Hot Servos

[Acetronics2]

Hi, The regulators are the correct way. I did make that mistake in the original design but luckily realised before i etched the PCB and plugged the servos in. They are running from a 12V supply so they probs wouldnt last long if the regulators were wrong.
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(12v - 5v) * ~350 mA ( servo w/ small load ) = 2W per regulator

is it enough to show ???

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The power supply is a desktop PSU (similar to a laptop PSU) and it is capable of outputting 7A. As far as i understand the voltage is correct so 7A shouldnt cause a problem but im not overloading them am i?

Ive opened the skull (yes, skull!) and turned the servos on. It seems that at least 3 of them are constantly twitching but its not noticable just by looking at them. I know that will cause a huge power drain through the regulators but i wouldnt have thought the servos would mind as they are designed to constantly correct their positions.

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your servos should have no load other than moving parts of your skull ... especially no STATIC load ... ( like compressing a spring coil )
so, all moving parts have to be balanced to create a minimum load on the servo @ " neutral position " or steady positions ...

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I think the slight twitching could be caused by my code but it is written in assembly and goes into a tight loop before the timer overflows so it should be accurate to 400ns.

I do not think it's a real problem ... theres a " deadband " around the steady position : about 2µs to 10µs ( from best to worse servos )

Now ... some servos draw not negligible current to hold position with very light loads ... some need quite a load to begin to draw current ...
This belongs to servo caracteristics ...

Say ... the smaller the servo, the Higher the current draw @ 1kg*cm torque i.e.

from 250 mA to 750 mA ... depending on the servo size and ... brand.

Alain

[The Master]

I dont think the regulators are the main problem. They are getting quite hot but they have heatsinks on them and ive had one running hotter for a few hours without problems.

The skull is more balanced than any other that ive seen. The jaw servo is in the center instead of at the front. One problem ive heard with this kind of project is that when the power is cut the skull will look straight down. Mine doesnt have this problem. It remains exactly where it was so the servos definately arnt struggling to keep the skull in position.

I did have a servo tester (now blown up after forgetting the 5V regulator) and all 4 servos worked fine with that. If its not my code thats causing the buzzing then there must be some fault on the circuit but i cant find one.

As my code listens for serial data i cant jump into the tight loop too soon but i calculated that it should jump in over 3,000 instructions before the timer overflows. Theres no way my program uses that many instructions in one iteration of the main loop.

Heres my tight loop for servo 1

Servo_Loop1							; Label: Servo1 tight loop
	
	; Check if Timer3 has overflowed
	BTFSS	PIR2,1					; If TMR3IF=1 Then: Skip the next line
	GOTO	Servo_Loop1				; Nothing to do yet so go back to the start of the loop
	
	BCF		_pServo1				; Turn the servo pin off
	
	GOTO	Servo_Next				; Goto the subroutine that sets up the next servo

The PIC runs at 10MHz with PLL (40MHz / 10MIPS). The servos were quite expensive but their deadband shouldnt be shorter than a few instructions.

Im going to go over my code again tomorrow because ive already found some stupid mistakes in it so there might be more. 1 thing i can think of is that i dont allow 18ms between pulses. There is a gap but i think its more like 10ms. That should be fine though right?

[Dave]

The Master , If I were you I would seriously look at the power supply for the servos. If it is not STIFF ENOUGH because of lack of current availability, the closed loop circuitry inside could be trying to compensate for it. I would take a scope or at least a AC voltmeter and watch the +5 volts that power the servos. If you are seeing more than a hundred millivolts or so, the supply is WEAK... This is also probably the case when you say that the servos make a buzzing sound when they are at a stable position, assuming the input pulse width is stable... Just my thoughts...

Dave Purola,
N8NTA

[The Master]

Heres the PSU i am using. http://www.rapidonline.com/Electrica...mode-PSU/79713. Even the smaller 2A one ive got provides a very clean power source. Do you think its worth adding a big capacitor just incase though?

[mackrackit]

If you think the power supply is not adequate test with one that you know is. In this case an automobile battery should do. Large and clean, power wise that is.

[The Master]

I think the power supply should be fine but i will try a large capacitor when i get home to see if that helps at all. I also have a PC PSU laying around that i can test with and ive run servos from that before so i know its clean enough.

[Dave]

The Master , You are missing the point... I said look at the +5 volts running the servos, not the main PSU... The servos are operated by +5 volts arn't they?

Dave Purola,
N8NTA

[ScaleRobotics]

[QUOTE=The Master;92102 1 thing i can think of is that i dont allow 18ms between pulses. There is a gap but i think its more like 10ms. That should be fine though right?[/QUOTE]

That might be some of your twitching. They should have 20mS between start of pulse and next start of pulse.

[tenaja]

Friction can contribute to servo heat. If the mechanism can't easily and smoothly move one servo step, then it can oscillate.

For this type of app, I've used digital servos. They have several benefits... the first of which is no heat when they are not moving. They are easier on the PIC mcu program, too, because you can give them a single pulse and they'll remember the target--instead of a continual string of pulses.

[The Master]

The regulators are hot but they arnt incredibly hot. Like it said, ive got another circuit where the regulator is running even hotter without any problems.

The servos im using are 3 X Hitec HS-645 MG and 1 X Hitec HS-5645 MG Digital.

Ive tried unplugging all but one servo then connecting a capacitor to it. There is no change at all.

There is almost no restriction at all in the mechanism. I can move it freely with my hand when the power is off. The servos only have to move the weight of the skull. (The mechanism)

Ive just done a test that i should have tried ages ago. I unplugged my PIC and powered up the circuit. All of the servos jumped to a random position (as they always do with no signal) then they stopped. No twitching or buzzing. The servos and the regulators are all remaining very cool. Im going to do some more maths and see if i can make my program a bit better

[The Master]

On the 20ms pause. Ive read that it should be on for 1-2ms then off for about 18ms. How sensitive are servos to that gap? My understanding is that the length of time the pin goes high for is the most important. The pause in my program will vary depending on how long the pins are on for. That has been fine in the past with a similar type of servo

[tenaja]

Sounds like a code issue. Have you looked at the output pulses on a scope? If the signal pulses vary at all, that can drain the batteries.

The 18mS isn't critical with the digital servos, but the analog servos are all different; some could just power down, and some could cause more drain.

[Acetronics2]

The regulators are hot but they arnt incredibly hot. Like it said, ive got another circuit where the regulator is running even hotter without any problems.

The servos im using are 3 X Hitec HS-645 MG and 1 X Hitec HS-5645 MG Digital.

Hi,

Bad news ...

The HS645 are rated 1 Amp each ... but their real problem is a very poor position holding caracteristics .
This servo is well known for its lack of precision AND its Hysteresis ...

That explains what you get as a behaviour.

Only solution is to change them ...

Try some old Futaba S9402 ( good discounts ... VERY good servos )

AND try to forget about Hitech products ... you'll really save your $$$ !!!

Alain

[The Master]

All of these servos worked fine when i was using the Hitec servo tester. I would have thought the digital servo would be the first to complain if there were any problems but thats the only one working.

The Futaba S9402 is £65.50. I need 3 of those so its a bit expensive. Ive been looking at some Hitec HS-5625MG digital servos. They are very slightly better than the analog ones im currently using but they are digital so i shouldnt have a problem with them. They are only £35.99 each which is still gonna be expensive but not as much.

I would like to know what went wrong though. I cant believe that 3 perfectly good servos would suddenly mess up at the same time like this. They were holding their positions very well when i was first testing them. There was no buzzing and they wernt struggling to move the skull at all.

By the way, i have completely removed one of the analog servos from the skull and powered it up. Its still twitching. I think its even worse than when its in the skull