SPFX Camera Dolly - Motor Acceleration Amps 1

[eckener]

Hi All... We are building a special effects film dolly. The motors have been sized and we are trying to estimate the current draw during the very fast accelerations... Doesn't seem to be too problematic for constant speed segments of our move profiles, but figuring out the current draw for accelerations has us a bit stumped.

Is there a relatively easy short-hand way to do this? perhaps using the torque constant (Kt)of the motor? or another formula that we have just been missing.

We have already figured out all our mechanical loads, wheel frictions, air drag forces, moments, gear- ratio, etc... We have this all in a spreadsheet where we plug in the acceleration rate and top speed. For simplicities sake, it spits out us a constant torque value on the motor shaft over that period of time to achieve that desired acceleration...

Now we want to find out how many amp hours that will approximately take.

We tried a couple motor sizing softwares, but they really didn't give us this information.

We did convert the whole shebang (including losses) into Work Done(Joules), then into Watts and then into Amps. When we compared that to the simple equation, Torque/Kt, the Torque/Kt was about 2-1/2 times higher... maybe this is because the voltage was not changing in this equation? Should we cut the voltage in half?

Its a brushless DC Servo motor (actually 4 of them.. Kollmorgen AKM series)
320Volt DC Battery Pack
Elmo Drivers
200 pound dolly with payload
3.91:1 gear reduction on motors.
6" diameter drivewheels.
27.4.mph top speed
14.65 ft/sec/sec accelerations (typical)

Thank you for reading.

 

[eckener]

You know... I looked at those direct drive motors very early on. They are very cool. But most max out at about 500rpm (some 800rpm). We'd either need excessively large drive wheels, or have to gear them UP 3:1 or more which I think would defeat the purpose.

 

[iop995]

Do you considered EVT as option?

http://www.tno.nl/content.cfm?conte...g1=894&laag2=914&laag3=105&item_id=646&Taal=2

 

[eckener]

Hi Iop995...

Thanks for posting that...
We hadn't seen those, Though we did take a pretty close look at a this mechanical CVT:Link

There were several concerns with it, namely input speed and torque, the fact that we needed a very compact and stout solution, and the notion that we would be adding another variable into a system that needs to precisely repeat. I suppose we could put additional encoders on the outputs, but it was getting to be quite complex, so we are trying to make work with a single gearing.


iop... in your spreadsheet (attached again with the fix), how do the Arms get converted back into Adc from the battery? or do they even need to?

 

[mikekilroy]

eckener (Electrical)
20 Aug 13 23:29
You know... I looked at those direct drive motors very early on. They are very cool. But most max out at about 500rpm (some 800rpm). We'd either need excessively large drive wheels, or have to gear them UP 3:1 or more which I think would defeat the purpose.

You need to ask an app engineer from their company; they do NOT max out at 500rpm but can go thousands of rpm. I recently showed them a market for 5-6000rpm motors in this arena and they are now available.

 

[eckener]

Do you mean ask someone from Kollmorgen about the ones in the link you posted?

 

[iop995]

eckener: current from battery is in Adc, no need for any conversion. Arms phase current is calculated only to find motor Joule loses (power at zero speed, Pmin) and Pmax is true power calculated by torque x speed (max speed at the end of profile); with these 2 values is calculed average motor power, then corrected by motor and converter efficiency and obtain average power supplied by battery and current. Sure, there are a lot of simplification but may correct result by a 10-15% factor to be more confident with battery capacity.
If you know battery discharge curve, may be more exactly by calculating power at each profile point (choose a step as you need, 0.01 - 0.1second) and needed current from battery will entry in look-up table that describe discharge battery curve (as discharging is not liniary with current supplied). Also may consider pulse (peak) current at converter switching frequency instead of average Adc but maybe will not be much result difference.

 

[mikekilroy]

Do you mean ask someone from Kollmorgen about the ones in the link you posted?

yes.

as you get more into battery systems you should consider how long this camera mover needs to last/work. If only a dozen times, then no need for a BMS system, if years, BMS is absolutely required to control charge/discharge/balancing those 100 cells. bms on my ev works well with my 108 cells as example:

http://lithiumate.elithion.com/

another battery supplier is ones in my ev:

http://www.gbsystem.com/Products_en.asp

both companies have been great working with me, answering my questions, and even updating/correcting their software with my suggestions/help. I have no affiliation with either other than using them and thus can recommend them based on that.

 

[eckener]

Iop995,

I guess my question then regards what current to use for comparing peak voltage of motor to the peak current rating of the controller (which is listed as Arms) ?

Seems like it would be Ibmax(Adc)/sqrt(2) to get peak current of motor in Arms.