The normalized nonlinear model of the electrostatic actuator given below will be used for the simulations that follow.
(
)
vs r q x r q q x x x 3 2 1 1 3 1 2 2 = − + = + + & & & & ς (5.1)It is important to note that since (5.1) is a normalized model the units of time are scaled along with the displacement, charge and control signal. The equations that govern the normalization are given in (2.35) and (2.36). The sensor noise source used in these simulations is given in Figure 60.
The first simulation compares the tracking performances of the three classic LADRC designs with different sets of tuning parameters introduced in Section 4.1. In this simulation, the electrostatic actuator is commanded to track several desired travel ranges which are set to 10%, 30%, 50%, 70% and 90% of the full gap. The simulation results for the three LADRC designs are shown in Figure 61. In Figure 61, all of the three designs have shown acceptable tracking performances. Figure 62 investigates the displacement responses of the three LADRC designs at 10% of the full gap in greater detail.
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Figure 60: Sensor Noise
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Figure 62: Displacement Responses of Three LADRC Designs at 10% of the Full Gap
From Figure 62, we can see that the first design, which has the highest observer bandwidth, performs the best with almost no overshoot. Designs 2 and design 3 perform well with a noticeable amount of overshoot, where design 2 has an overshoot of 4.8% and design 3 has an overshoot of 8.3%. For an electrostatic actuator, reasonable overshoot at small displacements is acceptable. However, the overshoot at large displacements is much more troubling since it may cause the two plates of the electrostatic actuator to crash into each other. The integrals of the squared errors (ISE) for the three LADRC designs as the desired travel range is set to 10% of the full gap are given in Table XIV.
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TABLE XIV: ISES FOR THREE LADRC DESIGNS AT 10% OF FULL GAP
ISE
Design 1 Design 2 Design 3
0.0127 0.0133 0.0144
From Table XIV, design 1 has the best tracking performance. The responses of the actuator to a desired traveling range of 90% of the full gap for the three designs (classic LADRC with three different sets of tuning parameters) are shown in Figure 63.
Figure 63: Displacement Responses of Three LADRC Designs at 90% of the Full Gap
In Figure 63, design 3 exhibits the largest overshoot percentage (2.2%). So it shows the worst tracking performance among the three designs. This is because design 3 has the smallest observer bandwidth. The effects of sensor noise on the outputs of these
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three designs are too small to tell in the figure. The ISE for the three LADRC designs when the desired traveling range is chosen as 90% of the full gap are shown in Table XV.
TABLE XV: ISES FOR THREE LADRC DESIGNS AT 90% FULL GAP
ISE
Design 1 Design 2 Design 3
0.8914 0.9074 0.9347
From Table XV, we can see that design 1 has the least tracking error among the three designs. Figure 64 shows the sensitivities of the control signals of the three classic ADRC designs (shown in Figure 60) to the sensor noise along with the equivalent control signals without sensor noise.
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From Figure 64, we can see that these three designs are highly susceptible to sensor noise. Design 1 is completely unacceptable due to a very noisy control signal. Design 2 performs better but is still problematic. Compared to the first two designs, design 3 is the least susceptible to the sensor noise, and can be considered for application in the real world. A close look at the control signal with sensor noise (noisy signal) of design 3, along with its noiseless equivalent signal (clean signal) is shown in Figure 65.
Figure 65: Clean and Noisy Control Signals of Design 3
From Figure 65 we can see that the control signal is highly affected by the noise source. The majority of the noise shown in Figure 65 is outside the bandwidth of the plant. In practice, the actuator itself is a very good low pass filter. In Chapter 2, Figure 16 shows how the actuator can be separated into an electrical sub-plant and a mechanical sub-plant. It also shows that the charge output of the electrical sub-plant can be viewed as a control signal for the mechanical sub-plant. The sensor noise at the input to the
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electrical portion of the plant is only a concern if it approaches the saturation level of the drive electronics. However, excessive noise at the mechanical portion of the plant can lead to excessive wear on the electrostatic actuator. The electrical charge acting as the control signal to the mechanical portion of the plant is shown in Figure 66. We can see for Figure 66 that the electrical sub-plant filtered out the majority of the high frequency noise. This control signal shows the steps in charge needed to track the displacement commands at 10%, 30%, 50%, 70%, and 90% of full gap respectively.
Figure 66: Actuator Charge Control Signal
In the following simulation results, the responses of the three classic ADRC designs to a reference input of 97% of the full gap will be investigated. We add a step
input disturbance with a magnitude of 0 to the input of the actuator at t = 15 seconds. .5
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Figure 67: Displacement Outputs of the Actuator for Three Classic ADRC Designs in the
Presence of Step Input Disturbance
From Figure 67 we can see all of three designs can accommodate a travel range of 97% of the full gap with small overshoots at the disturbance. Therefore it can be said that the classic ADRC design can achieve a maximum gap traversal of approximately 97% of the full gap in the presence of input disturbances.
Given the previous simulation results, design 3 is the only viable option for the classic ADRC strategy due to its excellent noise attenuation effects.
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