Seca 780 - Functional description; Introduction; Weighing; Overload protection device

Service Manual
Functional description
Functional description
DETF basic scale
Introduction
The DETF basic scale is a mechanical basic
module for various seca scales. It is used for the
electronic determination of the weight of adults.
For the electronic measurement of the mechanical
quantitiy “force” a double-ended tuning fork
sensor (DETF) is used.
The above principle of operation has been realized
on the one hand with a knife-edged load lever
system to meet higher accuracy requirements and
on the other hand with a more cost effective
bearing-determined load lever system to meet
slightly lower requirements.
Weighing
When a load is placed on the scale, the force due
to gravity is transferred from the platform (1) to the
load lever system.
The long load lever (2) transmits the force via the
bending element (5) on to the hook (6). The force
is reduced in accordance with the transmission
ratio of the load lever. The force applied to the
short load lever (3) is added to the force on the
long load lever via the centre suspension (4).
A bending load acts on the bending element that
increases the tensile force acting on the
pretensioned tuning-fork sensor (7). This increases
its resonant frequency in a non-linear way, which is
measured by the connected electronics (8) and
converted into a weight using the stored
characteristic curve. The weight is output on the
digital display (9).
together, but also exert an initial load which is
necessary to reduce the hysteresis.
In the bearing-determined load lever system two
springs exert a force which corresponds to a
preload of approx. 15 kg.
In the knifed-edged load lever system 4 springs
exert a force which corresponds to a preload of
approx. 11 kg. The platform is prevented from
being lifted by an additional locking device onto
which the long load lever hits if the platform starts
to rise (not for flat scales).
Vibration-sensitive switch (optional)
Any force exerted on the platform acts via load
lever system, hook and overload protection device
on the vibration-sensitive swich (13) which is fitted
between overload protection device and platform.
The load causes the vibration-sensitive switch to
generate an electrical pulse which starts the
connected electronics.
Overload protection device
The force acting on the hook is transferred to the
overload protection device (10). This device is
pressed against the platform by a pretensioned
spring. If the load on the scale exceeds a
permitted value, the spring yields and the load
lever system is lowered.
Platform locking device
The platform retaining springs (11) do not only
hold platform, frame (12) and load lever system
18.08.99 Har.
30-34-00-629A