"The Better Mousetrap" Servo Tester

Bill Henning

Hi guys,

I've decided that my new servo tester (based on RoboPi) deserved a page of its own on my site... as it is taking a life of its own, beyond its original mandate (helping me calibrate HexPi)

Here is its build log page:

http://www.mikronauts.com/the-better-mousetrap-servo-tester/

I'd love feedback from y'all! (and answer any questions you may have)

erco

And thus one complex project begat the next... Nice writeup, Bill! Looking forward to seeing HexPi in motion.

Bill Henning

Thank you!

I hope to get enough "Real Work" (tm) done today to start wiggling HexPi's legs later today

erco wrote: »

And thus one complex project begat the next... Nice writeup, Bill! Looking forward to seeing HexPi in motion.

Bill Henning

The servo jitter issue I was experiencing with the better mousetrap servo tested turned out to have two causes:

1) I was including the PWM object from the wrong directory, it was configured for a 80MHz prop, instead of a 100MHz prop

2) I needed to add a lot of capacitance to the MG-995's to calm them down, I suspect they drive the microcontroller in the servo off the servo power without filtering or regulating it! I'll need to order some bigger electrolytic caps for the lab.

More pics with scope captures at

http://www.mikronauts.com/the-better-mousetrap-servo-tester/

Right now I am packaging up the new RoboPi firmware / object / Pi libraries, will upload them to my site in an hour or two.

After that, it will be time to connect TBMST to HexPi and wiggle some legs!

erco

Bill Henning wrote: »

I needed to add a lot of capacitance to the MG-995's to calm them down, I suspect they drive the microcontroller in the servo off the servo power without filtering or regulating it! I'll need to order some bigger electrolytic caps for the lab.

Internal or external filter caps?

Bill Henning

I added 3000uF to 5000uF externally, I don't think there is enough room in the servo housing to add that much.

I'll be doing more experiments, I think I could fit maybe a 47uF + 100nF cap internally. I'll report my findings.

erco wrote: »

Internal or external filter caps?

erco

C'mon Bill, if the Chinese can fit caps in small places, surely you're up to the task!

https://images.search.yahoo.com/images/view;_ylt=AwrTcXeuQY1ThX0ATsiJzbkF;_ylu=X3oDMTIyYnR1Z290BHNlYwNzcgRzbGsDaW1nBG9pZANjYTM3YzI2ZGFjYTc4ZWM5OTA2OWMyN2M5N2Q5YzIwOQRncG9zAzIEaXQDYmluZw--?back=https%3A%2F%2Fimages.search.yahoo.com%2Fyhs%2Fsearch%3F_adv_prop%3Dimage%26va%3D%25C3%25A7hina%2Bcapacitor%2Bfake%26fr%3Dyhs-vz-vz%26hsimp%3Dyhst-vz%26hspart%3Dvz%26tab%3Dorganic%26ri%3D2&w=800&h=514&imgurl=img401.imageshack.us%2Fimg401%2F9649%2Fchinacapacitorek1.jpg&rurl=http%3A%2F%2Fcapacitor.web.fc2.com%2Fspecial3.html&size=63.2KB&name=http%3A%2F%2Fimg401.imageshack.us%2Fimg401%2F9649%2Fchinacapacitorek1.jpg&p=%C3%A7hina+capacitor+fake&oid=ca37c26daca78ec99069c27c97d9c209&fr2=&fr=yhs-vz-vz&tt=http%3A%2F%2Fimg401.imageshack.us%2Fimg401%2F9649%2Fchinacapacitorek1.jpg&b=0&ni=21&no=2&ts=&tab=organic&sigr=11atlhoar&sigb=14d6k0hd2&sigi=11mjkfs0e&sigt=11tdgh5mb&sign=11tdgh5mb&.crumb=wzjXR/q4zGl&fr=yhs-vz-vz&hsimp=yhst-vz&hspart=vz

Bill Henning

Wow... cool image! Terrible that some would sell fake parts like that.

I am going to tear down one of the servos - starting with the kentucky fried one - and see if they really left off filter caps!

erco wrote: »

C'mon Bill, if the Chinese can fit caps in small places, surely you're up to the task!

https://images.search.yahoo.com/images/view;_ylt=AwrTcXeuQY1ThX0ATsiJzbkF;_ylu=X3oDMTIyYnR1Z290BHNlYwNzcgRzbGsDaW1nBG9pZANjYTM3YzI2ZGFjYTc4ZWM5OTA2OWMyN2M5N2Q5YzIwOQRncG9zAzIEaXQDYmluZw--?back=https%3A%2F%2Fimages.search.yahoo.com%2Fyhs%2Fsearch%3F_adv_prop%3Dimage%26va%3D%25C3%25A7hina%2Bcapacitor%2Bfake%26fr%3Dyhs-vz-vz%26hsimp%3Dyhst-vz%26hspart%3Dvz%26tab%3Dorganic%26ri%3D2&w=800&h=514&imgurl=img401.imageshack.us%2Fimg401%2F9649%2Fchinacapacitorek1.jpg&rurl=http%3A%2F%2Fcapacitor.web.fc2.com%2Fspecial3.html&size=63.2KB&name=http%3A%2F%2Fimg401.imageshack.us%2Fimg401%2F9649%2Fchinacapacitorek1.jpg&p=%C3%A7hina+capacitor+fake&oid=ca37c26daca78ec99069c27c97d9c209&fr2=&fr=yhs-vz-vz&tt=http%3A%2F%2Fimg401.imageshack.us%2Fimg401%2F9649%2Fchinacapacitorek1.jpg&b=0&ni=21&no=2&ts=&tab=organic&sigr=11atlhoar&sigb=14d6k0hd2&sigi=11mjkfs0e&sigt=11tdgh5mb&sign=11tdgh5mb&.crumb=wzjXR/q4zGl&fr=yhs-vz-vz&hsimp=yhst-vz&hspart=vz

Bill Henning

I went back and re-tested one of the "jittery" servos with the GWS tester - and it was calmer, even though its 5V was just as noisy as TBMST.

The only real difference I noticed is that it provides a 5V servo control signal, where TBMST provides 3.3V servo control signal, current limited by a 2k4 resistor.

I'm going to try TBMST with less current limiting, and perhaps with conversion to 5V signal. (pics @ http://www.mikronauts.com/2014/06/06/more-tbmst-testing/)

Duane Degn

I've had several servos not work well when I used a resistor on the signal line. I generally don't use any resistor on the signal line in my Propeller/servo projects.

Bill Henning

Thanks Duane, I'll try that too.

I went to a local electronics supplier today and got some larger electrolytic caps to play with.

4x 3300uf
4x 4700uf
4x 6800uf

I'm like a dog with a bone - I will not let the cheap servos defeat me!

Duane Degn wrote: »

I've had several servos not work well when I used a resistor on the signal line. I generally don't use any resistor on the signal line in my Propeller/servo projects.

Bill Henning

I had an interesting thought... would the GWS MT-1 servo tester still behave better (ie almost no jitter) if it was powered the same way that TBMST was?

The answer is NO!

It is the the servo power supply!

Now I just have to come up with a clean enough power source, and I can resume my HexPi build!

More details at:

http://www.mikronauts.com/2014/06/09/tbmst-test-results/