. Five wires are used for different voltage power supplies, and one wire is used for the power supply ground. It is convenient if student laboratories are hardwired with data acquisition ports at workstations, but this is not required for use of the Process Control software. When used, each data acquisition port is wired with up to five different power supplies. Sensors are wired to connect to the power supply that they require. Thus, the sensor is powered and monitored through a single port. This scheme is used for a variety of sensors including pressure, strain gage, pH, dissolved oxygen, turbidity, and temperature. For several of the sensors the power supplies are also used to power a signal conditioning circuit to transform the sensor output into a voltage that can easily be monitored with a data acquisition system.Wiki Markup
Wiring
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standard
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used
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for
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combining
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power
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supplies
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and
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analog
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data
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acquisition
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in
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a
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Category
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5
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Ethernet
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cable.
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T-568B |
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standard | T-568A |
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standard | voltage |
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white/orange |
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white/green |
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S |
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- |
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orange | green | S+ |
white/green | white/orange | ground |
blue | blue | -5 V |
white/blue | white/blue | +5 V |
green | orange | +10V |
white/brown | white/brown | -15 V |
brown | brown | +15 V |
Sensors, Signal Conditioning, and Calibration
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pH sensors produce a voltage output in the range that would normally be easy to measure using standard data acquisition hardware. Unfortunately, the impedance requirement for a pH sensor is orders of magnitude higher than the inputs of standard data acquisition hardware and thus a signal conditioning circuit must be used to amplify the pH sensor output. The circuit consists of unity gain amplifiers that have less than 0.1 pA input leakage current (anonymous , 1993). A circuit diagram is available at http://ceeserver.cee.cornell.edu/mw24/Labdocumentation/pH%20Circuit.pdf. The ProCoDA software includes automatic pH buffer recognition and piecewise linear calibration between buffers.
Pressure sensors
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Differential pressure
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sensor.
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The ProCoDA software converts the voltage output from the pressure sensors into the physical units of water column height or pressure using linear conversion algorithms. The sensors can also be zeroed or set to a measured value using a one point calibration.
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A core feature of the ProCoDA software is the user programmable feature that facilitates customization of the control logic for specific tasks. The rule editor provides a programming environment for setting up states, control logic, set points, variables defined by links to external code, and selecting which user defined parameter controls each output in each state.
States
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View of the controls used to set all of the Stamp® Microprocessor outputs. These controls can have different values for every state. The "output settings" in the middle column are drop down menus containing a list of all the defined constants and variables.
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Rules
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Rule that ends the state named "BW filter after challenge" when the time in that state exceeds the set point "backwash (filter) time". The rule also indicates that the next state will be "Acid wash".
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External logic
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LabVIEW block diagram showing the external code that adds two variables or set points and returns the
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result.
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Wiki Markup
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Screen shot from the Process Controller showing how inputs are sent to external code. In this case the external code estimates an alum dose based on measured raw water turbidity and a simple model that relates turbidity and alum dose.
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Incrementing functions that increment linearly and that increment following a power law relationship are as follow:
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The parameter, x, is an integer that increments from zero to a maximum value set by the user. The output parameters, ylinear and ypower could be used to vary a flow rate, a chemical dose, or any other parameter.Wiki Markup
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Increment functions showing how the parameter varies as a function of the state. In this example the state cycled between states 1, 2, and 3. The increment state was 2, the number of replicates was 2, the reset state was 0, the y intercept was 200, the slope was 50, and the maximum value of x was 4. The power law relationship used a coefficient of 100 and a base of 1.5.
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ProCoDA can be configured to stop an experiment after the reaching the maximum value of the parameter. Otherwise it will reset the parameter to its initial value and begin the increment process again. It is also possible to systematically vary more than one parameter.
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By using both the data and state logs it is possible to reconstruct the events that occurred during an experiment and correlate sensor and variable values with the state of ProCoDA. The data can be analyzed to troubleshoot and to determine what event caused ProCoDA to perform unexpectedly.
Filter Test Apparatus
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Photo of the filter test apparatus.
Results and Discussion
Over the past 10 years ProCoDA software and hardware have been used by students to automate bench scale drinking water treatment plants, activated sludge sequencing batch reactors, temperature controllers, flow controllers, pH controllers, and a large number of other experimental configurations. These experiments were conducted as part of the Cornell University undergraduate curriculum in Environmental Engineering including the AguaClara program. In addition, the ability to automatically vary a parameter over a range of values has significantly increased our ability to characterize performance in research on unit processes for water treatment. As an example of this capability we present data from an experiment that was conducted to determine the effect of an aluminum hydroxide coating on porous media to enhance the removal of kaolin clay. The aluminum concentration in the feed during the pretreatment step was 2.5×10^-4^ mole/liter. The amount of aluminum applied to the filter was varied using the power law increment function to adjust the duration of the pretreatment state. Each of the pretreatment conditions was replicated and the results from the two tests were almost identical. The #data surface is from the particle challenge state from the 6 treatment levels and a replicate for a total of 12 tests.
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