Servo Current Comparison - Main Page

By Tom Paden

A recent thread on the EFLIGHT Mailing List where someone wondered about the newer S90 servos and how they compared to other servos in current consumption is the reason I conducted these experiments. I did four simple tests using an FMA S90, Airtronics 94461 and Airtronics 94141 servos. Please note that this is neither an endorsement nor a condemnation of FMA or Airtronics. I obviously use them both. I used these servos because they were lying around, waiting to be installed in my latest project(s). Another aspect to note: the sample size is much too small! It's just a quick test to get a feel for how these servos compare to each other. Your mileage will vary!

Equipment used for the tests:

  1. Airtronics Radiant Transmitter and associated receiver. My latest radio. Receiver battery fully charged before each of the tests.
  2. AEMC SL261 clamp-on, Hall-effect current probe. This probe converts current sensed via a clamp-on jaw, into a voltage that can be displayed on a multi-meter, oscilloscope, etc. It has two scales (100mV/A and 10mV/A). All readings were done on the 100mV/A scale. While this is not an inexpensive probe, it is designed for measuring higher current levels than encountered here, so the noise seen on the displays near the zero current level is due to the probe's inherent design. This shows up as a fuzzy line that obscures the servo's idle (servo motor off) current. The probe also showed a zero-drift that meant I had to re-zero it before any readings were saved.
  3. Tektronix TDS-420A oscilloscope. This is a digital scope that has built-in waveform measurement capability. The various servo waveforms were saved and displayed together to allow visual comparisons. Then the scopes' display was saved on the built-in floppy drive as a bmp file.

Picture format:
In all pictures, the top trace is the signal from the receiver to the servo under test (just to show as a reference). The next trace down is the S90 servo. Below it is the 94461 and the bottom trace is the 94141. The numbers on the right hand side of the picture measure the average (labeled 'Mean') current for the servo waveforms. The averaging occurred over the entire waveform, as seen in these pictures. They are in millivolts with the conversion to current (100mV=1A) shown immediately below. Ref1 is the stored S90 waveform. Ref2 is the stored 94461 trace and C2 is the 94141. With the scope set to 100mV/division for the servo waveforms and the current probe set to 100mV/A, 1 vertical division (on the lower 3 waveforms) equals 1 Amp. I added comments to the picture using MS Paint to help clarify things.

Continue to Test #1

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