Saturday, January 30, 2016

Low Cost Servo Motor Tester from eBay

For about a $1.50 USD eBay sellers will ship out to you the servo tester pictured above.   It's an amazing value to let you verify that the cheap servo you also got from eBay even works.  It's also great to help understand the MIN, CENTER, and MAX positions for any servo based RC or uC project. 
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Basically, servos motors are controlled by receiving pulse train signals.  The eBay servo tester is capable of producing this required pulse train.  Jameco has an easy to follow tutorial.  The image below shows that a 1mSec pulse width sends the servo full left.  1.5mSec centers the it.  2mSecs move the servo full right.  Jameco explains it all very well.
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The servo tester has three setting:
     MANUAL: allows the servo position to be adjusted with a knob.
     CENTER:   sets the servo to it's center position
     SWEEP:     continually sweeps the servo from it's MAX to MIN position (forever)

Connection is simple.  One side of the servo tester is for power.  The other side is where the servo under test conveniently plugs in.
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So.... let's use a Tektronix MSO5104B oscilloscope to look at the output of the servo tester and see what the heck is going on.  In the screen shot below the servo tester control knob is set to full left (minimum).  Sure enough, the pulse width as measured by the scope is 915uSec.  That's pretty darn close to the 1mSec width in the drawing above.
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With the knob set to full right (maximum) the scope cursors show a pulse width of 2.14mSec; pretty close to the ideal 2mSec in the drawing.
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Using the MSO5104B oscilloscope to zoom out on the signal we can see that the pulses repeat steadily every 20mSec.  If you check the tutorial at Jameco you will know that is exactly what is expected.
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Conclusion:  Splurge the $1.50 USD and treat yourself to this handy device.  It really helps to verify that a servo is actually working and in setting the movement limits on your project.  Below are two short videos of the servo tester in action.
 

 
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Here a short video where I use a Tektronix AFG3252C to send the control pulses to the servo.  The AFG3252C has infinitely more flexibility, but cost a few dollars more than the eBay option.
 
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Thanks for stopping by.  Build something.


Wednesday, January 13, 2016

5VDC Boost Converter Characterization

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Boost Converters are a type of switch power supply that magically turns a small DC voltage into larger DC voltage.  Learn about how that magic is preformed here.   Used properly they are great, but the magic of a boost converter does have it's limits.
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For the measurement setup a Keithley 2230 power supply is set to 1.5VDC (think AA battery voltage) to act as V(in) for the boost converter.  A Keithley DMM7510 HiRes multimeter is used to measure the ripple on V(out).  A Keithley 2450 SourceMeter SMU is used to simulate a load on the boost converter in static and swept mode. The Keithley 2450 also produces a V(out) graph under the swept load.

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The video shows how the 5VDC boost converter starts performing poorly at loads close to 100mA.  The output voltage begins to drop and ripple frequency increases as the boost converter works harder under the higher loads.  The Keithley 2450 is set up to graph the boost converter voltage output as it simulates the swept load.
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[EDIT: February 20, 2016]: We got a comment that the boost performance could have been effected by not having a constant 1.5VDC input into the converter during the entire load sweep due to lead resistance.  Turns out the comment was correct.  We retested the same boost converter with 4-wire measurement connections at both the INPUT and OUTPUT.  This 4-wire setup reduces any errors due to lead resistance.  The video below shows the improved result.  Measurement setup is everything!!!
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