The Implementation
During the implementation of our project design we developed better designs and workarounds for unforseen challenges. This page explains the processes and decisions that lead to the appearance and functionality of the completed mechatronics system. We have also included comments about budgeting and material procurement.
Actuation
The implementation of the actuation went according to design. The two major actuated subsystems are the furnace and the damper assemblies:
Dampers
The mechanical aspects of damper actuation involved use of four 3" galvanized steel tee ducts and six gearbox/12V DC brush motors assemblies mounted on the ducts. This duct and damper assembly used approximately 70% of our budget. All of the mountings and the dampers themselves were custom designed and manufactured. Six digital outputs from the PC104 stack were used to control the damper system. Three L293B ICs were used to drive 6 different h-bridge circuits for the 6 different motors in the system. The ProjectBoard 5V supply was used to power both the motors and the ICs. Control inputs were, by default tied high with a 1K resistor so that the digital outputs could pull them low. For one side of the h-bridge control, each motor had its own individual logic high or low control, and all shared control over the other side of the h-bridge. This configuration was made such that if the common side of the h-bridge was high, and the other side was floating or low, external dampers would close and internal dampers would open. Bi-directional control is further described in the software section, however the overall mechanism was arranged such that only individual motors could be moved at one time (in order for consistent current usage measurement), and single motors were moved by switching all h-bridge inputs, except for a target motor's, either high or low to drive that motor in a particular direction. Individual motors possessed their own flyback diodes, which is why they are not present on the protoboard image.
View of Basement and Damper Actuators
Dampers at End of Ductwork
Closeup of Damper Actuator
Damper Actuator Circuitry
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Furnace
The furnace was mechanically two steel boxes secured side-by-side with the peltier mounted between them and its heatsinks protruding into the boxes. At one end of each of the boxes we mounted a 70mm computer case fan to draw heat off the heatsinks and to force air through the ductwork and model. One of the boxes was connected to the ductwork system and the other simply blew the air out into the environment (called the exhaust). By placing the PC/104 stack near this exhaust output we were able to use it to provide some cooling for the computer. Electrical control and drive of the h-bridge was through a higher-wattage h-bridge (L298) with both h-bridges for the chip wired and commonly controlled to increase maximum amperage throughput. Two digital outs were utilized to control the direction of current flow through the peltier. Power was supplied for this portion of the device (and for the heat sink & fan attached to the L298) by the lab-supplied HY3003D-3 power supply set to parallel mode with voltage fixed at 18V (about 2.5 amperes drawn in steady state, 3.5 amps in transitional state when switching between heating and cooling). The internal and external 12V brushless fans for the furnace were driven by the 12V supply (ProjectBoard), and defaulted to an on state so long as power was on. A MOSFET was used to enable powering down of these fans.
The Furnace
Closeup of the Furnace
Furnace Circuitry
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Sensing
Electrical design of the project is divided between sensing and actuation. The former of these two categories represents the simpler aspect of the system. Inputs to the PC104 stack are all all analogue and are utilized for sensing of the temperatures in the two separate rooms, and determination of when a motor has hit the end of its range of motion. Thermistors depicted in figure below, where as depicted in the image below where the sensed voltage changed as a function of ambient temperature. This allowed for determination of current temperature in real-time after calibrations had been performed to convert voltage into temperature.
Thermistor Circuit Diagram
Thermistor Circuit Closeup
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The remaining sensor was used to determine damper motor current draw so as to determine when a motor had reached the end of its range of motion. To do so, a 10? resistor was placed upstream of the parallel grouping of h-bridges used to control the damper motors, and the potential drop across the resistor was measured to determine whether the motor currently being actuated was consuming a greater amount of current due to reaching an end position.
Status Display
Status indication was done using a pair of LEDs, which were driven using our 12V voltage source (ProjectBoard), and analog outs to control MOSFETs which gated current from the voltage source to the LEDs.
Satus LED Closeup
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Software
The major portion of the control software for the system is in the form of a Simulink model, which is compiled to run on the PC104 stack.
Inputs and outputs utilized are as follows:
- 3 Sensoray analog inputs for potentials pertaining to temperatures and motor circuit power consumption
- 8 Sensoray digital outputs for damper and furnace control
- 2 Sensoray analog outs for MOSFET control of status LEDs
- Goal temperature selection
- Damper state change by indicating a damper to change and the new state
Integer delays and linear fits within embedded MATLAB functions are used to convert potentials into temperatures for thermistor inputs. These temperatures are then fed to roomState and MainControl functions. roomState activates the room LEDs when an individual room has gotten within 1.5 degrees of the target temperature. Temperature data sent to MainControl are used to decide what state to put the furnace in (through FurnaceControl). If temperature is more than a degree above the target temperature, the system is put into cooling mode. If more than a degree below, heating mode is activated, and if the system is within one degree of the target temperature, the peltier is not active.
Simulink Diagram
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In addition to main furnace control, is functionality for controlling damper motors. Every 5 seconds MainControl checks if a new motor or state have been set, if this is the case, this information is passed along to MotorControl which receives this new information, and then over the next five seconds moves the motor to the new state. Only one motor can move at a time, and changes between 5 seconds must wait until that time is up to be considered. MotorControl keeps track of which motor it has been working on, and how far into the 5 second cycle it is. During this time, if current draw exceeds a pre-defined value, motor activation is shut off and left in the previous state. Further attempts to move the motor within the 5 seconds are made in case the motor has just become stuck, but the threshold current level is never exceeded for more than one cycle. To move the motor in a desired direction, digial outputs are set so that the current motor being operated is set to be in a different state (high or low, depending on direction) than the other 6 digital outputs (high or low) to only move one motor at a time, and to prevent other motors from moving spuriously.
About Project
The Home HVAC model was designed and built as an experimental test-bed for developing a smart HVAC control system. The model itself uses the design principle of independance of functional requirements to give as much flexability to the model as possibile; this allows for many interesting scenarios and experiements to be run in a controled system before implementing them in an actual home system. The purpose for the control system is to limit the temperature differential within a room and between rooms to make for a consistant and comfortable living enviroment. This is accomplished through the use of mechanical systems, electrical systems, and software. The idea for the project came from a personal dissatisfaction team members have with drafty rooms, hot and cold spots, and unresponsive thermostats in our own homes.
Team Info
- Ned Cameron
- Ned is a senior in the Mechanical Engineering department. He has concentrated in MEMS, Nanotechnology, and Mechatronics while fulfilling the requirements for his degree. After graduation, he will work as Application Service Engineer for Informance International in Northbrook, IL.
- Ted Reynolds
- Ted is a senior in the Mechanical Engineering department. He is a co-op engineer with ITW in Glenview where he plays on the volleyball team. He enjoys sleeping little and cooking lots. Considering he still has another year left, Ted has not given much thought to his future, but wherever he may be, he plans to have the best HVAC system on the block.
- James Snyder
- James is a first year graduate student in Biomedical Engineering. He has recently completed an undergraduate degree in biology at Northwestern University (concentration in Neurobiology), and currently works with the MacIver Lab studying sensorimotor integration in weakly-electric fish.