DE2337884A1 - MEASURING TRANSDUCERS FOR FORCES - Google Patents

Description

DESCRIPTION Force transducer

1. Area of application:

Transducer, for the implementation of mechanical forces or on such traceable measurands in the change of an electrical output signal.

2. Purpose:

With such transducers it is necessary to use mechanical forces with the smallest possible conversion error into electrical ones To implement signals that can be further processed in downstream equipment.

3 State of the art:

To measure forces with electrical force transducers, one mainly used resistance-) magnetoelastic, piezoelectric, capacitive, inductive and photoelectric converters.

(See "Mechanical - Electrical Measurement Technology" by Dr. Werner Gohlke, Carl - Hanser - Verlag, Munich 1965, page HO)

The strain gauge transducer belonging to the group of resistance transducers is very widespread.

The disadvantage of the transducers used hitherto for measuring forces is the fact that all of these transducers only emit an analog electrical signal, which is caused by temperature errors, calibration tolerances, leakage errors and reproducibility errors caused measurement uncertainty is afflicted, the size of which can only be reduced by a correspondingly greater effort.

According to the above literature, the total error (sum of all measurement uncertainties) is 2 percent of the measurement range.

4th task:

The invention was based on the object instead of the previously known Principles of finding a transducer for force measurement, which instead of the analog electrical output signal able to emit an electrical output signal is, which is afflicted with a frequency dependent on the amount of force.

At the same time, j | ls task was a substantial increase in Based on measurement accuracy.

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5th solution:

The object is achieved according to the invention in that a prestressed mechanical string (Fig. 1 number l) would take assistance electrical excitation and self-control, taking advantage of feedback circuits, stimulated to natural oscillations is, whose frequency is through a, preferably in the string longitudinal axis, on the string tensioning device and thus acting on the pretensioning force of the string, tensile or compressive force changed and thus a change in the string tension corresponding Change in the frequency of the electrical output signal to be obtained from the stimulating electrical circuit causes which can be processed by downstream electrical circuits.

6. Further embodiment of the invention:
See patent claims! 2 to 14.

7. Achievable benefits:

The advantages that can be achieved with the invention are, in particular, that one for digital measurement technology and processing suitable electrical signal is emitted by the transducer with high measurement accuracy at the same time, with suitable Construction of the equipped with the transducer according to the invention Force transducer, the temperature compensation is taken into account.

The temperature compensation should preferably either be achieved by using materials with the same temperature coefficient or by using thermostatically controlled heating devices.

By using integrated semiconductor systems, all for the operation and signal conversion of the measuring transducer Combine required circuits in force transducers, so that apart from the power supply no further additions are necessary.

8. Description of an exemplary embodiment:

An embodiment of the invention is shown in the drawing and is described in more detail below.

Show it:

Fig. 1 The basic principle of the transducer

Fig. 2 A force transducer for tensile forces with the inventive Transducers.

8.1. Mode of operation of the transducer according to Fig. 1:

The clamping device 2 is in a receiving device 3 attached string 1 pretensioned.

The permanent magnet 4 is attracted or repelled via the drive coil 6 wound on the soft iron core 5. In the feedback coil 7> which is also wound on the soft iron core 5, a voltage is induced, which on the exciting generator 8 is fed back. This creates natural vibrations, the frequency of which changes also results from the pretensioning force of the string. (In addition to the length and other constants)

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A change in the tension prevailing in the string due to an external force thus also causes a change in frequency of the electrical output signal «

8.2. Operation of a force transducer with the inventive Measuring transducer as an embodiment in Fig. 2:

The string 5 is pretensioned via the spring membrane 3 by the tensioning device k , which consists of three adjusting screws each with an internal fastening screw.

A tensile force acting on the eyelets 1 and 2 from the outside thus causes a direct change in the string tension and thus a Frequency change in the permanent magnet 6, drive - feedback coil S and the downstream generator existing Circuit.

Via the heating coil 7 a with a built-in temperature sensor the entire transducer is kept at a constant temperature by an external electronic controller. The thermal insulation 9 ensures an even temperature distribution and at the same time reduces the energy required for heating.

409886/0792

Claims (14)

-Jm- 2337884 7/17/73 ■ t- Claims Transducer, for the implementation of mechanical forces or Measured variables traceable to such forces in the frequency change of an electrical output signal, in particular for the construction of force transducers, characterized by that a pre-tensioned mechanical string (Fig. 1 number l) with the aid of electrical excitation and self-control, using feedback circuits, natural oscillations are excited, the frequency of which is preferably in the longitudinal axis of the string, on the string tensioning device and thus on the pretensioning force of the string, tensile or compressive force is changed and thus a change in frequency corresponding to the change in string tension of the electrical output signal to be obtained from the stimulating electrical circuit, which is further processed by downstream electrical circuits can. 2. Vibration transducer according to claim 1, characterized in that the preload for the string is generated by a preloaded spring membrane (Fig. 2), the string at one end is attached in the middle of the membrane and at the other end in the Vandler housing, in which the membrane is also stored, and the one to be measured Force in the middle of the membrane, preferably axially, acts on the longitudinal axis of the string and thus a change in the string tension acts. 3 · oscillating transducer according to claim 1, characterized in that the string is pretensioned by a spring-elastic mounted boom or bracket and the force to be measured, preferably in the longitudinal axis of the string, on this boom or bracket acts and thus causes a change in the string tension (Fig. 1). 4. Oscillating transducer according to the preceding claims, characterized in that the string used in the transducer is also different may have geometric shapes than the normal rectangular or circular shape, and that in the borderline case the string also as Cylinder, hollow cylinder, cone and ball can be designed. 5. vibration transducer according to the preceding claims 1 and 4 thereby characterized in that the string is tensioned in the direction of action of the external forces and directly experiences a change in its tension and thus a change in its natural resonance as a result of these forces. 6. Vibration transducer according to the preceding claims, characterized in that the string is preferably excited in the middle. 7. Oscillating transducer according to the preceding claims, characterized in that the excitation and feedback takes place on an electromagnetic basis, shown that at the Anre- 409886/0792 the drive and control coil as close as possible the string acts and at this point a small permanent magnet is attached in or on the string, which is attracted or repelled by the drive coil once by induction with each oscillation and also electrically Voltage pulses are generated in the control coil, which are used for feedback to the driving circuit, which thus becomes an oscillator, with the string acting as a frequency-determining element. 8. vibration transducer according to the preceding claims 1 to 6, characterized in that the excitation and feedback of the piezoelectric material (quartz, piezoceramic etc.) existing string is done in a known manner as with the crystal oscillators. 9. oscillating transducer according to the preceding claims, characterized in that the entire oscillating transducer is thermostatic is kept at a constant temperature via a regulated heater in order to reduce temperature-related measurement errors. 10. vibration transducer according to the preceding claims 1 to 7 and 9, characterized in that the string and the tensioning device by combining materials with a corresponding temperature coefficients are temperature compensated, preferably metallic materials are intended. 11. Vibration transducer according to the preceding claims 1 to 7 and 9 to 10, characterized in that the string is preferably made of metallic materials and is preferably made of one piece together with the tensioning device in order to avoid measurement errors due to material tension and clamping errors. 12. Vibration transducer according to the preceding claims 1 to 7 and 9 to 1 ± characterized in that the pretensioning of the string is achieved by defined material stresses in the clamping device, these material stresses preferably being generated by a suitable heat treatment and the string with the clamping device is made from one piece. 13. Oscillating transducer according to the preceding claims, characterized in that the oscillating transducer has additional electronic Components are added in preferably integrated form, which apart from the excitation and feedback circuit of the Schwimg converter all for measured value linearization and others Urneodization or normalization contain the necessary circuits, so that a direct connection to measuring and control systems is possible* 14. Vibration converter according to the preceding claims! up to 1%, characterized in that additional electronic components are added to the oscillating transducer in preferably integrated form which, in addition to the excitation and feedback circuit of the oscillating transducer, have additional circuits for comparing the signal frequency bits of a given frequency -6. and thus enable the use of the Schving converter as a programmed limit value indicator for forces. 409385/0792 Blank page