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Associated circuit diagrams:
1. A/D converter circuit diagram
2. Microprocessor circuit diagram 08-01-21-322
3. Display unit circuit diagram
4. Switched-mode power supply
circuit diagram
Measuring element
A platform load cell is used as the force
measuring element. 4 wire resistance strain
gauges are attached to its surface at suitable
points which are connected in a bridge circuit.
When a load is applied, the spring body is
deformed in such a way that the two resistors
forming a half bridge are extended and
compressed. This causes the resistance to be
increased or reduced so that the bridge is
detuned, causing a change in the output signal:
= ⋅
⋅
U
k U
F
U
a
S
In order to ensure a higher signal yield for the
seca 717, the measuring element and the A/D
converter are supplied with 10V; usually, a 5V
power supply is provided.
Analog to digital converter
The A/C converter directly processes the small
output signal of the strain gauge sensor. It
functions according to the principle of signal-
dependent pulse-width modulation. All digital
functions of the A/D converter are implemented
via software in a microcomputer. The reference
potential of the A/D converter is ≈ U/2, since the
positive input of integrator 402 is connected to a
bridge output via resistor 506. During a predefined
total time T, first the input voltage +Ue is
connected to integrator 402 via resistors 507 and
506 and then the reference voltage U/2 with FET
switch 600/1,2,13, via resistors 510, 517 (for
model 727 only), 602, 604, 511, 512, 513 and 514.
The components are selected so that the
integrator integrates up during this phase
whenever an input voltage is applied until
comparator 502 reacts. The response threshold of
the comparator is determined by resistors 410 and
411. Resistor 413 causes positive feedback and
prevents the comparator oscillating. The
microcomputer detects that comparator 502 is
triggered and switches FET switch 600/1,2,13 off.
The integrator now runs down until time T has
expired. The interval between T = 0 and the
moment the comparator reacts is a measure for
the input voltage Ue.
Trimmer 602 compensates for the local gravitation
(GAL value). Trimmer 604 is used to fine-adjust
the slope. As the adjustment range for 604 has
been deliberately kept small, the slope can be
adjusted using the combination 513, 514.
08.02.98 Law
Circuit description
Model 717,727,728,737,748,757
08-01-21-321
08-01-21-323
08-01-21-354
∝
F
a
Resistors 508 or 509, which can be used
alternatively, are used for coarse adjustment of
the zero point. Resistor 412 helps the output stage
of amplifier 402 to increase the negative output
range.
Resistors 515 or 516 allow characteristic curves to
be corrected.
For incubator scale 748 only:
For reasons of electromagnetic compatibility, the
analog to digital converter circuit is provided on a
separate board. Reducing the analog signal path
by mounting the board in the base plate and using
interference-suppression capacitors 400, 401,
501, 603 considerably reduce electromagnetic
susceptibility.
The 5V supply voltage for the board is fused on
the main board with a 100mA fuse to limit the
energy on the board in the event of a fault.
Temperature compensation
For temperature compensation of the strain gauge
sensor, a fixed-value resistor is connected in
parallel with a temperature-dependent resistor. To
compensate for the sensitivity's t.c. value, the
combination 511,512 is used (NTC). The positive
t.c. value of the test value can be compensated for
by the combination 405, 406 (PTC).
Zero point, sensitivity and test value are partly
interdependent. To determine these values, the
modules (electronics + sensor) are measured at
10°C and 40°C (zero point, slope, test value) and
the results are entered into a computer. The
computer uses a complex computing routine to
calculate the optimum temperature compensation.
Microcontroller and display
The central computing and control element is the
microcontroller (µC) 520.
It fulfils the following functions:
• Digital A/D conversion
• Calculation of the zero point
• Binary BCD segment conversion
• Display control using multiplex operation
• A/D converter test
• Testing the CPU and the memories (RAM and
ROM)
• Overload detection
• Monitoring the supply voltage (digital)
• Automatic zero point monitoring
• Taring function
•
Hold function
All functions are implemented via software in the
programmed memory (ROM) of the µC.
Sequence of operations
Blatt 1(3)
25-01-02-467 Index E
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