Servo Testing


 
J

JakeBrown

TVWBB Member
Just built the Heatermeter, and I'm testing the servo at the bench (not cooking).

How do I test the servo is functioning properly?
What is it expected?

I can adjust the fan output successfully. The servo sorta makes a noise, and it might have moved a tiny bit once. It doesn't do anything that I think it should be doing, like pulse open/close within a few seconds.

Are these servo's easy to damage by manually moving the gears?

Suggestions to troubleshoot?

Heatermeter Version 20141003B
Servo Tower Pro MG90S

PID Configuration:
hdahNWY.jpg
 
You have your servo set to move from 800 to 800...... not much action there! LOL
Change the second number to about 2000 and you should see it move about 80% of its range of motion...
 
I'm guessing mine doesn't work. It pulses a tiny bit, wants to move, but acts like it is stuck and won't move. If I try to move it manually, it will fight, make a tiny sound. If I move it enough eventually the sounds stops, and I presume that's where it was trying to get to.
 
Possibly a bad servo but also make sure you check your wiring. There should be a constant 5V between 5V and GND, and a multimeter should measure something like 0.2V on the signal line (5V with a 0.8/20 duty cycle).
 
JakeBrown said:
I'm guessing mine doesn't work. It pulses a tiny bit, wants to move, but acts like it is stuck and won't move. If I try to move it manually, it will fight, make a tiny sound. If I move it enough eventually the sounds stops, and I presume that's where it was trying to get to.
Click to expand...

Have you read my post and changed the SPD numbers in your setup??? If you set your SPD at 800-800 your settings are telling the servo NOT to move.....
 
I had tried a bunch of different SPD numbers before I took the screenshot (I can't remember what the default is). During this test when I did the screenshot I was wondering if I was misinterpreting the part about "start with 500us on each side of that center", and thinking the "-" in the UI is a minus, not a "thru", and tried 800 (instead of 500) at the time of the screen shot.

I now have 800-2000, and the servo still won't move. The servo makes a sound, and sounds like it is trying to move, but can't. When I manually move the servo through the arc, it seems like I get to one point and the sound stops. I presume this stop has something to do with the SPD.

I'll check the signal voltage later. Right now I presume the signal is working since the servo appears to be "pulsing" (but not moving).
 
I would leave your servo out on the table for now until you get it moving, so doesnt matter how or if you attach the servo horn. What housing are you referring to btw?
 
RalphTrimble said:
I would leave your servo out on the table for now until you get it moving
Click to expand...

okay. Did this, and now I have something working! I did this before, but probably when I had the 800/800 settings. Hate overlooking the obvious.
Or maybe it worked because I pulled on the axle and engaged something.

What I've found is this servo doesn't have much torque at all. I have to adjust the housing real loose.

One problem still remains. When the servo arms reaches almost to the point where it wants to stop, it buzzes. If I help it move to it's resting point, then it stops buzzing.
 
If you force the servo into a tight mounting it will bind.... its not impossible that you got a dud MG90s, they vary a LOT and sometimes I wonder if they aren't all knock off's... but, if you set the SPD settings telling the servo to land beyond its range of motion it will move in that direction until it can't any more and sit there and vibrate at you, saying, "really? you told me to do this?" LOL If it does it in every position then the servo is probably just lame... assuming you don't have a wiring issue or some issue with soldering or whatever on your HM board.
The MG90s can be hit or miss, well, more hits than misses but you can get a dud... If you look at the price for a single MG90, the price for 4 of them is usually about twice as much. I would encourage anyone building a servo damper to just buy the lot of 4, in case you get a dud or burn one up when you really need it working.
 
I get the buzzing from time to time. But, it could mean that you dont have the settings set correctly. What I usually do is start with it open, as its easy to see when you have both pieces aligned correctly. Change the 2nd number to about 2300 or 2400 and with the servo out still, turn the blower on, so that its at 100%.

The servo should move and stop, that stop is its maximum. Now take the 2300 or 2400 and move it down by 50 at a time, until you see the servo start to move. That will be your maximum number your servo will except. I like to give it some extra room a bit so I will lower the number down about a 100 and is around 2100 or 2200. With the blower still running at 100%, put the servo back into the rotodamper or whatever you are using and make sure it at the maximum opening and tighten it down, just enough so that it can still move freely.

Now, turn off the blower and the servo should find the minimum setting. Adjust it so that when it reaches the minimum your damper just overlaps and closes completely the opening, that should your lower setting. You may or may not get some buzzing from time to time, they are not the most expensive servos in the world. I found a shop near that has servo that have all metal gears and housing, a small servo is in the 30 to 40 dollar range, a little expensive as I have not had a mg90 servo go bad yet during a cook.
 
Yah, that's the thing... they're so cheap... get the 4 pack hope you don't need the extra 3 (but you will, eventually)
The thing about calibrating the MG90s, it has a tiny potentiometer inside it (I've dissected) that rotates with the shaft and indicates position to the controller. They are not all calibrated exactly the same so you really can't give out a valid set of SPD limits, they vary from unit to unit. With the roto damper you have about 15-20 degrees rotation beyond what you need with the MG90. You need to set your SPD numbers so you have a little bit more rotation available on both sides of 0% and 100%. But in a case where you are troubleshooting a HM board and servo setup choose safe SPD numbers like 1000 - 1500 which should be well within the range of motion for any MG90s...
 
RalphTrimble said:
if you let it set there and struggle I'm pretty sure you could kill it...
Click to expand...
I've done this!

Always leave a little extra at the end, that is don't keep increasing the numbers until it is right on the edge of jamming. They seem to have a small amount of tolerance so what works in the house might start jamming up against the end once you get it installed. By that time you've left it out there for a day with it trying to turn that last little bit and it's all fried.

The default settings are 1500 which is roughly centered then 500 on each side of that so 1000-2000 which seems to be about 70-80% of the full range of motion so it's a pretty safe default.
 
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