# Complementary Cooperation of Dual Power Circuits to Drive Active Magnetic Bearings
## 5 Experiments  
In this section, a few experiments are undertaken to examine the feasibility and capability of the proposed DC\({}^{2}\) in terms of tracking capability and ripple suppression. The photographs and the corresponding layout of DC\({}^{2}\) are shown in Figure 12. Besides, the experimental test rig, shown in Figure 13, is constructed. The schematic diagram of equipped facilities to conduct the corresponding experiments is depicted in Figure 14. The load to drive, with respect to DC\({}^{2}\), is any one of the axial AMBs of the 5-DOF rotor/active magnetic bearing (Rotor/AMB) unit. The electrical resistance and the nominal inductance of this AMB coil are 0.9 \(\Omega\) and 7 mH respectively. Two sets of direct current (DC) power supplies are employed. The first set, _ITech IT6332A_, is employed to provide +5 VDC/+15 VDC for DC\({}^{2}\). The other set, _Keithley 2268-40-21_, is applied to drive the AMB coils by +40 VDC. The DSP chip, _Texas Instruments_ TMS320F28335, in which the PWM-TPs are embedded, receives the signal from the CS (Current Sensor), \(\hat{u}_{AMB}\), and then generates the two control commands, i.e., _PWM#1_ and _PWM#2_, to DC\({}^{2}\). Figure 11: Output currents and tracking errors by two-state power amplifier, three-state power amplifier and DC\({}^{2}\) (dual cooperative drive circuit). (Reference signal: sinusoidal wave with 500 Hz and amplitude 0.1 A). Figure 12: Photograph and layout of dual cooperative drive circuit (DC\({}^{2}\)). Figure 13: Experimental setup. For the experiments undertaken, the desired output, a sequence of step-wise reference, is preset at the DSP chip. The amplitude and the duty ratio of the step-wise reference are set as 1 A and 50% respectively. The tracking performances under DC\({}^{2}\), DDM and ADM are shown in Figure 15. From Figure 15, it is observed that at the beginning of tracking the step-wise signal, i.e., starting 2.5 ms to 2.8 ms, the DC\({}^{2}\) operates by DDM. Under DDM, the duty ratio of _PWM#1_ (with respect to the Observation Point _OP#1_) is adjusted to its maximum, i.e., 95%, so that the output current is able to track the desired output as fast as possible. This is the major advantage of DDM at _transient state_ of the rotor position regulation dynamics. However, _M#1_ and _M#2_ operate both in on-off fashion synchronously under DDM which unfortunately induces severe current ripples embedded in the output current. In comparison, unlike the digital control command to _M#2_ under DDM (with respect to _OP#2_), the control command to _M#2_ under ADM (with respect to _OP#2_) is in analog manner. Accordingly, as soon as the output current to the axial AMB coil gets close to the desired quantity, DC\({}^{2}\) is switched to ADM. As DC\({}^{2}\) is switched from DDM to ADM, the mechanism of _M#2_, which is in on-off fashion under DDM, is changed to the voltage-controlled fashion to refine the output current more precisely. In addition, at _steady state_ of the rotor position regulation dynamics, compared with the duty cycle of _PWM#1_ under DDM (with respect to _OP#3_), the duty cycle of _PWM#1_ under ADM (with respect to _OP#3_) is reduced to a relatively low degree. As a result, the fluctuation of output current is greatly limited under ADM (see _OP#4_) so that the amplitudes of current ripples can be certainly much reduced. Figure 14: Schematic diagram of equipped facilities to conduct experiments. To be explicitly compared, the corresponding tracking performances under DDM, ADM and DC\({}^{2}\) are depicted by Figure 16. It is observed that the rise time under ADM, i.e., 1.08 ms from \(A_{0}\) to \(A^{\prime}\), is much longer than that under DDM or DC\({}^{2}\), i.e., 0.40 ms from \(A_{0}\) to \(A\). That is, the response of ADM is relatively slow so that ADM is inappropriate to be applied at _transient state_. On the other hand, the amplitude of the peak-to-peak current ripples under DDM is about 50 mA while it is merely 19 mA under either ADM or DC\({}^{2}\).