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.

 

[cswilson]

I haven't had much time to look at this, but I think a (the?) key thing you are missing is how the switching (PWM) amplifier processes the electrical power.

Let's say that at a particular instant, the amplifier is providing 5 amps to the motor at 80 volts. (With your 0.8 ohm [line/line] resistance, that's 4V for the resistive drop and 76V for back EMF.) This does NOT mean you are drawing 5 amps from your 320V battery pack. To first approximation, at least, you are drawing 5A * (80/320) = 1.25A. This ignores amplifier losses, so the draw will be a little (but not much) higher.

This would not be true in a linearly modulated amplifier, where 320 - 80 = 240V would be dropped in the amplifier, and a full 5A would be drawn from the supply.

The analysis does get tricky, because even for a constant acceleration, the back EMF voltage is steadily increasing, so while the motor is receiving constant current, the current draw from the battery back is continuously changing.

Very few servo systems are battery driven, or even care too much about total energy draw from the source, so this is not the focus of most setup software.

Curt Wilson
Delta Tau Data Systems

 

[Sparweb]

That's what I needed to zero in upon: a better "mission spec". I was struggling before to figure out why this all mattered. Now it's crystal clear.

Since the folks with much more electrical background helping you on that side, I think I can make a suggestion to help you with your "missions". In the aircraft world, mission specifications or "profiles" are used to simplify the definition of what the end-product aircraft must accomplish (and to make an even playing field if bidders are competing). It seems an appropriate analogy because in aircraft missions the typical flight profile assumes one fuel tank's worth of range to accomplish that mission. In your case I could see a handful of "mission profiles" for your camera system, each having to fit into the battery pack's charge. In each profile you define a typical routine of motions that the equipment must repeat. Each motion requires functions that require energy. Then you can add up the functions that the system experiences during the job to a sum of energy consumed.

This process may help you focus on things in proper order: I see you working through a lot of math and theory but there are aren't any concretely stated goals yet. Of course you could be unwilling to divulge this in the face of competitors, and no need to tell all here, either. I'm just suggesting that the "Superbowl" mission profile could be defined by a specific camera payload, moving X times on a player in "follow mode", Y times pan and zoom over the field, Z times rapidly traversing to track a pass... and so on. The mission profiles for the African safari and the Swedish rally race are all quite different.

STF

 

[eckener]

Dear iop995...

That spreadsheet is like a gold nugget for me! If a picture says a thousand words, than that spreadsheet is worth words*10^23!

I wouldn't have got all those calcs right in a million years... but at least I knew I was wrong!

My Ah's were coming in considerably higher, and was actually much closer to when I was simply calculating the work to be done, converting to watts, then amps, and then applying an efficiency to it.

For my own edification, and perhaps those that are following, the 5 (count 'em) big things I was totally incorrect on were:

[ol ]
[li] Motor Phase Current...your calcs use only 2/3 of the current because of the commutation, while I didn't even consider the phases and (incorrectly) used the entire amount!

[/li]
[li] Average of Min Current and Max Current... your calcs create an average current that considers the full torque at zero RPM as well as the torque at full speed, while i was only using the maximum torque number, and maybe considering cutting it in half.

[/li]
[li]Calculating Power in the Motor - Now I understand that power in this sense is Torque x Speed. You are finding average power inside the motor, and then using this to find average current inside the motor based on power. I was not even doing this step... and just trying to calculate amp hours directly from the current.

[/li]
[li]Current Conversion to RMS to DC current considering 120degree comutation. That one was really tricky. I thought all these equations were for DC, since we are on battery, but I guess all these equations are geared towards creating AC current, so you are converting it back to DC by multiplying by sqrt(2)... and the taking it further by dividing by the 2/3 for the commutation. Smart.

[/li]

[li]Basic Amp Hour Calculation. You were using going back and forth between watt hours in the motor and amps to determine the average number of watt hours drained from the battery over time and then calculated the amp hours based on this. I was just multiplying the amps used by the time of the acceleration, and skipping the power (watts) altogether. I was probably doing this completely wrong.

[/li]
[/ol]
If there were a part to question for me, it would be this last part above, though I'm not at all saying it's incorrect.

Also I did notice, you had "motor phase resistance" at .4 ohms.. where the spec shheet say resistance " resistance line to line" is .8 ohms. You probably had a reason to do that, but I am double checking. And of course, your calcs didn't have all the mechanism friction losses, air drag, etc... No problem for me to add those.

While I am going to continue to think about this on my own to see if I can get it to make intuitive sense to me, I am just going to take a guess for now, you being much smarter than I in this area, that you've pretty much got this all correct.

Now, if you are correct the batteries are certainly going to last a LOT longer than I thought.

Thanks so much....

P.S... I realize now that its probably NOT JUST ACCELERATIONS that I've miscalculated, but amperage use in general, when it is cruising with no acceleration. I will try to modify your spreadsheet for that as well....

 

[eckener]

Hey SparWeb...

Thanks for the insight... that is pretty much exactly what I'm doing, though my profile "missions" are only about 5 scenarios right now, and don't include bombs.

We have a very large spreadsheet set up, with every parameter of the system. There is a of course inputs for about 20 common parameters, and then of course about 500 outputs of all kinds of data. (obviously was having trouble in the amps department),

...tings like peak torque, RMS torque, rolling frictions, air drag, sprocket sizes, peak amps, inertia ratio, net torque to total inertia ratios, you name it.

Right now we are using this to facilitate the comparison of various motors.

I initially looked at seemingly hundreds of motors, finally selecting one of the Kollmorgen's that was giving us a decent inertia ratio, But now Mavilor has these new slotless stator servo motors that are really incredible. They basically have about two to three times the performance of the best of what we'd looked at, but of course this is in part because they are extremely fast... upwards of 18,000+ RPM as opposed to the 6000 RPM kollmorgens we were looking at. If we can figure out how to gear them around 10:1, and do this fairly quietly, we'd might go with them.

A great benefit for using the faster motors, is is that we could achieve a very large speed envelope... from very slooooow speeds to very fast... without the use of a transmission. As this is a repeatable motion control system, shifting gears would be undesirable, and good control at creeping speeds is essential.

 

[mikekilroy]

After figuring out the amp usage here, the next thing we are tackling is how to best stop the thing. (handle our back-EMF): deciding whether to use quite bulky and very hot dynamic braking resistors, or whether its worth it to regen into the battery.

realize you DO HAVE full batt recharge (regen) by default with batt powered drives: o resistors needed. regen power goes back to dc bus which is ur battery pack.

BTW, that Motioneering software does give amps during all the move segments as well as power required (front page top right - system power supply sizing icon). It would give you all these answers - ask your Kollmorgen supplier to go thru it with you and show you the small details maybe?

 

[iop995]

eckener: calcs are simple formulas and there are a lot of simplifications. Regarding your points:
1. For 3 phase BLDC motor torque is Tm=3/2*kt*Im; in motor datasheet I can't find details about switching method.
2. This average power allow to obtain an average battery current / capacity. Your battery type exibit almost same discharge rate for a wide discharge current, so errors are small.
3. Motor current is constant for a given acceleration / torque (neglecting drag forces and similar forces speed-dependent), but power developed increase as speed increase; driver must be albe to supply a constant current and increase output voltage up to reach maxim speed. From motor data, for this setup, maxim acceleration is 6.64m/s2 coresponding to a 17.99Arms phase current. This is one system limitation (from motor) and another limitation is maxim speed - due EMF, that reach battery voltage - 320V, at 23.4m/s.
4. This conversion is made considering 2pi/3 current commutation.
5. Here, need to know time working for such profile or/and add another profiles with their working time.
Motor line-line is 0.8ohm and I suposed star connection, so a phase have half.

cswilson: I considered converter like a black box; I have an output load (motor) and need to know power at input, so using efficiency obtain it. Converter is important in operation requested (constant acceleration - constant current capability for entire output voltage range).

mikekilroy: good point about regen brake. Battery used allow high charge current (about 1C or maybe 1.5-2C in pulse) so, may be a point to analize cost / advantages for such feature. Anway, regen must be combined with dinamic brage (resistor) and mechanical also.

 

[eckener]

Iop995...

I did run this through Motioneering real quick.

Your "Motor power at max speed" (P_max) does indeed exactly match the result for Motioneering's Buss Watts and Continous Power Watts (same thing i guess)...

But, in your spreadsheet, when you divide this (P_max) by the Maxim Battery Current (I_bmax), the watts shoots up quite a bit, even if you turn the efficiencies up to 100 percent... and then of course its considerably higher than the buss voltage in motioneering.

Just double checking that this is correct?


The formula for I_bmax (Maxim Battery Current) you are using is:

=Pmax/effm/effc/(2/3)^0.5​

Is this trying to account for the use of a DC power supply?

Curious what formula would be for I_bmax if it was an AC power supply instead?


P.S as you can see, am also using spread sheet to calculate amps during non-acceleration segments of profiles, by only looking at "max" numbers. Have incorporated formulas into our large mission spreadsheet and modified to calculate amp-hours for duration of individual segments... and of course are plugging in refined torque request rather than basic m*a*d.... Working great. File Attached

 

[eckener]

If this is correct, its weird that Motioneering wouldn't give the watts required by your power supply.

 

[iop995]

You right, above formula is not correct. Correct minim/maxim/average battery current with:
Minim battery current Ibmin =Pmin/effm/effc/Ub and corresponding. Now for 100% effm and effc will lead to same average power.

 

[mikekilroy]

whew......

rick armstrong and others (including me) spent over 20 years tweaking Motioneering equations. They take into account commutation. they take into account all efficiencies. Motioneering is the cumulative result of probably 3,000 hours of tweaking, not counting the 2,000 hours we each spent individually on our own spreadsheets that predated it - before even excel existed. I strongly suggest you go back and learn how to use it for this case rather than inventing your own now. How can this many hours from servo engineer experts not be the way to go?

to arbitrarily begin dividing Kt by 2/3 is wrong. If the Copley drive is NOT sinusoidal commutation, then it's trapazoidal commutation may need to be taken into account. Copley has some good sinusoidal commutation drives - are you not using one - to save money??

BLDC motors are designed specifically for trap waveform or sine waveform; your AKD is designed for sine current; if you are not using a sine drive, why?

You bought the akm motor from a Kollmorgen distributor Why not make them give you the proper help? There are a dozen high tech app engineers sitting at their desks at Kollmorgen in Radford, Va, some of whom would be tickled pink to walk you thru Motioneering, even with a not so good AKD drive.

Also, look into the real specs of your LiFePO4 cells: I know of none that cannot handle 3,4,5x (my 100amphr ones handle 10x for upto 20 sec) discharge rate, and also easily handle 1,2,3x or more CHARGE rate: use this information to decide if you can dump ALL regen (decel) energy back inot the batt pack real time of if you need to dump some of it; this should not be a 'guess;' you have the data to determine it properly.

Last, you said in multiple posts you can get to around IIRC 28mph, then in one said you want to chase a vehicle 0-60mph in 3 sec.... how can you do that?

 

[eckener]

Thanks Iop995, thats just got us a few more runs up and down the football field! Actually, this has whole excersise has had such positive impact, that maybe we will cut the capacity of our pack down... thus gaining, a lot of valuable space for electronics, whilst still providing for a sleek design.

mikekilroy:
Yes, we will probably go with the regen to battery, if for no other reason than those breaking resistors seem quite large and hot, especially for a piece of film equipment. I haven't calculated if the 1C charge rate of the LiFEPo4's can take the full amount of energy coming from regen, but if not then yes, as iop says, we may need at least some braking resistors in there. These calcs will certainly make those easier to size, as every drive manufacturer seems to size breaking resistors in a different manner.

We do have additional mechanical failsafe brakes on the motor shafts (not the kind that come built into the motors, but ones that can operate dynamically and repeatedly) for e-stops, and for holding the load when the motors are not spinning. They will be able to be temporarily disabled so they do not "click" when shooting a dialogue scene. There are four of them, one on each of the four motors/drivewheels. Beyond these however, we were not planning to have an additional mechanical brake system.

 

[eckener]

Hi Mike
Oh man, I did not mean to offend! sorry! No, the kollmorgen guys have been really nice and the motioneering definitely works and really helped me size the motor correctly, and I am VERY glad to see that you worked on that, thank you.

I did use it for motor selection, but now we are running lots of scenarios in a spreadsheet form, where its very easy to cut and paste and compare column to column, motor to motor, etc...

As a "beginner" also, it will say it was a little frustrating as there doesn't seem to be a real good mechanical "model" for a vehicle... conveyor belt was close I think, but I never could be really sure if was actually doing what I expected, especially with all the automatic computation of continuous and intermittent, and so ended up just calculating it all out and putting the numbers into Motioneering as direct data entry. (its good to have two techniques that corroborate) A more seasoned professional could probably use it with a lot more confidence than I.

Also, it ties you to a motor/drive combo, which I guess is no big deal, but we are not using the AKDs. They are far too large for this application, and can't take DC input, (from what I was told).

We were talking to the talking to the Copley guys, and though the Xenus might work for us, its still pretty large, and CANNOT regen into a battery. Copley does have a new Accelnet coming out soon that is for DC input, and is very small (a few inches by an inch tall) which will handle up to 20amps. That would be really good for us. Elmo has a similar model already out, called the Trombone, which has similar capabilities. Additionally Copley has suggested that they we can run their drives with one master drive running on position or velocity, and the other three slaving to it with torque.... torque assist, if you will. This will ease our programming burden, as this dolly is designed to go around curves and up and down hills, and in doing so, the drivewheels will see slightly different radii.

Regarding the 0 to 60 thing, the rig allows you to quickly swap out the gears for for a high-speed version of the dolly. The trade off is that the inertia-ratio suffers quite a bit, and the rpm at very low speed is kinda low... not ideal for precision motion control work for say, a product shoot, or slow moving camera shot.

 

[eckener]

Also we are not deadset on Kollmorgen AKM42j motors yet. We were all set, but at last minute were shown these Mavilor XtraforsPrime Slotless Stator Motors.

There are some drawbacks, but pound for pound, we get half the inertia ratio, 75% more RPM at very slow speeds (at the cost of running 30% faster...around 8500 rpm), and more head room on continuous and peak torques.

They do run hotter I think, and as if the 6000 rpm kollmorgens weren't fast enough, I'm still trying to figure out how to gear that high speed 6:1 or more and still keep things quiet.

 

[mikekilroy]

I didn't mean to sound offended, sorry. I wonder how you would use more than 6000rpm on such a camera slide - would have to be a timing belt arrangement I would imagine cuz most planetary gearboxes do not go over 6000rpm even intermittently. For noise be sure to pick helical gears too rather than straight cut if you do go gears.

whoever told u AKD does not accept dc input is wrong; we have lots running with dc input. the regen then to the batts is all automatic also; If one simply brings the dc in thru the ac input terminals on a drive, ok on maybe no regen, but any drive with access to the dc bus terminals like the akd allow full regen to the batts.

not sure how the mavilor motor can offer you anything over the akm; I don't see it. if you were talking about their older pancake design then yes, they can have higher inertia for a given torque. if you list exact part no I would be happy to compare them to the exact akm part no so you get apples to apples comparison.

 

[eckener]

Mikekilroy...

The Elmo Trombone definitely says "Vector control sinusoidal commutation"


The Copley Accelnet doesn't actually specify... I was just ass-suminng.

I thought the AKD's were also?

The sinusoidal makes a difference as to how we're calc-ing everything?

 

[eckener]

sure... I'll take you up on that.... here is a link to the Mavilor catalog pdf...

Page 17... FP-0207.8 (48 volt winding, running on 320 volts DC)

Attached is a curve...


even the next size down.. FP0105.8 is a very strong candidate.

 

[mikekilroy]

no link to page 17. I will not sell you the products since you are outside my territory, but I don't mind helping compare and such.
So please give me a link to the FP model - it is not shown on the link you posted earlier and I dont have time to search it out. You also made comments about pos mode to 1 wheel then torque command to the other 3... You should know all your options - that may not be the best one. Lots of drives can do electronic gearing in pos or velocity mode (what you would send to the other motors) - there are some big and some not so big pros and cons to each approach for what you are doing; if you pick drives that only can do 1 then you cannot try the others; I would pick a drive that can run any of these modes just by software changes. If you want to take this offline here that is ok too; maybe some of the small talk off line and good stuff on? Your choice. my email is mike -at- KilroyWasHere -dot com.

 

[eckener]

Guess it wont link to the .pdf page. Anyway, its just the product manual on that first link to Mavilor.

All the drives I mentioned do all three modes. We wouldn't be stuck with one, and may use more than one for different uses.

 

[mikekilroy]

hmmmm. either I am missing something drastic or some salesman is blowing some magic dust? Let's go to emails to compare.... The only data sheet I can find at Mavilor for this motor is here:

http://www.mavilor.es/pdf_products/FP_XTRAFORSPRIME.pdf

It is 2 pages and does not show windings or inertia or lots of other details. But what it does show is this motor is way smaller than the other you compared to, so I am not sure why you think it is equivalent.

Here is the basic data I can compare from the data I found on the Mavilor. Be careful if you go this route, remember iop995 showed over 7nm required for your accel; the speed torque curve you showed for the Mavilor motor is good for 2.2nm MAX - AT 31 AMPS! You won't get that in the 1-2" size musical FET drives upto 350vdc...

Again, I think you should have a servo person look over your shoulder to help guide you. I do go 0-60mph in 5 sec with 160kw worth of BLDC motors in my converted Chevy Equinox on my 350V LiFePO4 batt pack - drives are in torque mode as you suggested; there are definite resonant speeds that shake; electonic gearing might be much better method for you - it can still allow going around corners. I am sure my 8:1 planetaries to my 20 some inch dia wheels is a lot worse inertia match than you will have. Email me if you want.

Take care.

 

[iop995]

As I said, spreadsheet is an estimation tool, made with no complete info, much simplifications and no much time...
Regarding 2/3 factor, it's valid for sinus comutation and it lead to higher current values for trap comutation also (where is 1/2), respectively 32$ Ah oversizing.