Instruction Manual

Platform Wind speed Monitor

Manual written by

Jon Hagen

8-10-99

Arecibo Observatory

National Astronomy and Ionosphere Center (NAIC)

Electronics Department

Arecibo, Puerto Rico, 00612

Table of Contents

1. Introduction 3

2. Description 3

3. Operating Instructions 4

4. Electrical Connections 4

5. Description of circuitry 4

Appendix 1. Schematic diagrams 5

Appendix 2. Wind sensor data sheets 8

file: c:\hagen\wpfiles\windmon.wp6 jbh 8-9-99

Platform Wind speed Monitor

1. Introduction

This system consists of a commercial anemometer head (R. M. Young Co., Model 05103) which mounts on the platform together with a transmitter unit which sends a signal to a downstairs receiver unit. A front panel meter on the receiver unit displays wind speed, 0-70 mph, and a second meter to displays maximum Wind speed, 0-140 mph. A push button resets the maximum wind speed display to zero. Front and rear panel BNC monitor ports provide an analog voltage proportional to Wind speed (14V = 140 mph). A circle of 8 LEDs displays wind direction. Front and rear panel BNC monitor ports provide an analog voltage proportional to wind direction (3.6V = 360 degrees).

2. Description

The transmitter connects to the receiver via a single optical fiber or, optionally, by a single coaxial cable. When the wind sensor's propellor turns, the transmitter unit produces a train of pulses. The pulse frequency is proportional to wind speed: 640 Hz = 140 mph. The pulse width is a linear function of the azimuth: width in milliseconds = 0.1 + (az in degrees)/360. A sealed lead- acid battery in the transmitter unit provides back-up power for up to 20 hours in the event of prime power failure.

The receiver unit consists of a tachometer circuit to measure wind speed and a gated counter circuit to measure pulse width (azimuth). No back up battery is included in the receiver; it can be run from a standard UPS.

3. Operating Instructions

When using a coaxial cable, rather than an optical fiber, the shield of the coax should be grounded only at the transmitter end (platform). The BNC input jack on the receiver (which feeds an opto-isolator) is isolated from the chassis. The receiver can be tested alone for proper operation by providing a TTL input pulse train with accurate control of frequency and pulse width: A frequency of 320 Hz produces a full-scale reading (70 mph) on the wind speed meter, a half-scale reading on the maximum wind speed meter, and 7.0 volts at the analog speed monitor port. A frequency of 640 Hz pins the windspeed meter, drives the maximum windspeed meter to full scale (140 mph), and produces 14.0 volts at the analog speed monitor port. A pulse width of of 0.1 msec gives a direction indication of North and an direction monitor output of 0 volts. A pulse width of .51 msec gives a direction indication of South (180 degrees) and a monitor voltage of 1.80 volts.

4. Electrical connections to the transmitter box:

1. 120 VAC power cord

2. Anemometer sensor unit

3. One optical fiber optic to the control room (status information)

or

4. One coax cable to the control room

5. Description of Circuitry See the attached 3-page schematic.

The anemometer is an ac sinewave generator; both frequency and amplitude are proportional to Wind speed. In the transmitter unit this signal is filtered and clipped and then drives a one-shot to produce a 50 microsecond pulse. This pulse drives the "trigger" input of a 555 timer chip. The pulse produced by the 555 stays high until the voltage at its "threshold" pin goes high. Here, the threshold pin voltage is derived from a linear ramp with a constant slope of 4.09 volts/msec. The ramp begins to integrate up from zero volts at the start of each pulse. When the ramp voltage equals the voltage from the wind direction voltage divider pot, a comparator drives the threshold pin on the 555 high and pulse ends. The integrator is held at zero between pulses.

The tachometer card in the receiver uses the leading edge of the input pulses to trigger a one- shot. The pulse width from this one-shot is constant at 781 microseconds. The output of the one- shot is converted to a precise levels of 0 to -5 volts. The average voltage, which is proportional to wind speed, is produced by an op amp integrator with an RC time constant of 0.28 seconds.

This level drives the meter and the monitor ports. The max hold circuit uses a DAC to produce an analog voltage. A digital counter drives the DAC. Normally the clock to this counter is inhibited. Whenever the wind exceeds its previous maximum, the output from the DAC will be less than the wind speed voltage. A comparator detects this condition and enables the clock for the digital counter. The counter increments up until the DAC voltage is equal to the wind speed voltage. At this point the comparator inhibits the clock. The manual reset button simply zeros the counter.

The azimuth card in the receiver contains a free-running 32 kHz clock and a counter. While the input pulse is high, the counter is clocked. When the input pulse goes low, the contents of the counter are transferred to a latch and the counter is reset. The reset is actually a preset to 29. This is done so that the 0.1ms bias on the pulse width will zero the counter at the beginning of the each pulse. The output of the latch is connected to a 3-to-8 line decoder counter which drives the LED direction indicators. The latch output also drives a D-to-A converter to provide an analog monitor of the wind direction (3.6 volts = 360 degrees).