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Abstract 3892-Ciavattone De | Microcontroller | Input/Output

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  AControl SystemPlant Simulator utilizing the mbedNXP LPC1768 MicrocontrollerFor: NXP mbed Design ChallengeSponsors: NXP/mbed/Elektor/Circuit CellarDate: February 28, 2011By: NXP-3892    Motivation There are many tools available to the engineer to assist them with the analysis,design and tuning of control systems. Many of these tools are specially designedsoftware packages to model the controller and plant. In some cases, they createvirtual solutions without coming in contact with actual hardware. These methodsuse assumptions and software to create nice solutions at your desk. They mostlikely will need to be tweaked or perhaps redone when they are combined withthe actual system components such as a controller, sensors and actuators.One difficulty with this approach is the lack of understanding of the truecharacteristics of the plant. Along with this, the software solution doesn’t answerthe question of how the controller components will interface to the plant. Anotherconcern is that sometimes the characteristics of the sensors will play a role in theoverall solution. Three-mode or PID controllers are very common in industrytoday because of their low cost and ease of applicability. They seem to be thefirst choice of application toward a solution and work well when the system israther simple. What happens when parameters start to vary with time or changerelative to other parameters such as temperature or pressure? Do we retune? Dowe use gain scheduling, or perhaps do we use a system that is adaptive? Do weuse a more complicated control strategy? If we had a better understanding of theplant dynamics or process characteristics we could explore other possibilities inthe lab instead of at the customers expense.Furthermore, at some point during the development phase of a control system,the control strategy will need to be ported into the controller and tested againstthe actual plant. If the actual plant is not available because it has not yet beenbuilt, could we just use software to simulate the plant? The answer is NO. Wecan simulate the overall control/plant system using software tools but the controlstrategy warrants testing and debug when the final program is loaded into thetarget hardware. The project may not have enough time allocated to accomplishthis. If there were a piece of hardware available that interconnects and interactswith the controller that can simulate the plant/process operation, this would allowyou to test the overall system in a shorter time and place you much closer to thefinish line with a higher level of confidence.The mbed rapid prototyping tool for micro-controllers is an excellent way ofcreating an actual platform to simulate a process or plant that is to be controlled.In this approach, the engineer will use data acquisition to gather informationabout the plant or process and use it to derive an approximate model that can beused to develop a controller and/or control strategy. Once a physical plant modelhas been attained, the engineer can use it to asses different types of controlsthat are necessary to achieve their objectives. The control system can be testedin the laboratory under changing parameters (temperature, pressure, flow, etc.)and not during a time the customer is trying to maintain production.  This idea of creating an actual piece of hardware that simulates the plant canallow you to test the controller and achieve with absolute certainty that yourcontrol strategies will work and the system will interface to the plant without anysurprises. Hardware The NXP mbed design challenge hosted by Circuit Cellar provided an opportunityto take a look at a really great prototyping platform, the mbed NXP1768. Thissmall micro-controller board is based on the ARM Cortex-M3 operating at aspeed of 96MHz with 512K bytes of Flash memory and 64K of RAM. The mbeddesign places the ARM processor on a 40-pin DIP package with 0.1” spacing anda 0.9” width. The mbed board is designed to plug directly into a solderlessbreadboard. I chose a Futuelec protoboard to house the circuitry for this project.The mbed board hosts a load of interfaces including: (2) SPI ports, (3) serialports, (2) I2C ports, (6) Analog inputs, (1) analog output, (6) PWM outputs, (1)CAN, (1) Ethernet and (1) USB interfaces. (26) of the pins may be used as bi-directional digital I/O. The board can be powered through the USB connection orby an external voltage source in the range of 4.5 to 9V. The mbed micro-controller board is the heart of this project but provides quite a punch at a lowcost for any project. The pin-out shown below shows the capabilities of the micro-controller board. It can be found on the website:www.mbed.org. A visit to thiswebsite will immediately get you hooked on this great product.  Below is a block diagram of the circuit build to host this application. This blockdiagram along with a schematic for the system and bill of material may be foundin the appendix of this paper. The example that I used to explain this idea usesonly two analog inputs and a single analog output. I have added the MicrochipMCP4822 to provide an additional two analog output using the SPI bus.Below is a brief description for each element of the system.ANALOG INPUTSI wired all (6) of the analog inputs to the mbed micro-controller board to aconnector in the event that a more complex process involving multiple inputs is tobe simulated. In this example only two inputs were considered. AI0 is used toaccept the command signal from the controller. The second analog input can beused to simulate the effects on the plant for changes in temperature. Providingan analog signal on this input can simulate this condition..ANALOG OUTPUTThe Plant Simulator uses two analog outputs. Analog Output channel 1 is thesimulated transducer output with a range of 0-6V. The pressure transducer beingsimulated has a range of 1-6VDC. Analog Output channel 2 is used to monitorthe systems dynamic response with a scope. It is adjusted to have a 0 -10Vrange. As a future note, this second output can also be used to simulate a plantwith two outputs if need be.
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