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7 – ELECTRONICS step

7 – ELECTRONICS step response measurement of the uncontrolled MSTS (fig. 6.5). The stroke amplitude amounts to f ≈ 1.2 mm and is slightly below the required stroke. Therefor a peak current of IC ≈ 1.3 A must be applied, what can be deduced from the upper curve. As stimulus, a rectangular LVTTL signal was applied with a frequency of 10 Hz and a duty cycle of 20 %. The controller only consists of a P-contribution, what surprises at the first glance when considering the stroke measurements. Now the fact of the dead time in the feedback path, caused by the laser triangulator, must be discussed. This dead time is directly discoverable as the time shift of the two measured curves. There- fore, the set point (IC) spurts to around the half amplitude and then stays constant for ≈ 2 ms until the feedback signal reaches the comparator. The time constant of the Helmholtz coil can be calculated to τL= L/R ≈ 22 µs and has an insignificant influence to the blades open/close time. The control variable x, which corresponds to the blade stroke f, shall follow the set point as accurate as possible to reduce the error signal e to a minimum. Here, just a proportional controller 3 was applied, what typically does not suffices for well control- ling a control path with PT2 behaviour. Hence, undetermined capacitives occuring in the whole control loop optimize the controller, that the required behaviour just can be achieved. Further investigations must be performed for the controller design when an adequate sensor will be applied. 3 Note that the P-contribution was manually adjusted with help of a potentiometer until the required behaviour was achieved. 56

TS P-Control 25.05.2008 7 – ELECTRONICS Figure 7.2: Results of the controlled MSTS DM, which fulfills the required switching mode. The left part shows one cycle measured in t = 20 ms/DIV, whereas the right part is zoomed in to t = 5 ms/DIV. The lower curves of both parts show the stroke of the blade measured with the optoNCDT 1700 represented in f = 0.4 mm/DIV. The upper curves show the controlled current IC measured in 0.5 A/DIV. H:\MERTIS_HUA\MSTS DM Control Electronics\Measurements 57

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