DE2313758C3 - Method for the digital quadrature of the ratio of a variable measuring frequency to a constant basic frequency, in particular for weight measurement with string scales - Google Patents

Description

This invention relates to a method of digitally quadrating the ratio of a variable measurement frequency to a constant basic frequency, especially for weight measurement with string scales according to the patent 66 781 and for incremental calculation with two consecutive counting intervals using of presettable counters.

The invention is an advantageous development of the first additional application P 19 47 605.2 (I. addition to Patent 17 66 781).

In the case of string scales, the weight to be measured is determined by the natural frequency of the measuring string loaded by a preload V and a weight G. In the case of a slack string, the natural frequency is the well-known law of the square root

fx = {kK

with K = V + G and the factor k.

If the string is not completely slack so that the clamping conditions disturb this dependency Add correction terms so that the following very good approximation results:

fx =

fx

IJkK- l + m

k, 1, m are constants to be determined in such a way that the implicit function is fulfilled as precisely as possible by the real string. To calculate the quantity K , the equation has to be solved for K :

K = -

-fx + fx ² )

In the known string scales, the frequency measurement is usually carried out with the aid of a second string with a constant preload, ie a constant natural frequency, the comparison frequency fo . Then not fx is measured, but · = ^ -. Since fo is a fixed number, it can be expanded so that only ^ j- occurs instead of fx as a variable.

mfo ~ k ~

mfo (fx \ tf_ ffx \ ² k Kf ₀ J ⁺ k \ fo)

or with

a ■ - -.

mfo

T ~ ^;

c =

SA

G = a + bl ^ f-

(ΙΞ.) ²

KfO)

With the known arrangements, including those according to DE-AS 2148 323, only the case 6 = 0 can must be taken into account. The object of the invention is to calculate the weight by a

so complete second degree polynomial with one To enable method which manages with simple counting circuits.

The invention achieves the object according to the features defined in the patent claims. It is based on the division of the polynomial calculation into two counting intervals TX and T2, where the first counting interval TX is given by the quotient of a fixed number A divided by the constant comparison frequency fo and the subsequent second counting interval T2 is determined by the time in which in the first counting interval summed up counting pulses of the frequency fx with the frequency fo are subtracted again up to the value 0 using presettable counters and consists in that in the first counting interval 7Ί, a preset basic counter 1, pulses of the comparison frequency fo, a preset Control counter 2, pulses of frequency fx for counting up and a measuring counter

3 pulses of frequency fx are supplied for counting, the first counting interval Tl ending when the content of the basic counter 1 reaches zero and a second counting interval T2 follows, giving the measuring counter 3 pulses of the measuring frequency fx and the -> control counter 2 pulses of the comparison frequency Io can be supplied for counting down, the second counting phase ending when the content of the control counter 2 reaches zero and the measuring counter 3 then contains the desired result. ι υ

In the following, referring to FIG. 1 explains a first embodiment

Before the first counting phase, the basic counter 1 is preset to the number A, the control counter 2 to the number B and the measuring counter 3 to the number C. In the first \ -, counting phase Ti , the basic counter 1 receives the pulses of the frequency fo via the And- Gate U 1 and counts down. The phase of the first counting interval ends when the basic counter 1 crosses zero. This results in the duration of the first counting interval as Tt = -ß-.

fo

The control counter counts during the first counting interval 2 via the AND gate t / 2 pulses of the measuring frequency upwards. At the end of Tl its content is

During the subsequent second counting interval T2, the control counter counts down i / 3 pulses of the comparison frequency via the AND gate until the zero crossing is reached. This results in

Tl =

TX fx + B

fo

= A V- 2 + B

Counter 5 are carried out one after the other, with a longer measuring time must be accepted.

The saving of a counter requires a simple sequence control made up of a 2-bit counter and a few logic switching elements. This allows four phases to run in succession. In the first phase (duration of a few milliseconds), the control counter 2 is preset to the number B, and the primary and measuring counter (counter 5) to the number A Then the second phase, is the duration of Ti and through the zero crossing of the counter 5 starts is terminated. During this phase, the control counter receives 2 pulses of the measuring frequency fx via the AND gate Ui on the upward input and the counter 5 pulses of the comparison frequency fo via the AND gate i / 4 on the downward input.The zero crossing of the counter 5 initiates the third phase ( Duration a few milliseconds), in which the control counter 2 retains its counter reading and the counter 5 is preset to the number C. In the subsequent fourth phase of duration T2 the control counter receives 2 pulses of the comparison frequency fo via the AND gate [/ 2 on the down input and the counter 5 pulses of the measurement frequency fx via the AND gate i / 3 on the up input. The fourth phase is ended by the zero crossing of the control counter 2. The counter readings result from the following calculation.

The duration of the second phase results from Π —γ- · After Π the tax counter contains the number

During the counting intervals Ti and T2, the measuring counter preset to the number C counts up pulses at the measuring frequency fx . This results in a counter reading at the end of T2 zu

G = (Γ1 + Γ2) / χ

45

The default settings can be determined by comparing the coefficients with equation (*):

A = c; B = b-c; C = a

A second embodiment describes a circuit according to Figure 2, in which the function of the Basic counter 1 and the measuring counter 3 of one

B + TX -Jx = B + A

fifo

The duration of the fourth phase results in

_T2 " ± 7

fo

Tl = -.- (B + A

At the end of the fourth phase, the counter 5 contains the number

G = Tl fx + C

By comparing the coefficients with the equation (·), the quantities A, B and C can be determined as A = c; B = b; C = a.

1 sheet of drawings

Claims (4)

Patent claims: 1. Method for digital quadrature of the ratio of a variable measuring frequency to a constant basic frequency, in particular for weight measurement with string scales, according to patent 17 66 781 and for incremental calculation with two consecutive counting intervals using presettable counters, characterized in that the incremental calculation of a polynomial second degree from the ratio of two frequencies fx and fo with two consecutive counting intervals (TX) and (T2) takes place in such a way that before the first counting interval (Tl) a basic counter (1) on the number A, a control counter (2) on the Number Sund a measuring counter (3) can be preset to the number C and during the counting interval (TX) the basic counter (1) via an AND gate (UX) counting pulses of the comparison frequency fo to the down counting input, the control counter (2) via an AND Gate (U2) Pulses of the frequency / * on the up counting input, whereby the down counting input remains blocked by an AND gate (U3) bt, and the measuring counter (3) via an AND gate (U 4) counting pulses of the measuring frequency fx are fed to the up counting input, the counting interval (Ti) being ended when the basic counter (1) reaches the zero crossing and thus the second counting interval (T2) is initiated by the measuring counter (3) via the AND gate (U4) pulses of the measuring frequency fx and the control counter (2) via the AND gate (U3>) pulses of the comparison frequency fo are fed to the down counting input until the Control counter (2) reaches zero and thereby the second counting interval (T2) is ended and the result of the calculation is in the measuring counter (3). 2. The method according to claim 1, characterized in that the AND gate (UA) remains closed by a suitable circuit during the counting interval (Tl) and the measuring counter (3) is only preset at the beginning of the second counting interval (T2). 3. The method according to claim 1 and 2, characterized in that the functions of the basic counter (1) and the measuring counter (3) with the help of a simple sequence control (4) through a single Counter (5) can be realized. 4. The method according to claim 1 and 2, characterized in that the basic counter (1) counts up and the presetting results from the difference between the largest possible number in the basic counter (1) and the number A.