What Voltage does the servo actually "see" ?

[Paraprop]

In order to optimize the servo speed settings in the controller, I would like to be able to calculate what is the actual voltage the servo motor actually "sees" when controlled by a G320 in relation to the drives supply voltage.
Is there a formula ?

Thanks

Paraprop

[NC Cams]

Any PWM based servo drive applies FULL buss voltage to the armature.

The voltage the motor "sees" is based upon the ON duty cycle of the drive at any point in time.

Assuming the buss voltage is 100vdc and you give a 25% of max speed command, the averaged voltage will be 25vdc and the motor should approximate 25% of the speed the motor would run at if the 100vdc were applied to it.

However, the motor terminal voltage may not measure 25% with a vom due to the averaging algorythm that the meter uses. Give a 50% duty cycle, and some simple math and you have your "formula".

In reality, the motor "sees" the full 100vdc while the amp is in the ON mode BUT, BUT it only really "feels" this 100% voltage 25% of the time with the example.

[Paraprop]

Here I can not agree entirely.

At any time when the motor is energized there are 2 Mosfet's in series with it.
Those Mosfets behave like a little resistance when switched on.
I'd like to know what is the value of that resistance called Rds on.
This will allow to calculate the voltage drop if one knows the current flowing.
For the moment I as just assuming that the voltage at the motor terminals should not go lower than 90% of supply voltage but I am hopping that it is less.

[John3]

NC Cams is more right than you may think.

When the fets turn on, they are driving a complex load that includes resistance (armature, brushes, leads, etc), Inductance (from the same), a little capacitance and a voltage source representing the back EMF of the motor.

The inductance and back EMF both act to minimize the validity of the simple resistance drop calculations that you suggest.

I'm with NC, in that the voltage spikes (at the PWM frequency) to the bus voltage or slightly greater (due to resonances or inductive spikes if the controller is in Dynamic braking mode, absorbing the motor's back EMF).

The average voltage across the motor is determined by the PWM duty cycle. This could be zero to something very near the bus voltage.


John

[Al_The_Man]

Paraprob, I think this article may answer your questions, page down to servo amp/motor.
I Keep a downloaded copy for reference.
http://www.elecdesign.com/Articles/P...ArticleID=7635
Al.

[Torchhead]

RDS On is the resistance of the FET when it is turned on (typically very low) for the N Channel power FETS used in the drives. The RDS_ ON effects the heat generated with the FET is on and is basically a series resistor into to a complex load as was stated. With gate drive removed the main body of the FET (where the resistance is measured) goes up into the meghohms (essentially open circuit). Dissipation across the FET is a function of the RDS on and the turn-on and turn-off (Rise/fall) times of the gate signal. Anytime the FET is not fully on or full off it dissipates lots of energy.

If you want to know the RDS on it's a normal part of any spec sheet for a MOSFET part. You can use the simple formula of E^2/R where RDSon is R and E is the power supply volts then multiply times the duty cycle (always equal to or LESS than 1) to model base power dissipation. Then you have to calculate the switching losses and finally any dissipation from the drain/source body diode during reverse recovery. To get proper (exact) numbers you need the know the characteristics of the load being driven. What you find in practice is you can use the base model, design good gate drive circuits and add a fudge factor for the other stuff and get a good idea.

If not knowing the numbers is making you lose sleep at night then go to the IR (International Rectifier) website and find the application notes about designing with HEXFETS and gate drive circuits, snubbers and driving highly inductive loads. There are many hours of interesting reading there.


tomCAUDLE
www.CandCNC.com
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[NC Cams]

Not only does the Vgs affect the Rds but also the instantaneous current flow as well. Check the specs for the curves should you be inclined to disagree.

If the Vgs is 50% of the max allowed Vgs, you'll have a higher Rds than if you hit the gate with the max possible Vgs that it can/will tolerate.

When you're trying for THAT infinitessimal reduction in loss to/thru the fets, find ones with the lowest possible Rds at offered in at the particular die size. Then hit them with as much Vgs as you can and still be within the Vgs limit for your fet.

For a typical SMP60n05 or similar fets, we hit them with 18volts although they should be well enhanced at 10-12 volts. For an IRLZ44 we hit them with 10 volts even though they should be well enhanced at 5volt logic levels.

The IRLZ44's could be run heat sinkless at Vgs = 10 but would be warm/hot to touch at 5v with current draws at/near rated capacity in R/C car use.

[cjmerlincnc]

Hi, I read the article suggested in Al_the_Man's post as I am building a 4-axis servo based machine for the first time. I've done stepper systems and feel well knowledged in the art and until I got down to the bit in the article where it explains about dumping the stored energy that the motor sends back to the servo driver when decelerating.

From all the posts and info I have read I suddenly realised that there has been no mention of this extra 'bit' , Do I need it in my new design?.

All I have so far is a basic power supply (transformer,rectifier,caps) oh and fuses of course.

Gecko servo drivers and motors, tie it all together and tah dah.

My question, Do Gecko drivers need this bit or have they something to handle decel energy.

If so does anyone know a circuit design or parts I need to build this.

Regards

[Al_The_Man]

Almost all motors regenerate on fast deceleration, and although going larger with power supply capacitors places a higher demand on supply transformer VA sizing, it will help absorb the back emf generated, but if this back emf is too large or lasts for more than a second or two, then other means are usually required, like dynamic braking circuitry to disipate the energy.
See the A-M-C Engineering bulletins for more.
Al.

[cjmerlincnc]

Thanks Al, an interesting read. I'll keep an eye on the PSU voltage when i'm up and running. You're the man!

Regards

[NC Cams]

Fanuy literally has a NO/NC relay on the output of their "velocity controls" (3 phase AC fed SCR generated DC output to the servo drives). When the motor generates back EMF, it can drop out the drive and shunt the back EMF into this massive, low resistance resistor to dissipate the induced voltage. We've not traced down all the logic yet to see if the relay does drop out all the time or only drops out when the servo drive is disabled for whatever reason.

On my mosfet PWM drives in my Bridgeports, they simply shunt the fet bridges with external diodes. YES, you can use the epitaxial diodes built into the fets - they're free

But, we learned via our R/C car speed controls (PWM mosfet drives) that even the dumping of 7 or 8 volts backwards across 6 fets in parallel will eventually toast some fets. Its the current, not the voltage is what I came to realize.

The two "brake" fets in R/C ESC's were easily cooked if your didn't use brakes which were there whether you used them or NOT to slow/stop down the cars. Hence, I'd be inclined to want to find a way to properly dissipate back EMF in a PWM servo drive than to simply dump them on the H bridge fets and hope for the best - which some designers seem to find wholey adequate.