# Development of a Magnetorheological Damper with Self-Powered Ability for Washing Machines
## 5 Experimental Validation
### Damping Force under Constant Magnetic Field  
The damping performance is tested when the EH part is open and different constant currents are applied to the exciting coils of the MR damping part by an external power supply. Figure 15 shows the force-velocity and force-displacement curves under the often-resonant frequency of 2.4 Hz, amplitude of 30 mm, and different currents of 0, 0.2, 0.34 (the maximum value in Table 3 obtained from theoretical modeling), and 0.4 A. It is noted that the damping part of the self-powered damper in this research are similar to that of the MR damper in [12]; therefore, the experimental results in Figure 15 are almost the same as those in [12]. From the figure, it can be realized that the damping force significantly depends on the external magnetic field. When there is no current applied to the exciting coils (\(I=0\)), the force-displacement curve is almost elliptic and the force-velocity relation is approximately linear, which indicate that the behavior of the MR damper is nearly similar to that of a pure viscous damper. However, the forces in this case are not concentrated in the vicinity of zero due to the presence of the frictional forces between the shaft and the O-rings. There is a slight difference  
Figure 14: Comparison between the simulation and experimental results of the generated power under 2.4 Hz frequency and 30 mm amplitude.  
Figure 13: Comparisons between the experimental and modeling results of the induced voltages of coils under 2.4 Hz frequency and 30 mm amplitude: (**a**) coil 1 and (**b**) coil 2.  
between the zero-field force obtained from the experiment and that obtained from the theoretical modeling. The average value of the zero-field force at the steady position (the position relatively far from the stroke ends) is 19.5 N, which is around 106% of the modeling value (18.4 N). This results from the inaccurate estimation of the friction force of the O-rings. It is believed that the accuracy would be improved with further tests on the O-rings.  
When the magnetic field intensity increases (\(l>0\)), the area of the force-displacement loop increases considerably, meaning that additional damping forces are produced. As shown in the figure, when the current of 0.34 A is applied to the coils, the maximum damping force increases from 19.5 to 76.2 N, which is around 95% of the simulated one (80.2 N). The difference mainly comes from the loss of magnetic field to the ambient and at the contact between the magnetic parts. However, when the current is increased to 0.4 A, no more significant change in force is found. Thus, it is proved that the damping force almost reaches to saturation at 0.34 A of the applied current.  
Moreover, as shown by the force-velocity responses in Figure 15a, the self-powered MR damper experiences two particular rheological regions: the preyield (rectangular mark) and postyield (the rest of the loop) regions. The preyield region exposes a strong hysteretic phenomenon while the postyield one almost exhibits a linear relationship. It is observed that in the preyield region, a sudden decrease/increase in value of damping forces corresponding to small velocities appears. This roll-off effect has been investigated in many researches.  
Figure 15: Experimental responses under a frequency of 2.4 Hz, an amplitude of 30 mm, and different currents of 0, 0.2, 0.34 (continuous lines from inside to outside), and 0.4 A (dash line): (**a**) force vs. velocity and (**b**) force vs. displacement.