Automatic weighing method and device

For the automatic weighing of a beam balance with electromagnetic force compensation, substitution control weights and digital recording are used. An indicator signals the position of balance beam to a microcomputer. The load compensation is carried out with a limited number of variable electromagnetic compensation forces of constant amplitude, which are produced by means of a corresponding number of discrete constant currents. The currents are graded according to decreasing powers of a numerical system of a suitable basis; as a rule, just like decimally organized figures according to units and powers of ten of these units. Starting out from upper to lower decades, a count is made in each decade as to how many compensation units are needed in order to overcompensate the load. For this purpose, a decision is made merely in the case of every compensation step--without determining precisely the rest position of the balance beam--as to whether the load applied is larger or smaller than the compensation force. Whenever the indicator signals an overcompensation, then the preceding counting step is regarded as the weighing result of this decade. Numerical correction values are assigned to each compensation step of each decade. The weighing results develops from the sum of the weighing results of all decades and the sum of correction values which had been assigned to the evaluated compensation steps of each decade.

BACKGROUND OF THIS INVENTION 
1. Field of this Invention 
This invention relates to a process for automatic weighing, whereby the 
deflection of the balance beam is compensated within the scope of a 
regulating process electromagnetically and/or by other methods, for 
example, by substitution weights, and the compensation force needed for 
this is used as a measure for the mass to be determined. The apparatus of 
this invention for executing this process is built up according to the 
principle of the electromagnetic force compensation, possibly in 
connection with other compensating systems (for example, substitution 
control weights) and with the use of a microprocessor as well as of an 
arrangement for signaling the deflection of the balance beam from the rest 
position, and it has a digital recording. 
2. Prior Art 
Electronic balances have been known, for example a sensor, attached to the 
balance beam in the manner of a closed control loop, which signal the 
non-existence of the rest position to an electromagnetic force 
compensation system. A current of sufficient strength is produced in 
dependence on this deflection in order to compensate for the distortion of 
the balance beam. At the same time, the sensor is returned automatically 
into the zero position. The current needed in order to bring about the 
zero position is measured and is used as a measure for the weight to be 
determined (for example, Gast, Feinwerktechnik 53, 167-172, 1949). More 
recent developments provide microprocessors which are able to store and 
process the measuring values obtained from the control loop. At the same 
time, the microprocessor merely serves for the evaluation of the 
measurement results and has no function within the control loop. 
Furthermore, substitution balances are known where a weight compensation is 
accomplished by lifting off of control weights. 
The force compensation systems of conventional balances must operate with a 
precision which corresponds to the entire weighing range to be resolved. 
In the case of analytical balances, for example, the error must not amount 
to more than fractions of one per mil. Temperature differences and other 
possible disturbing values must be compensated over the same range by 
physical measures. Thus, for example, control weights of 100 g must be 
made precisely to 10.sup.-5, as a result of which the production process 
becomes more expensive. In the case of such systems, furthermore, the zero 
indicator must be capable of indicating the exact rest position of the 
balance beam since any deviation will cause a more or less severe 
measuring error. 
BROAD DESCRIPTION OF THIS INVENTION 
This invention is based on the object of providing a process and a device 
with which (while avoiding the disadvantages of the above mentioned 
methods and devices) disturbing factors--for example, temperature 
influences--may be precisely compensated in a simple manner. Furthermore, 
merely the deflection of the balance beam is to be signaled and not its 
precise rest position. 
It has turned out that this object may be solved with a process of the 
initially described kind whenever compensating forces are used which are 
graded according to decreasing powers of a numerical system of an 
appropriate basis, whereby the weight value of each power is always 
compensated by (n-1) quantization steps, whereby n signifies the number of 
quantization steps within the numerical system used which causes an 
overcompensation, and in that the weight is determined from the sum of the 
compensation forces of all powers. The weighing result is determined by 
adding the sum of the compensation forces of all powers and from the sum 
of the electronically stored correction values according to numbers, 
whereby the correction values correspond to the compensation errors which 
were determined for each compensation step once within the scope of a 
calibrating process. Advantageously the compensation forces are scaled 
decadically. Also advantageously the correction values are taken into 
account both in case of electromagnetic force compensation as well as in 
case of compensation by other methods, for example, by substitution 
weights. 
The device for carrying out this process, that is; the arrangement for 
signalling the deflection of the balance beam from the rest position, is 
connected with a microprocessor which controls a current distribution 
network, which in connection with a precision current source produces 
currents of quantized strengths which feed at least one electromagnetic 
force compensation arrangement. Preferably, one separate force 
compensation arrangement is provided for every power. However, it is also 
conceivable to combine several force compensation arrangements. 
Furthermore, there is an electronic storage connected with the 
microprocessor in which correction values may be called up, whereby the 
correction values correspond to the compensation errors which were 
determined once for each compensation step within the scope of one 
calibrating process. 
The weight compensation of this invention may be accomplished 
electromagnetically exclusively or by other methods--for example, by 
substitution weights. However, the weight compensation may also be 
accomplished partly by electronic means, according to the substitution 
principle. 
In the case of this invention, the otherwise customary continuous control 
process, up to the return of the balance beam, is changed over into a 
limited number of discrete steps--for example, whenever the compensation 
steps are sealed by decades, 10 steps per decade. The fact that a weight 
correction value may be assigned to each of these steps constitutes a 
special advantage. It is therefore sufficient to produce a substitution 
control weight of 100 g precisely only to 10.sup.-2 and to determine the 
remaining error by a single exact weighing. This remaining error, as a 
numerical correction value, is stored electronically as belonging to the 
pertinent weight. The same applies also for electromagnetic force 
compensation systems, which are assigned to individual decades. The 
precision requirements for these production systems are thereby reduced to 
the percentual range. The weighing result consequently develops from the 
addition of the sum of the weight compensation values--precise, for 
example, up to 10.sup.-2 --of all decades and tbhe sum of the stored 
correction values assigned to the participating compensation steps. This 
invention reduces the hitherto customary manufacturing expenditure 
therefore practically to a precise calibrating weighing, i.e., 
determination of the correction values. The arrangement for signaling the 
deflection of the balance beam does not have to indicate the precise rest 
position. Generally, a binary decision is sufficient as to whether the 
weight is too large or too small. In the special case, whenever the exact 
residual weight is already compensated by the n.sup.th step, (i.e., no 
definite binary decision takes place), the weight is found to be too large 
in the (n+1).sup.th step, and nevertheless reduces it by the algorithm to 
(n-1), i.e., the exact value. 
Another advantage of this invention is that disturbance factors, especially 
temperature influences, may be compensated precisely by using variable 
correction tables which are dependent on the magnitude of the disturbing 
parameter. The behavior of the balance may be non-linear to any extent. 
The weight might be determined precisely by introduction of corresponding 
correction tables. The electronic storage capacity for this, at about 80 
locations per correction list, for example, is very small according to the 
present state of microcomputer technology.

On the left-hand side of the balance beam 1 there is a loading pan 2 with 
the mass M to be weighed. An indicating arrangement 3 signals the 
deflection from the rest position to a microprocessor 4, i.e., an applied 
load. The right-hand side of the balance beam is provided additionally 
with electromagnetic force compensation arrangements 5, 6, which may be 
developed for example, as plunger or rotating coil systems. The force 
compensation arrangements 5 and 6 are fed from a current distribution 
network 7. The current distribution is controlled by the microprocessor 4. 
The distribution network 7 obtains current from a precision current source 
8 of high time constancy and from a second current source 9 of higher 
power, of which low requirements of constancy are made. In principle, 
however, this second current source 9 could also be dropped. Additional 
control outputs are connected with a substitution weight circuit 10, as 
well as the weight indicator 11. In addition, the device has an 
arrangement 12 for the electronic storage of current or weight correction 
values. 
The weighing processes are repeated cyclically. A weighing takes place as 
follows: 
The arrangement 3 signals to the microprocessor 4 that a load has been put 
on. The microprocessor 4 produces a current I.sub.o rising in 160 equal 
steps by way of the distributor network 7 and the current source 9, which 
feeds the force compensation arrangement 6. The compensation force 
develops proportionally to the product of current strength and magnetic 
induction, so that in the case of constant induction B.sub.o, the current 
strength I.sub.o represents a measure for the compensating weight. The 
compensation arrangement 5 is designed such that one current step I.sub.o 
corresponds to a weight of 1 g.+-.1%. 
Whenever the reversal of the arrangement 3 is signaled to the 
microprocessor 4, then the latter causes the weight circuit 10 to lift the 
substitution weights G.sub.s corresponding to (n-1) g, whenever n was the 
number of the steps I.sub.o, which dissolved the indicator reversal. Thus, 
the mass M is determined in its gram range. The microprocessor 4 now 
produces decadically staggered currents I.sub.1 to I.sub.10 by means of 
the precision current source 8 via the current distributor network 7, 
which might produce with the induction B.sub.1 compensation forces in the 
range of 10.sup. -1 g, with B.sub.2 compensation forces in the range of 
10.sup.-2 g, etc. Each decade, beginning with the first digit after the 
decimal point, is tested in steps of 1/10 by increasing the current 
(I.sub.1 to I.sub.2 to I.sub.3 etc.) to the effect as to whether the 
indicator signals a reversal. Whenever that is the case after n steps, 
then this decade is compensated by a holding current corresponding to 
(n-1) steps and subsequently the next decade is examined. 
The values of all decades obtained thus are added by the microprocessor 4 
after a correction value, stored in the storage 12 and pertaining to each 
value, has been taken into consideration. Work with correction values 
decreases the requirement for the technical balancing of the force 
compensations to the percentual range. However, the tolerances must be 
developed such that one decade is reached safely or is easily exceeded. 
The microprocessor 4 then produces the indication of the weight and starts 
a new weighing cycle whenever the load put on changes. During this new 
weighing process always the last indication remains unchanged. 
By way of summary, for automatic weighing of a beam balance with 
electromagnetic force compensation, substitution control weights and 
digital recording are used. An indicator signals the position of balance 
beam to a microcomputer. The load compensation is carried out with a 
limited number of variable electromagnetic compensation forces of constant 
amplitude, which are produced by means of a corresponding number of 
discrete constant currents. The currents are graded according to 
decreasing powers of numerical system of a suitable basis; as a rule, just 
like decimally organized figures according to units and powers of ten of 
these units. Starting out from upper to lower decades, a count is made in 
each decade as to how many compensation units are needed in order to 
overcompensate the load. For this purpose, a decision is made merely in 
the case of every compensation step--without determining precisely the 
rest position of the balance beam--as to whether the load applied is 
larger or smaller than the compensation force. Whenever the indicator 
signals an overcompensation, then the preceding counting step is regarded 
as the weighing result of this decade. Numerical correction values are 
assigned to each compensation step of each decade. The weighing result 
develops from the sum of the weighing results of all decades and the sum 
of correction values which had been assigned to the evaluated compensation 
steps of each decade.