Digital data acquisition system for measuring wind effects on tall buildings

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Digital data acquisition system for measuring wind effects on tall

buildings

DIVISION OF BUILDING RESEARCH

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No . .

533

PREPARED BY W. A. Dalgliesh CHECKED BY W.R.S. APPROVED BY N. B. H.

ｾ March 1969

PREPARED FOR Inquiry and record purposes

SUBJECT DIGITAL DATA ACQUISITION SYSTEM FOR MEASURING

WIND EFFECTS ON TALL BUILDINGS

The .Building Structures Section of the Division of Building Research has been rl1easuring wind pressures and suctions on a 34- and a 45 - stor ey offic e building in downtown Montr eal, Canada. The main objective of the measurements is to acquire field data to be correlated with wind tunnel tests on small-scale models, in which the major flow characteristics of the actual wind are also simulated.

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replaced by a digital data acquisition system recording on magnetic tape in a form compatible with digital computers. This note is intended to describe the reason for replacing the former recorder and the main features of the new digital system.

REASONS FOR REP LAC EM ENT OF OSCILLOGRAPH RECORDING SYSTEM

In the early stages of the project oscillograph records on direct print photographic paper were very useful because the traces were available for immediate checking and adjustment of each channel of information.

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-Afte-r some experience had been gained in the acquisition and analysis of records, the necessity for immediate visual records in analogue forrn diminished and the disadvantages inherent in the con-version of analogue records to digital form for computer analysis became more apparent. In addition to the slowness and the tedious nature of even a partly automated procedure, there were the problems of loss in accuracy and possible mistakes caused by difficulty in sorting out overlapping oscillograph traces.

MAIN FEATURES OF DIGITAL SYSTEM

1.

As in the oscillograph recording system, the input on each channel is a varying voltage proportional to the difference in air pressure through the wall at a particular orifice, or to the windspeed, dir ection or any other parameter that is to be measured. Twelve of the 16 channels now in operation accept low-level voltages, ranging from ±_{5 millivolts to} ± ₅₀

millivolts full scale. The other 4 channels accept high -level voltages of

± _{10 volts full scale. The difference is in the provision of two stages of}

pr eamplification for low -level signals, providing (a) a gain of from 100 to 1000, and (b) a gain of 2 combined with active filtering which effectively removes any high-frequency components in the signal (which would normally be caused by unwanted llnoise").

2. Sequential Sampling of Channels

In the oscillograph recording system. each channel was recorded in analogue form as an individual, continuous trace on the paper. The new digital system, however, sarnples each channel in sequence, converts the analogue l 'eading to a digital value, and l 'ecords in digital form. on magnetic

tape. Strictly speaking, the channels are no longer sampled either simul-taneously or continuously, but the time delay between the sampling of the first channel and the sampling of the last channel is very short (less than 1/10 second). The sampling rate per channel can be selected by a dial on the front panel to be one of 1, 2, 4, or 8 per second.

There are other possibilities for faster recording rates, but with certain limitations on the number of channels and the recording form at on tap e.

'3. Automatic Recording of Data Files

Whereas the oscillograph recording system could not be operated unattended, the digital system is continuously sampling all the data

During this time no data are recorded unless recording is initiated by the signal on a particular monitored channel exceeding a preset limit value. When the monitor ed signal (usually wind speed above the building) exceeds the chos en level (say, 50 mph) the sampled

readings are recorded on tape for a preselected period of time to produce a data file. Recording then stops until the next time the signal on the monitor ed channel exceeds the limit value. A data file usually consists of nine blocks of information on tape, each separated by a "record gap" (3/4 in. of blank tape) and corresponds to 15 minutes or half an hour of real time operation (depending on whether the sampling rate is 4 per second or 2 per second). One 2400 -ft r eel of tape has a capacity of about 30 hr at the 4 per second rate, or 60 hr at the 2 per second rate ..

4. Physical Characteristics of the System

A reasonable degree of 'Iportability'l was required for field installations. The digital system is at present mounted in one large cabinet (about 24 in. square by 48 in. ) and one smaller cabinet (20 by

24 in. by 24 ｩｮｾｨｩｧｨＩ resting on top of the first which is mounted on castor s (Figure 1). Over -all weight is about 500 lb including mountings.

The total power requirement is about 500 watts, 115 volt -60 cycle a. c. This includes signal conditioning equipment for providing power, resistive balance, and calibration for each of the pressure transducers.

The whole system is enclosed in an outer cabinet of sheet aluminum within which some cooling is provided by a fan drawing filtered air in at the bottom. As protection against overheating due to malfunctioning, two thermo-stats are provided inside the top of the cabinet, one of which is set to cut

all power if the temperature in the inner top cabinet should rise above 110°F, and the second to cut off all power and also to ring an alarm bell, if the first thermostat should fail to ｦｵｮ｣ｴｩｯｮｾ at 160°F.

FUTURE EXPANSION OF SYSTEM

The major limitation of the system at present is in the tape recorder which is operating at the upper limit of its capacity in sampling, for example, 18 channels at 8 samples per second each, or 3 channels at 16 samples per second each. It is possible to sample more channels at slower rates; for example, 50 channels can be sampled at 4 samples per second and the confi-guration of the system is such that up to 128 channels could be sampled 'without major redesign. In _{fact, 4 more low-level channels are soon to}

be added to the system, and only a limited amount of rewiring is said to be required.

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-The sampling rate can go as high as 8134 samples per second, but with the present tape recorder these readings must be recorded in

a l·synchronous" ｭｯ､･ｾ such that no inter -record gaps are provided. The absence of record gaps presents problems for computer input. It is also possible to incorporate a digital computer into the system to provide program.ming control over the course of an experiment and also to do preliminary data processing.

APPROXIMATE COSTS OF SYSTEM

The main components' of the complete system are:

I. Statham pressure transducers (approximately $250 each) or other sensors for measuring physical parameters and converting to an electrical analogue signal.

2. Signal conditioning unit (approximately $300jchannel) for trans-ducers operating on the Wheatstone bridge principle. Type used: Band F Instruments, ModelI-700SG.

3. Redcor Model 611950 Digital Data Acquisition System composed of 3 units:

(a) Model 663015 -4211 Analogue to Digital Converter. (b) 103726 Control Unit Assembly.

(c) DS370RH5 Kennedy Tape Unit Assembly

(approximately $33,000 for system with 12 low-level, 4 high-level channels).

4. Additional low -level channels, approximately $500 per channel up to about 30 channels. Mor ethan 30 channels may involve the cost of an additional cabinet.

stalled in 45- store y office building in downtown Montreal, October 1968.