Author Topic: Regarding Motor Selection (Read 2901 times)

ukesh · « **on:** December 07, 2009, 11:12:35 AM »

Hello!
One of my friend is building an battle bot, so for deciding the motors i used admin's RMF calculator. These are my specifications.

Mass: 35Kg
Velocity: 0.5m/s
Acceleration: 0.25m/s^2
efficiency: 75%
wheel dia: 0.08m
No of powered wheels: 2
The expected RMF was 1.52

I decided on this motor:
http://robokits.co.in/shop/index.php?main_page=product_info&cPath=2_3&products_id=186

motor specs:
torque: 0.3 kgm
speed: 450 rpm
The motors RMF was 2.25. So, is this motor enough?

waltr · « **Reply #1 on:** December 07, 2009, 03:02:58 PM »

You didn't spec the Incline Angle so I set this to zero since Battle site are flat and level. Or are they?
Any bumps in the battle surface is an incline so you would want to take this into account.

The required RMF = 1.52 so you need a motor that is equal or larger value than this.

The motor RMF = 2.25 which is enough as it is larger than the requirements.
The motor RPM is too high. Look at the RMF result for the motor speed. to obtain 0.5m/s with 0.08m diameter wheels the motor only needs to turn at 119 rpm. So this motor is not the most suitable but will move the bot.

If you set the incline angle to 5° then the required RMF goes up to 6.7 and the selected motor would not be enough.
A larger motor would be desirable in a battlebot so the bot can muscle its way out of trouble and not break when attacked.

ukesh · « **Reply #2 on:** December 07, 2009, 08:44:05 PM »

the battle arena is flat. The rpm needed to obtain 0.5m/s with 0.08m diameter wheels is only 119. But this motor provides 450 rpm which is way more than required. Is higher rpm bad?

waltr · « **Reply #3 on:** December 07, 2009, 09:29:05 PM »

120/450 *100 = 26% PWM duty cycle would give you the top speed you want. Or the other way to state this is that the Bot could go about 4 times faster than you want. This may not be a bad thing.
The motors are not that expensive so they could be a good starting point. Also if the Bot's weight can be reduced you would have a quick and nimble Bot.
In the long run only experience will determine if those motors will work well.

ukesh · « **Reply #4 on:** December 07, 2009, 10:23:32 PM »

Thanks waltr, I'll go with that motor then

ukesh · « **Reply #5 on:** December 08, 2009, 09:46:49 AM »

Guys, For controlling the two motors, We'll be using an sabertoooth motor driver. For powering the two motors I"m choosing an 11.1V/850mAH/3S/25C Li po battery pack. Each motor draws 9.5A(max), so totally for both the motors the current draw is 19A(max). The battery provides 21.25A. So is this battery pack wise? Or should i go for an battery pack that provides higher current?

And does an 11.1V rated li po battery pack hold higher voltage than the rated voltage when it is fully charged like Ni-mh?

waltr · « **Reply #6 on:** December 08, 2009, 01:50:45 PM »

The 9.5A per motor is the stall current. That is when you apply power to the motor but the motor can not turn due to being jammed.

Your design does need to take the stall current into account so your drive circuit doesn't go up in smoke. You may see close to this current on starting the motor from stopped but only for a fraction of a second. The processor/drive circuit should also have a way to determine if the motor is stalled for any longer period. The battery pack seems to be able to deliver the stall current but not for too long (0.85Ahr/19A = 2.68 minutes). Do ensure that the wires and connector to the motors are heavy enough to safely handle this current. An install a fuze (10A per motor should be ok) as the last resort to protect everything.

You may want a larger capacity battery just for a longer run time. The no load motor current is 800mA per motor. That would give a MAX run time of less than 31 minutes and more like 10 minutes moving the robot around. In combat the motor currents could become quit high which decreases to battery life.
You could use two of those battery packs, one for each motor and then a third for the control system.

I'm not familiar enough with LiPo batteries to answer your last question but a little web searching should.
Do you have a link to the battery?

ukesh · « **Reply #7 on:** December 08, 2009, 06:37:33 PM »

Quote

Do you have a link to the battery?

yes, I do
http://cgi.ebay.in/DESIRE-POWER-LIPO-BATTERY-11-1-850mAH-3S-25C_W0QQitemZ150388307182QQcmdZViewItemQQptZLH_DefaultDomain_203?hash=item2303d774ee
two of theese would be better u say?

Or I should opt for an SLA battery. An 15AH rated Sla battery provides, 15AH/19 around 47.3 mins rite? The main reason I'm opting for li po is to reduce the weight of the bot.

waltr · « **Reply #8 on:** December 08, 2009, 07:59:39 PM »

That battery looks like it would work. The question is for how long will it run your Bot? Next question is how long do your need your Bot to run between charges?

If you need a longer run time then use one battery on each motor and the run time should double.
It is up to you to decide what is really need. So you can start with one battery and if you need more run time then split the motors to one battery each.

Do get a proper charger for that battery and don't discharge it below its 'cut-off'.

ukesh · « **Reply #9 on:** December 09, 2009, 05:32:01 AM »

Quote

That battery looks like it would work.

By that battery which u mean to say waltr? the SLA or the Lipo?

waltr · « **Reply #10 on:** December 09, 2009, 08:32:38 AM »

Since weight is a concern go with the LiPo.
What does the SLA battery weigh?

Design has everything to do with trade-offs.
For example: The LiPo battery is small and light but only has 850mAhr of capacity whereas the SLA to bigger and heavier but has 15,000mAhr capacity which will run to Bot for 17 time longer.
Which is the best choice?
If the Bot only needs to run for 5-10 minutes on a charge and needs to be quick (light) then the LiPo is the best choice.
However if the Bot needs to run for an hour but can take its time going somewhere then the heaver SLA is the better choice.

Without a lot of experience you take the best guess then try it. If it doesn't work try something else. If it works but not well find out why and make changes until it does work well.
Good luck and have fun.

ukesh · « **Reply #11 on:** December 10, 2009, 11:23:32 AM »

Quote

What does the SLA battery weigh?

15AH rated SLA weighs around 4Kg's. an 4.5Ah SLA weighs 1.5Kg's. 4.5AH rated battery will give me about 13 mins (approximately) of run time if my calculation is correct. Consider the losses etc, it should last for at least 8 to 10 mins. I think i'll be going with this battery.
Thanks for all advice waltr!

waltr · « **Reply #12 on:** December 10, 2009, 03:30:31 PM »

You are welcome. Please do post back with progress reports on your Bot.

ukesh · « **Reply #13 on:** December 11, 2009, 05:23:54 AM »

Sure man! And hey another small doubt. I heard that SLA's have slow discharge rates. Is that true? If it is, is that a real problem that should be looked into?

waltr · « **Reply #14 on:** December 11, 2009, 08:52:39 AM »

In my experience Sealed Lead Acid batteries can provide fairly high currents.

What they do not like is to be discharged too far, even by self-discharge from sitting. They they will permanently lose capacity.

And like any battery always use a properly charger designed for that particular battery chemistry.

ukesh · « **Reply #15 on:** December 11, 2009, 09:42:47 AM »

For an lipo pack u multiply the mah value and the capacity of the pack to get the max current that the battery can give. Is there any way to calculate the max current that an SLA battery can give?

waltr · « **Reply #16 on:** December 11, 2009, 11:54:27 AM »

Not really, you should use the manufacturer's data sheet the determine this.
In SLA batteries there are several types. Some are designed for very large current draw from a short period, these are normally rated with a "cranking current" which implies that they are for turning a starter motor on an internal combustion engine.
Another type is a "Deep cycle" which is designed from lower current draw than a "Starter" battery but can be discharged to a lower voltage without damage.

Here is a good battery tutorial:
http://www.batterystuff.com/tutorials_index.html

ukesh · « **Reply #17 on:** December 13, 2009, 12:38:52 PM »

hm..IL check with the data sheet. Since the battery is manufactured locally, the data sheet isn't available online. I'll have to check with the manufacturer in person. But I guess an SLA wouldn't have much problem of providing around 20A current. Once again, Thanks Waltr!

webgeek · « **Reply #18 on:** December 17, 2009, 09:08:53 AM »

Good SLA batteries are tanks. They are very commonly used to drive the cheap DIY electric bikes and such. My bot is using some small SLA batteries as well - Werker brand. They don't need special charging for the most part - just get a couple batteries and snag a couple chargers for em online and you are good to go. For instance, if you are running 24v or such you can use import electric scooter chargers and save money:
http://tncscooters.com/product.php?sku=101135

I have a funky scooter cart thingee I built this summer using electric scooter parts and four 12v deep cycle marine batteries (Duralast from Autozone) and I charge them over the course of a couple of days with one of those 48v import chargers. Of course those batteries are ~100AH

Mike

ukesh · « **Reply #19 on:** December 17, 2009, 11:00:51 AM »

Guys, Having a problem regarding SLA's. Since the batteries i have are not branded and are manufactured locally finding the data sheet is difficult. Is there any practical way to find the max discharge current? The batteries i have is similar to this:
http://www.robotmarketplace.com/products/0B-PS5-12.html

waltr · « **Reply #20 on:** December 17, 2009, 12:37:13 PM »

I general those types of batteries can supply quite a large current draw, maybe even in the 100's of Amps.

If you have the battery you can perform tests to measure this, or at least measure the battery's internal resistance.
Links: http://www.google.com/search?q=battery+internal+resistance+measurement&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a

Also you may put a small Ohm load, high current and measure the voltage drop. This wiil give you a rough idea as to the battery's max current draw.
Links: http://www.google.com/search?q=battery+maximum+discharge+current&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a

ukesh · « **Reply #21 on:** December 19, 2009, 11:23:51 AM »

I tried to measure the internal resistance of the battery using one of the methods in the links provided by you.
http://answers.yahoo.com/question/index?qid=20080214215136AACzjrF
I tried the first answer in that post. I used an 820 ohm resistor. First i measured the battery voltage. Then connected the resistor and measured the voltage drop, which was 0.01 volts. But the drop was not sudden, it took a couple of seconds. Then by using the formula mentioned in that post, I got the resistance of the battery as 0.638 ohms. So, using ohms law I got current as 20A. Are my calculations and method correct?

waltr · « **Reply #22 on:** December 19, 2009, 11:36:24 AM »

Your numbers sound to be 'in the ball park' so they are believable. With the motors you have chosen that battery should work.

ukesh · « **Reply #23 on:** December 19, 2009, 11:04:50 PM »

Hmm..I'm a bit concerned about the low voltage drop. Shall i try this method with an resistor with an higher value? Or Am i worrying too much?

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Author Topic: Regarding Motor Selection (Read 2901 times)

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