Build a two-at-once test jig:
Connect the two side by side separated by a convenient handle. This could be done as simply as by using a few pieces of wood or plastic and some tape. Do not use metal.
Ensure that there is enough separation that they do not interact. The width of a hand between them should be very adequate.
Arrange "handle" such that when handle is held each shaker is orientated in the same way that it would be if it was being held instead of the handle. This is so that the hand shaking motion shakes both at once in much the same way as it would if they were being held directly.
Wire outputs via wires and a rectifier to two identical loads. This could be a large capacitor (maybe a super cap, or a resistor with an oscillocope monitor or an LED or whatever.
Compare both at once:
Operate the pair in differing ways and note the outputs. This allow direct comparison without changes in shaking pattern, speed, stroke etc having to be standardised.
Use of a large cap as load with low leakage allows voltage to be slowly "pumped up" over time - a clear winner should emerge.
Question - Julio asked
- If I could be sure that the same input energy would be used on each test, how could I use the oscilloscope to measure the efficiency
Use a dual channel scope.
Connect one side of each generator to ground.
Connect a load resistor of the same value across each output.
Connect a scope probe / channel to each output.
Shake it all about.
Observe two waveforms on scope.
Make decisions about which looks "best".
Try different resistor values.
For DC comparisons, rectify to DC and use output capacitor perhaps 100 uF per channel.
My "large capacitor" charging test is arguable one of the best as
- it can be used to simulate battery charging,
- you can see the result at different voltages
- and its easy to measure with just a voltmeter (or two voltmeters).
You can go from eg
0 to 3 V
or 4V to 5V etc
as suits you (to eg simulate battery charging).
You can use a power supply and battery to precharge the cap.
One at a time testing:
With a large enough capacitor you can test one shaker at a time with a reasonably good chance if comparison.
Say it takes about 1 minute to go from Vstart to Vend.
Test each in turn at the same level of shaking and see which is faster or which charges higher.
You should be able to match tests to within about 10%.
Try it several times on the same shaker to see.
Then anything outside that difference is probably due to relative performance.