The equivalent circuit of the piezo-electric tuning fork. This is essentially due to the fact that the effective force exerted on the twoįigure 3. According to the theorem proved by Butterworth, the tuning fork is equivalent to an inductor, a capacitor, and a resister connected in series, which is stunted by another capacitor, as shown in Figure 3. In the setup of this report, we used the electrical excitation method. There are two ways to excite the quartz tuning fork, the mechanical way, and the electrical way, as illustrated in Figure 2. In the setup of this report, we use the qPlus sensor with a tip on the moving beam, as shown in Figure 1(b) and Figure 1(c). This is called the qPlus sensor, which was first introduced by by Giessbl in 1998. Alternatively, one can fix one of the branches to a firm base to make it a single quartz beam. To maintain the symmetry of the tuning fork, balancing mass should be added to the other branch, which is obviously very difficult job. Įspecially when the tuning fork becomes asymmetric when a tip is added to one of the branches to make a probe microscope. (c) Schematic arrangement of sensor and sample. They might oscillate symmetrically or asymmetrically, adding complexity to the measurement,įigure 1. In addition to that, we tested our equipment by measuring the resonance frequency shift when the sensor is in contact with Polydimethylsiloxane (PDMS) sample.Īs shown in Figure 1(a), the quartz tuning fork has two branches. We achieved a more compact design by using the op-amp chip instead of the transformer as introduced by Grober. In this report, we designed a phase compensator consisting of a tunable inverting operational amplifier connected in series with a capacitor to try to eliminate the non-harmonic effects of the quartz tuning fork. There have been many ways to cope with this issue, including a theoretical approach, usage of tunable compensation circuit, and using another identical but immobilized tuning fork for compensation. However, for the electrical-excitation design, the outcome is not so desirable because of stray capacitance intrinsic to the tuning fork and its wiring. It is clear to us that the latter is simpler because it does not need an actuator. The material will deform according to the voltage applied. In a mechanical-excitation design, the input current drives a piezoelectric actuator attached to the fork which then drives the fork, while in an electrical-excitation design, voltage is applied directly to the quartz tuning fork. There are mainly two ways to drive the oscillation of the fork, namely electrical excitation, and mechanical excitation. Although there always exists thermal excitation of the quartz tuning fork, the intensity is too small to be useful. To use the tuning fork to measure atomic forces, we need to make it oscillate. The biggest advantage of using quartz is its piezo-electric property which integrates easily into electric circuits and allows data to be obtained electrically. Many variations of this design exist including combining the tuning fork with a diamond with NV center to probe the magnetic field in the vicinity of the sample surface. They are small in size, having low damping effect, and very versatile. Quartz tuning fork has been used as the sensing component of atomic force microscopes (AFM) for various reasons.
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