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Now that HexPi has all his hair servo extensions, I’ve decided I need to make a better power distribution board for TBMST than my first effort (shown below) for the NiCd R/C battery packs.

This time, I plan to use an ATX power supply with separate power feeds for servos 1-8, 9-16 and 17 & 18 as the next experiment. It will be interesting to see if the MG-995’s behave better with more instantaneous current being available.

I also made two modified servo extension cables that allow me to replace the power from the servo connector with external power so I can use the normally Pi 5V powered servo connectors with the ATX supply.

RoboPi can already accept separate power sources for the two servo banks (P0-7 and P8-15) on the svt2 and svt3 headers.
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 (click on image for larger version)

I’ll be posting the results of this latest experiment as they become available!

I finally had time to work some more on HexPi.

I added 50cm extensions to all the servos, cleaned up the wiring, and gave HexPi three pairs of new shoes!

(click on the image for a larger photo)

For about what I just added, and the whole build log, please click on the HexPi link below:

Related Links:

• HexPi – Hexapod RoboPi + Raspberry Pi Robot: https://www.mikronauts.com/robot-zoo/hexpi-hexapod-pi-robot/
• The Better Mousetrap Servo tester: https://www.mikronauts.com/the-better-mousetrap-servo-tester/
• RoboPi advanced robot controller for Raspberry Pi: https://www.mikronauts.com/raspberry-pi/robopi/

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Or to put it another way – I wiggled his front two legs with “The Better Mousetrap Servo Tester”!!!

(click on the photo above for larger image)

If you are interested, here is a YouTube video (HD) showing the test:

Related Links:

• YouTube link: https://youtu.be/9tLrE_4NFAo
• HexPi – Hexapod RoboPi + Raspberry Pi Robot: https://www.mikronauts.com/robot-zoo/hexpi-hexapod-pi-robot/
• The Better Mousetrap Servo tester: https://www.mikronauts.com/the-better-mousetrap-servo-tester/
• RoboPi advanced robot controller for Raspberry Pi: https://www.mikronauts.com/raspberry-pi/robopi/

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Today I tried using an inexpensive LM2569S power module to step down the 7.2V NiCd RC battery pack to 5V.

After verifying that the Tower Pro MG-995 still jittered with the GWS MT-1 servo tester (powered from the step down module), I tried four different servos to see if they would also jitter. NOPE. 

(Remember the Kentucky Fried Servo? I guess that showed their level of quality control!)

The four well behaved servos were:

• Parallax Standard Servo
• MOX-M38N Standard Servo
• MOX-M9A Micro Servo
• and surprisingly, Tower Pro 9g Micro Servo

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I guess in order to get to the bottom of *WHY* these MG-995’s jitter, I’ll have to dis-assemble and reverse engineer one. Maybe I will test all 49 working servos and determine their jitter quotient, and just use the 18 quietest for HexPi 🙂

Sometimes it really helps to take a step back, and think through whatever problem you are working on.

I’ve been having significant problems quieting the jitter – the servos would shake rattle and roll on the table – when using inexpensive MG-995 servos.

That nice big spike showed a clear power supply issue. I was powering the servos from a NiCd battery pack, using a few diodes to drop the servo voltage to 5V.

So I kept adding electrolytic capacitors until I cleaned up the power.

Adding 3000uF of filtering helped considerably, but the servos were still more jittery than when tested with the GWS MT-1 servo tester.

The GWS MT-1 servo tester provided a 5v servo control signal. In order to eliminate the possibility that the 3v3 signal from TBMST was the culprit, I divided the MT-1 servo testers servo control signal down to 3V … and it still performed admirably (ie almost no servo jitter).
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Next, I thought let’s see what would happen if I powered the MT-1 with the same supply (the NiCd R/C battery pack).

THE MT-1 JITTERED THE MG-995’s LIKE THE  TBMST WHEN POWERED LIKE TBMST!!!!

I feel much better now that the problem is clearly identified. I just have to make a really good, fast response (to increased load) power supply for HexPi, and I won’t have servo jitter issues. As a bonus, when looking at the pulse generation software, I did increase the timing resolution 🙂

I was 99% sure that the issue would turn out to be needing really clean power for the MG-995’s – the scope traces of the issue don’t lie – but I was surprised that heavy filtering of the power supply was not enough. In hind sight, the large capacitors probably could not handle the “instantaneous” load changes fast enough.

I suspect the reason the MG-995’s do not have a voltage regulator for the microcontroller in them, nor for the potentiometer used for feedback. Therefore, moving the motor draws more power, dropping the power to the microcontroller and the feedback pot… thereby messing with the ADC results, making the microcontroller over-correct for non-existent position changes, producing jitter.

Looks like I’ll be able to get back to HexPi soon… I just need to clean up the servo power.

I have some nice DC-DC converter modules, time for me to find out which will work best for TBMST.

Stay tuned!