Motors & Servo's

[Reloadron]

Today's Science Experiment. So I drove this little servo motor and got the stall current. I used a Fluke 87 to record the current. The average unloaded current was just about a constant 230 mA. Locked rotor stall current averaged 530 mA. I used a well regulated bench supply set for 5 volts with a 3 amp current capability. The voltage never dipped. I was a little surprised as I really expected a higher stall current. For a little toy motor I was surprised at the torque it delivers. While I didn't measure the torque the feel was strong. I thought I might strip it's little plastic gears but it hung together through several test. I really expected a higher locked rotor current, like about an amp but that didn't happen. I drove it using a little PICAXE because I had one sitting here on the table.

So a newly minted penny weighs in at 2.5 grams +/- .05 gram, that is good to know. I have a standard metric weight set that I seldom use.

Ron

 

[Sceadwian]

The stall current does seem low. Was the servo arm locked at it's mid point and was the test signal a full deflection? The torque (and subsequently current) are proportional to the error signal, so the further away from it's set position it's at the greater the torque/current will be. It's also partially related to the update rate. You'll increase torque/current if you increase the update rate from the standard 20hz to say 50-75hz, I've tried 100hz but some servo's start to tweak out at full deflection updated that fast because the pulse stretcher starts to overlap.

As far as the Penny goes that's a safe assumption but I wouldn't call it fact. The US mint on their website states that the weight of a penny is 2.500 grams. Typically speaking the zero padding means it's accurate down to that last digit however there is no tolerance listed on the mints page and a precision of .001 grams is a little tighter than I think is practical in a mass production scale even for new so the value I gave I think is a safe bet although I'd worry a bit about compound error with the tolerance being a guess.

I wasn't able to get a new roll of pennies at the bank today but I'll see if I can without any hassle if it comes up in the future just to satiate my curiosity. I tried a couple of random pennies I had it my pocket on the balance at work and didn't find any that were off by more than .03 and that's from pennies 10+ years old. The work balance is good down to .0005 grams and calibrated every 6 months.

At any rate well more than accurate enough for solid practical standards to a hobbyist at least.

 

[Reloadron]

Here is what I did, keep in mind this does not qualify as a more advanced science experiment. I used a small PICAXE chip to drive the servo with the following code:

init: servo 4,75 ‘ initialize servo
main: servopos 4,75 ‘ move servo to one end
pause 2000 ‘ wait 2 seconds
servopos 4,225 ‘ move servo to other end
pause 2000 ‘ wait 2 seconds
goto main ‘ loop back to start

I used the chip simply because I have several and a little kluge card I made for screwing around with them. I initialize the servo by driving it filly in one direction. Every two seconds it transitions from one extreme to the other. I let it run like that unloaded for awhile with the DMM taking the Hi and Low current and averaging them. That unloaded current ran pretty much inline with the data sheet. Also, as I mentioned I used a pair of Fluke 87 DMMs which are more than accurate enough. I monitored the voltage simply to make sure the voltage was not dipping, it remained a constant 5 volts. I wanted to be damn sure the applied voltage was a constant.

Next I allowed the servo to fully transition and locked the rotor by grabbing it. Oh yeah, I had the servo locked in a small panavise I like using. So yes, we had a command for full travel in the opposite direction while the servo pot would see the greatest delta. Again, taking the average current of several passes. I did this several times with pretty much repeatable results. I posted the highest average current I saw, so go figure. What? A little over twice the unloaded current. I expected about four times or more the unloaded current but that just didn't happen.

I am obviously getting old and comfortable with life. I can not remember using part of a day off work and doing something like this and finding it amusing. Then too, being winter and snow outside I guess I am easy to please. Quality beer is well stocked and wife makes great food so I guess as long as the furnace works life is good.

Ron

 

[Sceadwian]

I don't disbelieve your results but one last thing before it's chiseled in stone so to speak. How does the Fluke you're using handle pulsed DC current? I know that the pulse stretchers that a servo use are square wave at the same frequency of the servo update so what it's drawing is actually a 20hz square wave with a variable duty cycle (you can feel it with your fingers with a mild deflection it become dramatically obvious if you decrease the servo update rate.)

Got a scope and a current sense resistor you can throw on there to verify? I know it's anal of me, but I'm curious as to the Fluke's readings compared to the actual servo draw. I made a bad choice and parted out the last working scope I had due to space constraints so I can't test stuff like this right now, which is ironic because I now have an 800 square ft basement and more storage than Fort knox for equipment and bits.

 

[Reloadron]

Sceadwian, you're killing me here.

Yeah, I have a few scopes. I guess I could dig one out. Actually you make a good point as to the pulsed current. I never gave it much thought till you mentioned it. I am not sure how the Fluke DMMs treat that. Not sure what I'll use for a current sense resistor. Hell I have 10 amp, 50 amp and maybe some 100 amp shunts but nothing great for an amp. That sucks. Depending on noise I guess I could try a 10 Amp / 100 mV shunt. A 1 Amp would be 10 mV which I hopefully can see. If I don't get to this tomorrow (Sunday) it will have to wait as I have a busy 5 day work week on tap. While not worth much, the small analog meter on the power supply I used was bouncing around about .5 amp of the 5 amp range it shows.

These little science experiments generally consume Kathleen's kitchen table. This would not normally be bad but my computer tables are overflowing, my entire workbench is overflowing and in general I have one hell of a logistics problem. When my projects stray into her domain she calls it encroachment.

I'll try and get some pictures.

Ron

 

[Sceadwian]

I know I know, I'm sorry too! But my innate desire for data is overwhelming I hate assumptions and I worship statistical analysis =) one of my favorite phrases is 'the devil is in the details' toys aren't always 'toys' =) The fundamental nature of even trivial things is a deep interest of mine (and the cause of many headaches and lost sleep I might add)

The current shunt could be anything that will tolerate the power without heating significantly enough to effect the reading. I'd suggest a short stub piece of Cat3 cable. It's solid core of a known wire gauge and the voltage drop across the low resistance is subject to nothing more than a quick Google search for the effective voltage drop at the expected currents, you could tune one to any mv/amp range, up to even 100amps if you have the patience to wire strip and bind the ends, assuming again the nominal wire gauge is what is expected.

I have low value resistors sufficient to be used as shunts for 1amp easily but my basement is however in the process of a massive effort of organization. If I don't get my wifes side cleared out this weekend there'll be hell to pay, and aside from that I'm very quickly coming to terms with the fact that organization and simply having what is needed when it's required where you expect it to be is the only limiting factor in creating pretty much anything the imagination can come up with.

When we bought the house last year (around this time) I posted on here that it'd be about a year before I had my first major project going. I can tell you this... I was truthful, but it's not electronic in nature (aside from some mundane wiring), it's management all the way.

There's an adage, "When you have a hammer everything looks like a nail"

Try sweeping the floor with a hammer.

The **broken link removed**applies so fundamentally to even basic actions that I've become moderately enamored of understanding the fundamental minimum of what I need to do what I want. If I ever get to sleep tonight I'll be working on that tomorrow with some power tools to make a tool housing that doesn't require me to rummage around for 30 seconds to get the one tool I need.

I'll use a day of construction to save 10 seconds on something that is repeated 100 times a day, because it will save 15 minutes a day. In 1 year, the time saved will be 3.802 days vs the 1 day spent on construction.

I'd be grateful for pictures, I love this kind of sharing of how people do DIY type stuff at home.

I've put in a request to have the last few posts of this thread moved to a new thread in the members lounge to avoid contaminating to original posters thread worse than we already are (hopefully they are experimenting rather than posting)

 

[Reloadron]

Yeah, while somewhat on topic we sure as hell likely left the OP wondering. Who knows, maybe someone else will weigh in with constructive advice or criticism. This would have went much easier if the manufacturer provided a little more data in their data sheet. Interesting enough, I saw a post in another forum today where the poster was using one of these. Interesting because the manufacturer list the torque and speed at different voltages. If you are still awake go to bed!!!!!!!!!!!!!!

Ron

 

[ericgibbs]

Hi Sc,
It would be better to stay On Topic rather than ask Moderation to Create a New Thread and move some posts.

Moderation.

 

[jpanhalt]

@Reloadron,

What brand and model servo did you test? I probably have something similar and can confirm the stall current with a scope easily.

John

 

[Reloadron]

Hi John

The motor was one of these little Parallax motors made by Futoba for Parallax. All of this discussion spawned from another thread. I have seen a few assorted data sheets on this little motor which seem to vary. The question was stall current or locked rotor current.

When I tried getting stall current I locked the little motor in a panavise and ran it. I drove it with a PICAXE from 0 to 180 and back to zero at two second intervals. I used two Fluke 87 DMMs, one for current and one for voltage. The motor was powered with 5 volts. The current meter was set for Min Max recording. I ran it several times, maybe about 5 transitions unloaded then 5 transitions with the rotor locked down. Then I averaged the Max currents from all the runs I did. I was getting currents around .5 amp with an average of about 530 mA.

Any information you could shed would be most welcome.

Ron

 

[jpanhalt]

That looks very much like a Futaba S148 servo based on its dimensions and torque, which is its standard utility servo. I happen to have one and will test it later this morning.
John

 

[Reloadron]

That would be great John and much appreciated. Curious to see what numbers you get.

Thanks
Ron

 

[jpanhalt]

Results from testing Futaba S148 servo

Setup:
1)TEK TDS210 oscilloscope
1:1 probe
50 mV/division
25 mS/division sweep
2)Agilent E3630A power supply
5.00 volts
Current is limited to 2.5A
Display showed V and A
3)Current sense: 0.1 Ω Ohmite 1% (i.e.,10 mA/mV)
4)Servo driver: old 555-based unit

Results:
1) Condition: idle, mid-range of motion (no torque on servo)
Scope current: buried in noise
Supply current: 20 mA

2) Condition: full CCW against stop with driver
Scope current: 500 mA (small down pulses)
Supply current: 500 mA

3) Condition: full CW against stop with driver
Could not do; was beyond drive capability of servo driver.

4) Condition: Unloaded back and forth sweeping
Scope current: about 460 mA peak, settled quickly to noise level
Supply current: 140 mA indicated

5) Condition: Mechanically locked rotor, mid-range position
Scope current:initial spike to 680 mA observed on 10 mS/division scale rapidly settled to 500 mA
Supply current: 480 mA

Conclusion: Consistent with Reloadron's data

John

 

[Reloadron]

Can't thank you enough there John. Interesting how it worked out. I was getting ready to start looking into my stash of power resistors to come up with a shunt.

Thanks
Ron