Vibrating string receiver

A vibrating string transducer is a sensor for measuring quantities that can be transferred to the tension of a string . These include stretch , mass , force , pressure and temperature . The vibrating string transducer converts the tension on a string into mechanical vibration , which in turn is converted into an electrical frequency signal.

Physical explanation

A string that is stretched with a force F can vibrate with fixed natural frequencies .

Mechanical integration of a vibrating string in a transducer. Electronics and magnet have been removed to make the string easier to see.

${\displaystyle {f}_{n}={\frac {n}{2}}\cdot {\sqrt {\frac {F}{m\cdot l}}}}$

in which:

${\displaystyle {f}_{n}}$ ... natural frequencies of the string

${\displaystyle n\in \mathbb {N} }$

${\displaystyle F}$ ... exciting power

${\displaystyle m}$ … Dimensions

${\displaystyle l}$ ... length

A hard hit on the string would result in a superposition (superposition) of all natural frequencies. The energy decreases sharply with higher frequencies.

technical realization

Vibrating wire transducer for fast load changes, which is operated with the second harmonic (n = 2). The vibrating string is covered by the magnet.

A string that vibrates at several frequencies at the same time is difficult to control as a measuring instrument.

Therefore, individual natural frequencies are specifically stimulated. Technical implementations with n = 1 or n = 2 are common.

The string is usually excited inductively or piezoelectrically, while the vibration frequency is recorded inductively, capacitively or piezoelectrically.

A real vibration ( quality factor of the string is not infinitely large) is subject to damping . A stimulation is therefore necessary to maintain the oscillation.

Two operating modes are possible:

• Excitation with an impulse and subsequent absorption of free oscillation
• continuous operation with forced vibrations (the string sensor is part of an oscillator )

By supplying energy, the free oscillation becomes a forced oscillation , the resonance frequency of which is close to the natural frequency with weak damping.

properties

resolution

The resolution with a single vibrating string is given as 1: 4,000,000.

When combining two vibrating strings to compensate for the varying gravity of the earth (mass measurement), a resolution of 1: 2,000,000 is specified.

Linearity

The relationship between the applied force and the square of the measured frequency is highly linear. In the literature, a deviation of 0.02% from the ideal straight line is given.

Long term stability

The long-term stability of vibrating wire transducers is an outstanding property. In the literature, the deviation is given as 0.01% per year. Vibrating wire transducers are therefore ideally suited for applications in which regular zeroing is either not possible (silo scales, storage scales) or not useful (geological movements or observation of buildings).

dynamic behaviour

The limit for the measurability of dynamic force curves is currently at a bandwidth of approx. 1000 Hz.

Measuring with little travel

The frequency of the vibrating string is not increased by stretching it, but by simply increasing the force. The extension of the string is very small with a suitable choice of material. The movements in the measuring elements are therefore also very small. In comparison, a strain gauge needs a path (lengthening of the material with simultaneous thinning) in order to generate a measurement signal.

If a load cell based on strain gauges is subject to frequent load changes, it is prone to material fatigue and breakage. By using a vibrating string, only minimal deformation of the load cell is necessary and breaks due to material fatigue are prevented. This is one reason why load cells based on vibrating wire sensors have become established on refuse collection vehicles.

Signal yield

A vibrating string transducer with a lowest frequency of 9000 Hz can be operated non-destructively down to 21000 Hz. In practice, only half of this range is used in order to be able to tolerate high overloads.

Overload resistance

A vibrating string transducer can be loaded non-destructively up to twice its nominal load.

Active transducer

An error in the transducer leads to the loss of the frequency signal. An existing string frequency is therefore a clear indicator of a properly functioning transducer.

Low power consumption

The excitation electronics of a current vibrating string transducer consume less than 15 mW of electrical power. Because of this low output, there are no problems with self-heating.

Disturbance variables

temperature

Temperature effects are largely offset by the fact that the construction that holds the string is made of a material with the same thermal expansion. For applications that require high levels of accuracy, calibration values ​​are saved and used accordingly.

Crawl

A great advantage of vibrating wire receivers is their low tendency to creep. Since all materials used are either metallic or crystalline, creep can be reduced to a minimum. Modern vibrating wire transducers are designed in such a way that no plastic deformation (flow) occurs in the intended measuring range and in the near overload range.

Cross sensitivity

The vibrating string itself has no cross-sensitivity, as a cross-load does not change the tension of the string. However, improperly holding the string can destroy this positive property.

Hysteresis

The vibrating string itself has no measurable hysteresis. The mechanical retention of the string can, however, bring this about. A good choice of materials and skillful construction can keep the hysteresis at a low level.

humidity

Non-condensing moisture has no effect. Condensing water is thrown away by the string vibration. However, condensing water can lead to short circuits in the measuring electronics, which can be prevented by applying paint. When completely immersed in a liquid, the mechanical vibration of the string comes to a standstill.

Nuclear radiation

The principle of the vibrating string cannot be influenced by nuclear radiation. The usability is determined by the nuclear radiation sensitivity of the electronics used.

Electromagnetic radiation

Since the string is made to vibrate inductively and, depending on the measuring principle, the frequency is also measured inductively, alternating magnetic fields can have an influence. The electrical loop that the vibrating string and electronics form must therefore be kept as small as possible. This works very well by placing the electronics and the string close together. In practice, vibrating string transducers without a shielding housing pass all common EMC tests.

application

Inventory recording: each individual box is weighed

Because of their high robustness and low creep, vibrating wire transducers are used in a variety of applications, such as:

• Body scales on commercial vehicles (the cargo space rests on vibrating wire transducers)
• Elevator control (the vibrating wire sensor is screwed onto a structural element with bending loads or shear loads)
• Belt scales
• Dosing scales
• Structural supervision (dams, bridges, buildings)
• Geotechnology (early warning systems for rockfalls)
• Inventory recording (material stores that constantly monitor the number of stored or removed items using built-in scales)
• Garbage disposal (the hydraulically raised garbage containers are weighed during the lifting movement)
• Silo scales (high demands on long-term stability)
• Steel casting robot (the pan with the molten steel is continuously weighed for exact dosing)
• Drive-over trolleys for rail vehicles and road vehicles (high demands on robustness and response speed)

swell

1. ↑ K-Tron Smart Force Transducer Weighing Technology for Gravimetric Feeding, Batching and Metering. (PDF; 436.61 kB) In: pfe.pt. May 2009, accessed on June 23, 2020 . 
2. ↑ PESA Waagen AG - 2-string technology. In: pesa.ch. Retrieved June 23, 2020 . 
3. ↑ ^{a b} Dr. Closely. Dan Mihai Stefanescu: Handbook of Force Transducers, Springer-Verlag, Berlin Heidelberg 2011, ISBN 978-3-642-18295-2 , pages 203-223
4. ↑ ^{a b c} [1] DIGISENS AG, data sheet Strain Monitor, Dec. 2018, accessed on February 22, 2020
5. ↑ Digisens - KL66. In: digisens.ch. Retrieved on June 23, 2020 (DIGISENS AG, data sheet Strain Monitor). 
6. ↑ Digisens - inventory recording e-nventory. In: digisens.ch. Retrieved June 23, 2020 . 

Web links

Commons : Vibrating wire  - collection of images, videos and audio files

• Digisens - vibrating wire sensor. In: digisens.ch. (Animation of a vibrating string receiver).