Project/Product Requirements
The purpose of this project was to design and implement a reliable and cost effective wireless temperature sensing system for the recreational spa industry. This wireless product has potential applications in many other areas including, salmonid enhancement hatcheries, private/public tubs, and indoor/outdoor pools. The product should be easily adaptable to include multiple sensors for future implementation; and if time permits, a datalogging capability is an attractive feature that should be possible from an output pin on the receiver to a PC data acquisition card. Below is a list of specifications that were followed during the design of the product.
·Minimum 1°C accuracy
·Minimum temperature range of 5 °C to 65°C.
·Active display in Celsius and Degrees
·Minimum refresh rate: 2 samples per minute
·Reliable and insensitive to noise.
·Long battery lifetime under continuous use (Greater than 4 months)
·Transmitter (Spa) unit runs on batteries
·Receiver (Home) unit runs on wall AC-DC adapter.
Product Functionality
The product consists of a transmitter (Spa) unit and a receiver (Home) unit. Both units employ a PIC16F684 microcontroller.
The transmitter unit is responsible for acquiring sensor values, conditioning the data using a cyclic redundancy encoding technique, and transmitting the encoded packet using FSK at 433.92 MHz. The transmitter is also responsible to power-down the microcontroller and transmitter circuit to reduce the duty cycle of operation to less than 1%. This power-down feature greatly reduces the energy consumption and results in an exponentially extended battery lifetime. Please refer to Figure 1 for a transmitter software state diagram.
The receiver unit is responsible for decoding the incoming data while employing noise immunity algorithms including signal threshold and cyclic redundancy checks. The receiver employs a custom serial communication protocol that is defined within the transmitter. If the data is valid or can be corrected, the receiver displays the value in Celsius and Fahrenheit on a Hitachi 2 line LCD. Please refer to Figure 2 for a receiver software state diagram.
Figures 3-6 illustrate the circuit schematics and PCB prototype layouts for both the transmitter and receiver modules. For a further explanation of the product, please see the Project Report .
Figure 1: Transmitter Software State Machine.
Figure 2: Receiver Software State Machine.
Figure 3: Spa (Transmitter) Module Schematic.
Figure 4: Spa Module PCB Layout, Prototype 1.
Figure 5: Home (Receiver) Module Schematic.
Figure 6: Home Module PCB Layout, Prototype 1.
Current Research
Present research at Aquatic Systems includes investigating more energy and cost efficient methods to perform the wireless communication.
Reliability enhancements are also being tested in the recent implementation of a new wireless protocol in the LC300 model. Knowledge acquired from courses such as Elec 466 and Elec 405 were implemented in the power consumption enhancements within the transmitter and the error control (noise immunity) within the wireless communication. The realistic power consumption of the product is being investigated. Only theoretical and worst case battery lifetime approximations currently exist.
More intensive part searches are being performed to find more cost effective components to integrate into the next prototype. The microcontroller memory on the receiver side (currently at 4 % available memory) is under revision to fit more sensor data functionality. It may be necessary to implement a PIC microcontroller with more variable and instruction memory. This memory increase upgrade would decrease the time-to-market and development time. The transmitter's microcontroller does not require memory management as the original implementation took less than 30 % of the available memory.
Hardware layout and component specifications are being reviewed. An antenna ground plane is also being investigated in an attempt to increase the range and reliability of the product. The ground plane would give a more proper ground reference for the dipole antenna being used. A smaller antenna with a larger radiation pattern (specifically in elevation) is also being researched to further increase the wireless reliability when the transmitter and receiver are at different levels above the earth's surface.
Future implementation of chemical acquisition systems and eventual active/programmable temperature and chemical control will be discussed within the description of future ASI products.
Elec 499 Project Group 12 - Wireless Spa Temperature Sensing System
