Control System - Hardware
The aim of the control system is to monitor the flow temperature at the diffuser head and to switch the relays - and thus the heaters - on and off in such a manner that a near constant flow temperature is maintained at the diffuser head. This is a form of simplistic feedback control.
The system works by taking temperature readings at the inlet to the diffuser head using a LM35 temperature sensor (the controlling temperature sensor, or T10) which sends a signal to a USB6008. The function of the National Instruments USB6008 is to relay signals between the hardware (the sensors and relays) and the software (LabVIEW). In LabVIEW the voltage signal from T10 is scaled to convert it to temperature and then compared against the target value. If the controlling program decides that T10 is in a state where the air requires further heating then a signal is sent automatically, using LabVIEW, from the USB6008 to the relays.
These signals open the relays, allowing the mains supply to flow through the transformer inducing a current in the heaters - causing them to be activated - until T10 registers that the measured temperature is higher than the reference value. The exact method employed in LabVIEW will be further discussed in the software subpage linked here.
In addition to this temperature control feedback system the air velocity required control. This was done through a frequency controller which ‘pulsed’ the DC current: by reducing the frequency of these pulses the fan speed was reduced. The flow velocity was to be measured through the use of a differential pressure sensor and orifice plate set-up however this proved impossible during the calibration stage and so has not been included as part of the system here. Further details of why this was the case and the solution reached can be found in the fan speed sub page linked here.
Overview
A circuit diagram of the finished system is shown here. It should be noted that only one of nine op-amps have been included for the sake of clarity. All of these are arranged in the same manner as that which is depicted in the diagram. Furthermore, the dashed lines indicate the existence of the plastic housings which keep the electronics (in particular the transformer) safely sealed away during operation and have been shown in the physical system linked here. The use of two housings also separates the system into the high voltage (HV Housing - mains/24V) heater ring and the low voltage (LV Housing - 12V/5V) sensor and fan ring. All wires ending in ‘’To 6008’’ or ‘’From 6008’’ are destined to/from the USB6008. The pin out wiring for the 6008 can be downloaded from the link provided here. Lastly, as is common practice, the black dot indicates the position of pin 1. For individual component data sheets, including complete pin out information, see the component data sheets provided.
Completed System
1. Temperature Sensors - LM35
These are the temperature sensors selected for the system. They are rated as accurate to +/- 0.2˚C. There are 10 in the system in total: one of which is used to measure the temperature of the flow at the diffuser (T10) and 9 to measure the temperature in the flow field. They output a voltage signal which can be converted to a temperature in LabVIEW. Data sheet found here.
2. Humidity Sensor
There is only one of these in the system and is purely for recording the humidity in the flow field. Much like the LM35s they output a voltage signal which can be converted to a temperature in LabVIEW. Data sheet found here.
3. Darlington Array
This is an array of transistors that uses the control signal from USB 6008 to switch the 12V supply. The 12V supply is of high enough current to switch the relays and allow the heaters to turn on and off as required. Data sheet found here.
4. Pull up Resistors
These resistors were required to help the system settle on certain logic values. It should be noted that the USB 6008 has inbuilt pull up resistors however, these proved to be insufficient.
5. Op-Amps
The impedance difference between the LM35s and the USB 6008 mean that they cannot communicate effectively and require the operational amplifiers to act as impedance matchers. As such they are arranged in a non-inverting unity gain setup. It should be noted that ten were required as the system contained ten LM35s. Data sheet found here.
6. Power Brick
The operational amplifiers require positive and negative 5V supplies to be powered. This component takes the 12V supply and converts it to the required levels. Data sheet found here.
7. Frequency Controller
This component provides fan speed control by pulsing the DC 12V supply. Lower frequency pulsing decreases the fan speed. Physically the speed is controlled by turning a dial.
8. Solid State Relays
The relays are controlled by the control signal sent from the 6008 via the Darlington Array. When this signal is high the current going to the heaters from the transformer is allowed to flow. Data sheet found here.
9. Transformer
The transformer steps down the mains AC voltage to the more manageable 24V AC required by the heaters. Data sheet found here.
10. USB 6008
The function of the National Instruments USB6008 is to relay signals between the hardware (the sensors and relays) and the software (LabVIEW). It takes in voltages from the LM35s and the temperature sensor and out puts a 5V DC digital signal based on the value of T10 to switch the relays.