victorbouvet / weather_system

Weather system prototype

8 hours of practical work (VHDL code, analysis of simulations, wiring and testing of the prototype) + writing of a summary report

The prototype is an electronic system allowing the display of data from a wind vane or anemometer.

We mainly worked on the processing subsystem part (development in VHDL of the entities integrated in the CPLD, analysis of the results, wiring and laboratory testing of the prototype).

Material needed

• a XC2C64A CPLD
• a mechanical encoder in GRAY code 25LB22-G from GrayHill,
• a cup anemometer
• two micro-switches
• two LED bars (bar-graph) type DC-10YWA from Kingbright,
• resistors, capacitors and other laboratory components.

Schematic of the prototype

The CPLD XC2C64A is only available in a VQFP SMD package, so we will use a C-MOD_C2 board allowing wiring on a board. On this C-MOD_C2 board we find the CPLD, a regulator allowing to manage the component power supplies and a JTAG connector allowing its programming.

Resistors are added to the mechanical encoder, to the anemometer and to the micro-switches, in order to ensure, on their outputs, logic levels allowing to correctly drive the CPLD. A filtering capacitor of the parasites is added to the anemometer.

The schematic of the INVERTER oscillator is a classical schematic using 3 INVERTERS 74HC04, two resistors, a potentiometer and a capacitor. The frequency of the oscillator will be set at about 15 kHz.

The 100 nF capacitor placed on the INVERTERS power supplies is a decoupling capacitor. The whole set is supplied in 3.3 V.

Project specifications

The project aims at realizing the prototype of an electronic weather system processing and displaying information from a wind vane and an anemometer. The selection between the display of the data from the wind vane or the anemometer will be done with a first switch.

For the wind vane, only 16 wind directions will be identified (N, N-NE, NE, E-NE, E,...). Each of these positions, captured with a mechanical encoder in GRAY code, will be visually indicated to the user with an illuminated LED. It will also be possible to visualize with the help of LEDs, either the Gray code, or the equivalent code in natural binary. This selection will be made with a second switch.

For the anemometer, the LEDs will be used to indicate the intensity of the wind, ie its speed. The higher the wind speed, the more LEDs will be illuminated.

The meteorological system to be realized is made up of 3 subsystems :

• a subsystem ensuring the conditioning of signals from the analog input elements,
• a subsystem ensuring the processing of information,
• a subsystem allowing the visualization of output information.

Input subsystem

The rotation of the vane's arrow causes the rotation of the shaft of the mechanical encoder delivering a GRAY code on 4 bits. Thus, each position of the wind vane corresponds to a code among 16.

The analysis of the wind speed, measured by the cup anemometer, will be managed through sequential logic elements. These elements will be clocked by a square clock signal, with a duty cycle of 50% and a frequency between 10kHz and 20kHz. This signal will be obtained using an inverter oscillator.

A first micro-switch will allow to choose between the GIROUETTE mode and the ANEMOMETER mode. A second micro-switch will be operational only in GIROUETTE mode and will allow to choose between the display of the GRAY code and the display of the NATURAL BINARY code.

Processing subsystem

The processing subsystem will be named weather_system and will be integrated in the XILINX CPLD XC2C64A. The component has 44 inputs/outputs and its number of macrocells is 64.

The GRAY code obtained at the output of the mechanical encoder will be transcoded into NATURAL BINARY and each of the 16 possible combinations will be decoded in order to indicate the wind direction from the illumination of one LED among 16. The GRAY code or the NATURAL BINARY code will be displayed on 4 other LEDs. The signal from the cup anemometer will be processed to display the wind intensity on 20 LEDs.

The input signals are :

• weathercock: 4-bit signal from the GRAY mechanical encoder that corresponds to the position of the wind vane. The wind vane has only 16 possible positions.
• code_select : signal coming from the second switch allowing to display on the LEDs the GRAY code or equivalent natural binary (0 = GRAY, 1= natural binary).
• mode_select : signal coming from the first switch allowing to select the type of data to be displayed (0 = wind vane, 1= anemometer).
• anemometer : signal coming from an all or nothing sensor. Depending on the position of the cups, the wires will be connected (1) or not (0). The faster the anemometer rotates, the faster the sensor will change state over the same time period.
• clock : square periodic signal with a frequency between 10 KHz and 20 KHz.

The output signals are :

• Anemometer mode:

  • The code_wind_speed and wind_dir_speed outputs form a single bus named wind_speed defined on 20 bits that displays the wind speed. The more LEDs are lit, the higher the wind speed.

• Wind vane mode:

  • code_wind_speed: 4-bit signal that displays the GRAY or natural binary code equivalent of the mechanical encoder on the LEDs.
  • wind_dir_speed : 16 bits signal which displays the wind vane position (1 position per bit).

The 3 blocks of weather_system are :

• Decoder : Module that decodes the weathercock input and returns the wind vane position on wind_dir and the Gray or equivalent binary code on code_display output.

  • gray_bin_4bit: Transcoder which outputs the binary_code signal encoded in natural binary from the weathercock input signal from the encoder.
  • mux_2x1x4bit: Multiplexer that transmits on the not_code_display output defined on 4 bits either the weathercock signal in GRAY or the binary_code signal in natural binary depending on the code_select value (0 = GRAY, 1 = natural binary).
  • gray_decoder_4bit: Decoder that sets one of the 16 bits of the wind_dir signal from the received weathercock signal as output. The decoder outputs are set to LOW. North is associated with weathercock = "0000" and wind_dir(0).
  • inv_4bit: 4 inverters that invert the not_code_display signal and output the code_display signal.

• Speed_counter : Module that analyses the number of all or nothing signals received during a clock stroke and deduces wind_speed.

  • risingedge_detector: Module that compares the clock and anemometer signals and provides the counter_ena signal as output. When there is an edge on the anemometer signal (rising or falling), counter_ena goes to 1 which will create a pulse edge to the rhythm of the anemometer's rotation.
  • freq_divider: Frequency divider of the clock signal to provide an output counter_rst signal. This signal periodically emits pulses to indicate a reset of the counter_5bit.
  • counter_5bit: Synchronous binary counter that sums the number of counter_ena pulses between two counter_rst pulses (reset). The faster the anemometer rotates, the greater the number of pulses between two counter_rst pulses. The sum of these pulses will form the speed_code signal defined on 5 bits.
  • transcoder_speed: Transcoder which provides a wind_speed signal defined on 20 bits according to the speed_code value. The wind_speed output is active in the low state.

    0-9 : 10 LEDs so 2x 0-9 (see above) correspond to the 20 LEDs to display the wind force

• mux_2x1x20bit : Module that receives the outputs from Decoder and Speed_counter and will, depending on the value of mode_select, either display the gray/natural binary code and the position of the wind vane or the wind speed on the LEDs of the bar graph.

Visualization subsystem

The visualization of the signals is ensured by two LED displays or bar-graph (2*10 LEDs). An LED will be illuminated when the signal to be visualized is in LOW state.

Appendix

GRAY mechanical encoder

This mechanical encoder, 25LB22-G from Grayhill, is made of a disc rotating on 360° with a copper comb (see on the right of the picture). The teeth of this comb are put in contact with 5 electrical tracks of a printed circuit board (PCB). The tracks of this PCB have been designed to correspond to 5 concentric circles, 4 of which are discontinuous (see left of the picture). By a spring effect, each tooth of the comb is associated with one of the discs. The continuous circle in the center of the PCB is connected to a positive potential. In case of contact, a tooth of the comb will connect the central track to the corresponding track constituting a kind of switch. Thus, according to the design of the tracks, the voltage levels at the output of the mechanical encoder will evolve according to a Gray code type coding on 4 bits (16 possible positions). The position of the encoder is fixed from a ball and a spring.

Anemometer

The anemometer is a rotating device with cups. It consists of an "all or nothing" sensor connected to two wires. Depending on the position of the cups, the two wires will be connected together or not.