A data acquisition systemfor soil mechanicsby L R. DAVISON*, MErMI, CEng, MICE at J. M. HILLS, BSC

IN 1978 the data-logging equipment inthe soils laboratory at Bristol Polytechnicwas gathering dust, because no one couldget it to do what they wanted. The sim-ple reason was that it had not been de-signed for the sort of tasks required in asoils laboratory and was unsuitable forthem. After talking to people in other la-boratories and to several equipment sup-pliers, it became clear that this problemwas widespread and that the equipmentneeded was not available at a price thatcould be afforded.

The solution was to design and buildwhat was required within the Polytechnic,a course of action made possible by thenature of higher educational establish-ments which have the required range ofexpertise. With the collaboration of a col-league in the electronics section, an intel-ligent logging system has been produced,based on the Commodore PET desk-topcomputer. This system has been taken upand is exclusively marketed by EngineeringLaboratory Equipment Ltd.

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HardwareFrom the start, the idea was to produce

a system of sufficiently low cost to be ableto dedicate it to an individual test andyet to be sufficiently flexible to be ableto be used for several different types oftest. It needed intelligence to enable it tobe switched from taking readings on alogarithmic time scale during consolida-tion of a sample to taking readings atconstant increments of strain (or stress)during shearing of a sample. The resultsshould be in engineering units —withoutthe need to make fiddly adjustments of"span" and "zero" controls of a signalconditioning unit.

It was decided to use a microprocessor-based system, with some memory to storethe readings. Both a key pad to instructthe unit and a display device to show howthe test was progressing were also re-quired.

Initially the intention was to build thewhole thing from scratch, but it soonbecame clear that a small desk-top com-puter contained the necessary features atroughly the same cost as they could beput together. These computers costing lessthan E1,000 were new and somethingof an unknown quantity. Since the depart-ment already owned one Commodore PETand it was one of the cheapest available, itwas chosen as the basis for the system-the intention being to learn from it andto move on to a different machine whenits limitations were reached. Since theoriginal design the only change neededhas been to move to a larger PET (32Kfrom 8K); this has been achieved withoutspending a larger amount of money be-cause the price of desk-top computers hasfallen by so much since 1978.

The transducers used in the soils lab-oratory all require a 10V dc supply butthe output range varies greatly. Displace-ment transducers are generally of the vari-able transformer (LVDT) type and have

*Senior Lecturer, and g Research AssIstant, De-partment of Construction, Bristol Polytechnic

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Fig. 2. Performing a quick undrained triaxial compression test. Dataface System 3controls machine operation and collects and analyses the data commg from the trans-ducers. Also shown is a Tritest 50 load frame and 50mm triaxial cell with submersibleload cell, a strain transducer and a pressure transducer

January, 1983 15

an output in the range —5V to + 5V.Load cells and pressure transducers aregenerally of the strain gauge type andhave an output in the range 0 to 100mVor less. Some form of signal conditioningis necessary to deal with these extremes,but to provide it for individual channelswould add significantly to the cost andthese would require resetting each timea different transducer was connected. Thechosen solution was to make the signalconditioning programmable and then toposition it in the circuit at a point whereone signal condit'oning unit could be usedfor all channels. The general arrangementused to achieve this is shown schematic-ally in Fig. 1. In this way, all settings arecontrolled by the computer; instructionsin the computer program perform theappropriate adjustments at the time achannel is selected.

The errors due to non-linearity of thetransducers is typically 0.2% of the fullscale output. To convert the voltage fromthe transducers to a binary signal requiresan analogue-to-digital converter (ADC).Either an 8 bit or a 12 bit converter couldhave been used; a 12 bit converter waschosen as this gave a resolution of 1 in4096 (0.025%). In choosing the numberof channels for the system, it was decidednot to produce a system to run dozensof tests simultaneously, the flexibility ofindividual systems being preferred to onelarge system, It was convenient to choosethe number of channels as a power of twoand so the system was designed for 8channels, this being the maximum likely tobe required for a single test.

Having specified the system it was timeto think about safety. Two additional fea-tures were incorporated —a horn to at-tract the attention of the operator if needbe, and a programmable mains power sup-

ply to drive a loading frame. The program-mable power supply enables the loading tobe terminated automatically at the endof a test or at some earlier time if theload exceeds a safe value. Both the hornand the power supply were designed tooperate for four seconds and then switchoff. To maintain operation an instructionto continue must be received at frequentintervals; this reduces the risk of a pro-gramming error resulting in either of thembeing left on.

Special computer routinesThe Commodore PET uses a language

called BASIC which is very easy to learnbut which does not contain any direct in-structions to select a channel, set theconditioning unit and input the readingfrom the ADC. The necessary instructionshave now been created and added toBASIC so that programs to control thesystem are easy to write; and to correctif there is an error.

Hidden from the casual observer is themachine's own language, or code: BASICis really a computer program written inthis machine code and stored in 'ReadOnly Memory'hich is not lost when thepower is switched off; for this reason itis often called firmware. The user's pro-gram is stored in 'Random AccessMemory'. The contents of random accessmemory can be modified, hence the namesoftware, and data can be stored anddeleted when no longer required. A mac-hine code program has been written towork in conjunction with BASIC and per-form all the special tasks required to workthe system. These routines can be storedas part of the user's program on cas-sette tape or floppy disk and loaded intorandom access memory along with theuser's program. Alternatively, it is avail-

able on 2 'chips'hich fit in the socketsprovided inside the computer for this pur-pose. The use of the 'chips'ree some ofthe random access memory for data stor-age and further simplify the task of de-veloping software.

One of the things the machine codeprogram does is to input the results of24 analogue to digital conversions eachtime it is called upon to read a trans-ducer. These 24 readings are ranked andthe top and bottom 4 are discarded, theresult finally stored in memory being theaverage of the remaining 16. The resultsfrom this form of averaging are called12% trimmed means and the effect of theprocess is to reduce the fluctuation in thereadings due to electrical noise.

There is bound to be some electricalnoise in a system of this kind, althoughvery small; the problem becomes apparentwhen the signal from a transducer witha low output is resolved to 1 part in 4096.The use of 12% trimmed means on groupsof 24 conversions reduces these errors tonegligible proportion on all except thelowest output transducers. For this reason,we prefer to avoid transducers with anoutput of much less than 100mV full scale.The penalty for using this process is thatit slows the machine code program downto 50 readings per second but in soilmechanics this is generally acceptable.

SoftwareWriting programs in BASIC requires

little tuition and is well within the capa-bilities of most recent graduates; indeed,the equipment is now being put to use bystudents for their projects. Developingsoftware which is fool-proof and exhaus-tively tested is a different proposition.Once the decision had been taken to makethe system available commercially, it be-

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Fig. 3. In Soil Mechanics'aboratory at Bracknell, Dataface has been used to collect and analyse data from a series ol eight consolida-tion tests

16 Ground Engineering

came necessary to produce software toan appropriate standard and this hasproved very time-consuming. The prog-rams which have been tested to thepoint at which they can be made avail-able to others are for:

Transducer calibration,Undrained triaxial tests (one is being

performed in Fig, 2),Oedometer consolidation (up to 8 oedo-

meters running asychronously as seenin Fig. 3),

Shear box (Fig. 4) with consolidation(2 boxes); and

General purpose logging program.The last of these gives some idea of

the flexibility of the system. Any com-bination of channels can be grouped to-gether and designated as a single test. Alltransducers in the group will be readsimultaneously at either prescribed timeintervals (not necessarily constant inter-vals) or at prescribed increments of theoutput of one of the transducers. Up toeight such tests can be run simultaneous-ly and independently; for example it couldbe used for one drained triaxial test (4transducers) and 4 oedometers, the read-ings being taken at log or root time in-tervals during consolidation and at equalincrements of strain during shearing. Thescreen can be used to display the pro-gress of any one of the tests and thedata can be stored on cassette tape orfloppy disk for later analysis, or it can betabulated with appropriate headings ona printer. All output is in engineering units.

With test data in the computer or oncassette tape, the idea of writing analysisprograms becomes very attractive. Thedanger is that engineering judgement inthe interpretation of the results may be lostin such a process. A number of printer/plotters are now available and one of theseis being used to plot the results grap-hically in the traditional manner. Fig. 5 is atypical example taken from the oedometertest analysis program. The policy adop-ted is to use the computer to perform allcalculations which can be clearly definedbut to require the user to enter those valueswhich entail an element of judgement—helped where appropriate by a plot of thedata. The final printed output has a spacefor a signature and date to show clearlywhere the buck stops.

Once a small computer is available itis surprising how quickly it assumes arole comparable to the role of calculatorsin the early '70s. The extent to which it isused for, say, liquid limit calculations isa matter for individual laboratory managersalthough it does depend on the extent towhich it can be spared from its primarytask. The experience of one of the lab-oratories which are already using the sys-tem may not be typical because they hadthe enthusiasm to take it on untried; none-theless, they have purchased a secondcomputer for analysis. As another labora-tory manager put it, his job included look-ing over the shoulder of his staff and ask-ing the sort of questions which ensuredthat they did not take short cuts. He want-ed the computer to save technician time,but he also wanted to ask those same ques-tions. When he got his computer he wasprepared to spend some money on tailor-made programs; after a very short timehe found one of his staff had the inter-est and the enthusiasm to stay late anddevelop new software, not only to playstar wars but also to help in the generalwork of the laboratory.

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Fig. 5. Typical example taken from the oedometer test analysis program

Fig. 4. The system used in connection with an ELE mororiaed shear apparatus, to givecomputer controlled recording and analysis of three versions of the shear test. Alsoshown is a transducer load ring and linear and horizontal strain transducers

January, 1983