The tuning fork system developed by Shinko Denshi is now well known as an ideal sensor for weight measurement technology. The tuning fork sensor has also been adapted for use in the active support mechanism of the Subaru telescope, the world's largest, most accurate optical telescope.

1.Why use a tuning fork sensor?

The three major force measuring systems currently used in various measuring instruments are: load cell, electromagnetic system and tuning fork sensor. The reason why a tuning fork sensor is preferred is its excellent performance, including reliability and stability, as explained below in comparison with the other two systems.

Load cell scale
This measures distortion of metal and a strain gauge affixed to the metal according to the loaded weight, and then displays the weight. This type of scale needs an A/D converter due to its analog output. The structure is simple and inexpensive. However, the accuracy is lower than the other two types, and especially in terms of high resolution, the tuning fork sensor demonstrates its superiority to the load cell, as detailed later.

Electromagnetic balance
This keeps the balance against loaded weight by the electromagnetic force from a magnet and a coil, measuring the electromagnetic flow to the coil. This type of balance also needs an A/D converter as well as a load cell. It is highly suited for high-resolution balances, although it needs much care with regard to temperature and magnetic force changes.

Tuning fork balance
A tuning fork vibrator is monolithically formed in the shape of a pair of common tuning-forks joined at each of the tines. When the vibrator made of metal is strained or loaded, the frequency increases/decreases according to the loaded weight, which is counted and output digitally. As a tuning fork is standardized for a use in a musical instrument or a clock, the frequency is extremely stable and accurate. The stability of a tuning fork sensor is unrivaled and it's output is digital, threfor no addiditional circuitry is necessary.

2. The structure and principle of the tuning fork sensor

Next we will give you more details in structure and principle of the tuning fork sensor.


Fig. 1 shows the operating of a common vibrating force sensor. Generally speaking, the natural frequency of the beam vibrator shown the figure is calculated as
F=A(1+BxF)

A and B are fixed numbers determined by the dimensions of a beam vibrator (L, b and T in Fig.1), the density of the material and Young's modulus. The beam vibrator is made from metal, so its dimensions and density are stable. In addition, Young's modulus is also stable by utilizing a special elastic material those temperature characteristics is less than 10ppm/Ž. Consequently, the frequency of a beam vibrator is stable enough to obtain extreme accuracy without distortion by the load or force of metal material which influences signals, magnet drift or the A/D converter.

Utilizing these principles of a beam vibrator, a tuning fork vibrating sensor is shaped as if two tuning forks used for music instrument tuning are combined upside down in order to draw out the characteristics easily. Moreover, the lever and fulcrum are also united so that it can be used as a force or gravity sensor easily.
Fig.2 shows this tuning fork sensor unit.Fig.3 is the mechanism unit of a tuning fork balance. When load "W" is loaded on the pan, the force "F" is transmitted to a vibrator through the united transfer mechanism.

Fig.4 shows a balance that adopts a tuning fork sensor. The sensor has two piezoceramic elements close to the lower ends of the vibrating beams. As shown in the figure, being connected to the output and input terminal of an amplifier respectively, the elements act to sustain the vibration, one for exciting and the other for sensing. The element's impedance is several hundred kĦ and it is several kĦ less than a load cell; compared with an electromagnetic system, it is 100 times greater. In addition, the electricity consumption is very low. This system can minimize the total electricity consumption as it does not need an A/D converter and the circuit is simple. This big advantage is utilized for specialized needs, such as an explosion-proof.

Fig.5 shows a structural example of a tuning fork balance. This balance consists of a mechanism unit with a tuning fork sensor, a circuit board for sensing, and a major circuit board with a micro-computer.

Below is a summary of a tuning fork sensor's characteristics:

1. Repeatability: Excellent as the system does not require any distortion of metal.
   
2. Linearity: Although the original signal is non-linear, it is stable and can be linearized in a computer. It greatly minimizes non-linearity.
   
3. Temperature characteristics: Very good, as the greater part of the mechanism is a durable elastic material.
   
4. Generation of heat: Minute due to the low power consumption of the sensor and circuit board, and the sensor needs no warming up time.
   
5. Long-term stability: Superior, as the tolerance factor is little as mentioned above.
   
6. Structure: Small and compact

3.Tuning fork balance that more than meets expectations

As stated above, the tuning fork sensor has many advantages compared with the load cell or the electromagnetic system. On the other hand, it is also a fact that the load cell and the electromagnetic system have been utilized as the major mechanism of electronic scales or/and precision balances for a long time.

However, the tuning fork sensor, unlike different system from others, offer very low power consumption is needed to drive the tuning fork vibrator, and the tuning fork balance needs no warming up time, although a load cell and an electromagnetic system needs 10 to 60 minutes for this.

The most notable characteristic of tuning fork sensor is that the zero point and the span are extremely stable against temperature changes. This is obviously proved by the world's largest telescope, Subaru, and it is true even after you use our products for a long time. This means that warming up and calibration are not required usually if you do the specific gravity adjustment at the place of use. Pursuing actual and potential user needs, we develop unique products day and night.