Conventional weighbridges are expensive items and well out of the finanoperators,

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Norah Pitts
6 years ago
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1 ! Design VMIST: Manchester UK 4pp June 1982 A Low-Cost Microprocessor-Controlled Weighbridge DG Whitehead* RMF Goodman* KA Welsh* Conventional weighbridges are expensive items and well out of the finanoperators however have frequently expressed the need for a low-cost moderate accuracy ~leighbridge system which can be operated by un- Conventional weighbridge system~ are expensive because of the high accuracy transducers required and because of the complex analog electronics that these transducers demand The use of a microprocessor however provides the intelligence to enable costs to be reduced dramatically in the following ways Firstly the operation of the weighbridge can be made extremely 'user-friendly' so that operation by unskilled personnel including the lorry driver is possible at the press of a single button transducers' to be used by eliminating skilled setting up operations For microprocessor allows flexibility in ticket printout displays and weighing method (eg axle "eights tare weight static weight or rolling In which is based on the 6502 microprocessor In particular we will indicate where the microprocessor is being used to eliminate or make automatic processes that previously required complex analog electronics The weighbridge system has been fully built and two prototypes are currently and acceptance of the report work began on the design of a prototype weigh- An essential ingredient of the low cost nature of the system is the singleaxle platform (3 metres by 075 metres) that is used The platform is supported by four load cells one at each corner Before proceeding with the system design it was necessary to carry out a pilot: experiment on the '1 I * Department of Electronic Engineering University of Hull! ri

2 l without i 216 I Initial trials showed good correlation between the weight obtained on a Public Weighbridge and that measured using the platform However it was observed that whilst static tests gave excellent correlation inaccuracy did occur when the vehicle was allowed to pass over the narrow platform stopping The errors were not consistent and can be shown to be due to three major effects Firstly there is the inevitable overshoot which occurs when the vehicle rolls onto the platform The dynamic system comprising of the platform and vehicle is critically dependent upon the characteristics of the suspension tyres and weight of the vehicle As! such it cannot be accurately determined and rolling weight measurements must alw~ys be less accurate than static measurements Errors due to bounce effects can be reduced to acceptable limits by (a) keeping the data OnJva limited amount of software processing can in fact be achieved This is due to the time available between axle readings However work on this problem is continuing I The second source of errors involved the design of the platform In this design the load cells were placed within the perimeter of the platform occurred Whilst it is to be expected that the load distribution will outside the perimeter of the platform and modifying the design of the tie-bars The third source of error is related to the second The load cells used in the tests were not a matched set Subsequent tests showed a difference of almost 3% in sensitivity between them Using a matched t set inequality in the load distribution providing it is not so great as to cause overloading of anyone cell will have very little or no effect i on the combined output However for the pilot experiment this was not; the case i : gauge amplifier to minimise costs This obviates the need for three more (expensive) amplifiers With four separate amplifiers unequal load 1 cell sensitivities can be accomm:odated by adjusting the gains of the f amplifiers With the single amplifier this cannot easily be done nor is \ it desirable to have many adjusting points to be set 'on test' However ; it has been found possible to set economically acceptable limits on the ; load cell and platform specifications which if met remove the necessity : for adjustment t 3 System Specification forward as possible Since microprocessor control is used jt is possible accommodate drift can be made automatic as can a simple calibration check i An important feature of the system must beprovision of 'hard copy'!

3 " 217 included The printer format using 6 cm wide A capacity of 9999 tonnes is the maximum allowed for the totaliser with legal limit for axle weight but the system could conceivably be used for other purposes Whilst a four-figure display is used giving a resolution ofool tonnes an accuracy of ~ 20 kgms is considered to be adequate In order that the system can be operated by a single person eg the" moves over the weighing platform Under these circumstances the large i robust construction A drawback is that special paper is required though trosensitive paper with the added advantage that it can be written on with pen or pencil As far as we were aware at the time of development the Rockwell AIl-1 65 was the only readily available low-cost system on the market with an integral printer of any sort As such it is already used unit price compares very favour ably with the cost of developing a special purpose system A further point to bear in mind when incorporatexisting monitor was adapted for use Also as an aid during development and modification 'in the field' the program though held in EPROM is downloaded into RAM for execution on power-up The total size of the 2K words the extra RAM requirement being of negli- I d) Display

4 " c"'"-v"c""c"" '"'"-""w~'-' : 218 i j zero adjustment prior to each weighing operation With the proviso that j weighing sequence the arrival of the 'start' signal causes the microor 'datum' level A calibration resistor set to simulate full load is A simple calculation gives calibration and zeroing for subsequent axle ing the quoted accuracy to be easily maintained Automatic operation is ensured because after the datum has been measured and calibration has been system then tracks the increase in load and when full load is definitely at any time It is in this static mode that the accuracy of ~ 20 kgms 6 Operation of the System c) print-out of preamble - manufacture's name et~ d) start light switched on mode axle weights will be recorded and displayed as the vehicle/s move recorded can be found by pressing 'totalise' This initiates a print out any drift due to temperature changes throughout the day or long term A microprocessor weighing systeln has been described which by means of an be taken by unskilled operators Inherent to the automatic calibration is the tolerance of low-cost load cells which do not need to be accur-

5 !! ed By basing the design on an existing microprocessor system the AIM-65 development costs have been kept to a minimum and for medium-quantity production such an approach a}so offers considerable economic advantages * 7 ;

6 ?'>fj ~~ \ ~;~)t 1 - [ II r:1 ';' e G; '" " '" " \~ (c- i:' ry"j; '" "'<" 'O'lj" t (>'" e r ] )! j : j! r \" t :'" ~ ~~~: ~' W!g"1!'i"""" " "":_-"" "'t ~~";~I'" r{ t' I ij; c~~i~y1:)ii ;ci;;i~~r;;;j c- Figure 2 Showing the AIM-65 microcomputer mounting

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