Difference between revisions of "Characterizing the response of a solenoid"

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[[Image:Resonance.JPG|right|thumb|500px|Resonance]]
One resonant frequency of the setup is at 20Hz

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These scope traces show the response just before, just after, and at the resonant frequency of 20Hz


[[Image:15V15Hz50%(JBResonance).JPG|left|thumb|300px|Just Before Resonance]]
[[Image:15V15Hz50%(JBResonance).JPG|left|thumb|300px|Just Before Resonance]]

Revision as of 23:33, 11 February 2009

Original Assignment

Your job is to characterize the response of a solenoid to on-off pulsed forcing. The solenoid should be equipped with a return spring so that the solenoid returns to a home position when the coil is de-energized.

You should provide the user with the ability to control three parameters of the on-off pulsed forcing: the frequency of the on-off pulsed forcing, the duty cycle, and the amplitude (the voltage or current applied to the coil when it is energized). You will mount an accelerometer on the shaft of the solenoid and use an oscilloscope to simultaneously look at the control voltage to the solenoid and the acceleration of the solenoid. Try frequencies from 10 Hz to hundreds of Hz. For what frequencies, duty cycles, and amplitudes does the solenoid shaft hit stops, and for what values do you get approximate square wave acceleration profiles? Summarize your results in text and post clear images of the oscilloscope for a few representative choices.

Note: your PIC may not be involved in this assignment at all. You could use a function generator to generate the frequency and duty cycle you want, and use the amplitude to (perhaps) control a transistor operating in the linear regime to deliver different amounts of current to the solenoid. It may make sense to always energize the coil a little bit, even in the "off" state, so that the return spring is always in a bit of tension.



Overview

Circuit

Accelerometer: [[1]]

The Accelerometer Rig
Solenoid Driving Circuit
Set up for the lab


Oscilloscope Graphs

The upper trace is the accelerometer, the lower trace is the voltage across the solenoid. On the accelerometer trace 0g corresponds with 2.5V, +18g is 0V and -18g is 5V.

3 Hz Driving


Region A-B-C-D The accelerometer reading near A indicates a downward acceleration. This coincides the falling edge of the square wave, and the solenoid becomes unpowered which indicates some unusual effect.

The region above the baseline at B indicates an upward acceleration caused by the return spring. The subsequent oscillation from B-C-D is typical damped oscillatory motion. Since the solenoid is unpowered in this region the result makes sense.

Region E-F-G Since E coincides with the rising edge of the square wave and the solenoid is powered the upward acceleration at E is also unexplained. The region E-F which shows a downward acceleration is consistent with the solenoid pulling the plunger down with a positive jerk. At point F, maximum downward acceleration the return spring is compressed enough to induce negative jerk. The region near G of upward acceleration (above baseline) is the result of the plunger overshooting equilibrium between the spring compression and the solenoid until the solenoid can retake control and stop the oscillation.


These scope traces show the erratic vibrations at low frequencies. At these frequencies the solenoid plunger hits both end stops. The erratic acceleration is due to the spring becoming completely unloaded as the plunger exits the solenoid and the subsequent recontact between the plunger and spring.

10 Hz Driving
25 Hz Driving
50 Hz Driving


These scope traces show the effects of varying the duty cycle with constant amplitude (24V) and frequency (200Hz). At this higher frequency the plunger retains contact with the return spring and does not hit either end stop. Note that at 75% duty cycle the acceleration profile approaches a square wave.

25% Duty Cycle
50% Duty Cycle
75% Duty Cycle


These scope traces show the effects of varying driving amplitude at constant frequency (300Hz) and duty cycle (50%)

15 V Amplitude
20 V Amplitude
24 V Amplitude


Resonance


These scope traces show the response just before, just after, and at the resonant frequency of 20Hz

Just Before Resonance
Resonance
Just After Resonance