Author Topic: Question about motor torque, etc...............  (Read 6068 times)

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Offline RITESH29Topic starter

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Question about motor torque, etc...............
« on: November 23, 2011, 11:02:17 PM »
Hello everyone,

I want to clear one doubt that is our team want to make robot as shown in video the manual one......
[youtube]ABU Robocon 2012 Hong Kong Game Rule[/youtube]
the wt of robot will not exceed 25Kg and a person is to be sited on it.........app. of 65Kg so the total wt will be of 110Kg max.

for this we require a motor the problem is the cost of motor and wt of motor is very high ( app 15kg per motor) so I am thinking to use this motor whose torque is 30Kg ...
http://robokits.co.in/shop/index.php?main_page=product_info&cPath=2_3&products_id=186&zenid=ea3264b3432e30fb2fb8781c2d060a47
so if i will use these motor 2 at back 2 in middle and 2 in front total 6 motor do you think that will work .......( can lift that much load)????


Please suggest..!!

Thanks
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Offline newInRobotics

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Re: Question about motor torque, etc...............
« Reply #1 on: November 24, 2011, 01:12:24 AM »
Robot wheel radius is required to tell You what You want to know  ;)
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Offline RITESH29Topic starter

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Re: Question about motor torque, etc...............
« Reply #2 on: November 24, 2011, 04:55:59 AM »
HI NIR,

I am not sure about radius i think it will be just 7Cm....
as i already mention the cost and wt of motor is very high i decide to use these 6mtor  for robots of high torque so do you think will they work or not???
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Offline newInRobotics

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Re: Question about motor torque, etc...............
« Reply #3 on: November 24, 2011, 07:04:36 AM »
O say no. Here's my reason: to be on the safe side (and not to loose any RPM), I calculate required torque imagining that bot has to go up 90deg slope (a vertical wall) times two. So, to do that You need to have 110Kg * 7cm * 2 = 770Kg.cm * 2  = 1540Kg.cm of torque. Your motors can deliver stall torque of 30Kg.cm, hence 6 motors will be able to deliver 180Kg.cm torque which is not enough based on my calculations.
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Offline RITESH29Topic starter

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Re: Question about motor torque, etc...............
« Reply #4 on: November 24, 2011, 08:18:13 AM »
Quote
O say no. Here's my reason: to be on the safe side (and not to loose any RPM), I calculate required torque imagining that bot has to go up 90deg slope (a vertical wall) times two. So, to do that You need to have 110Kg * 7cm * 2 = 770Kg.cm * 2  = 1540Kg.cm of torque. Your motors can deliver stall torque of 30Kg.cm, hence 6 motors will be able to deliver 180Kg.cm torque which is not enough based on my calculations.
HI,

Please explain me the calculation..........
and in the arena there will be no slope as shown in above video.
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Offline newInRobotics

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Re: Question about motor torque, etc...............
« Reply #5 on: November 24, 2011, 08:32:17 AM »
Quote from: RITESH29 on November 24, 2011, 08:18:13 AM
Please explain me the calculation..........
Stall torque required to hold 110Kg at 90deg angle with an arm of 7cm is 110Kg * 7cm = 770Kg.cm. On flat land You need less than that, however, RPM will be very low and motor will be running near it's stall torque drawing loads of current. So, to avoid killing Your motor and to get full RPM available, You have to go for motor with higher torque, hence multiply minimum required torque (770Kg.cm) by 2 (to be on the safe side), but that's only on flat land; if one needs good RPM going up the slope, even higher torque motor would be required.

Quote from: RITESH29 on November 24, 2011, 08:18:13 AM
and in the arena there will be no slope as shown in above video.
Video link is invalid.
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Offline RITESH29Topic starter

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Re: Question about motor torque, etc...............
« Reply #6 on: November 24, 2011, 08:39:52 AM »
Quote
Stall torque required to hold 110Kg at 90deg angle with an arm of 7cm is 110Kg * 7cm = 770Kg.cm. On flat land You need less than that, however, RPM will be very low and motor will be running near it's stall torque drawing loads of current. So, to avoid killing Your motor and to get full RPM available, You have to go for motor with higher torque, hence multiply minimum required torque (770Kg.cm) by 2 (to be on the safe side), but that's only on flat land; if one needs good RPM going up the slope, even higher torque motor would be required.

I don't know why are you calling it arm and at 90*...??
ABU Robocon 2012 Hong Kong Game Rule
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Offline RITESH29Topic starter

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Re: Question about motor torque, etc...............
« Reply #7 on: November 24, 2011, 09:18:35 PM »
help required..........
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Offline newInRobotics

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Re: Question about motor torque, etc...............
« Reply #8 on: November 25, 2011, 01:22:14 AM »
Quote from: RITESH29 on November 24, 2011, 08:39:52 AM
I don't know why are you calling it arm and at 90*...??
In this case arm is radius of the wheel. 90deg means climbing vertical wall. I hope diagram below explains it better  ;)
« Last Edit: November 25, 2011, 01:23:28 AM by newInRobotics »
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Offline RITESH29Topic starter

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Re: Question about motor torque, etc...............
« Reply #9 on: November 25, 2011, 05:13:18 AM »
Quote
90deg means climbing vertical wall.

Hi, but why the robot is assumed to climb on wall it it will follow surface not any slope..........
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Offline newInRobotics

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Re: Question about motor torque, etc...............
« Reply #10 on: November 25, 2011, 05:30:25 AM »
Logic behind my calculations is as follows: If motor is strong enough to hold weigh of the robot, it will be strong enough to move it on flat land without much sacrifice of RPM. Does that make sense?

Quote from http://www.societyofrobots.com/actuators_dcmotors.shtml
Quote
Torque
When buying a DC motor, there are two torque value ratings which you must pay attention to. The first is operating torque. This is the torque the motor was designed to give. Usually it is the listed torque value. The other rated value is stall torque. This is the torque required to stop the motor from rotating. You normally would want to design using only the operating torque value, but there are occasions when you want to know how far you can push your motor. If you are designing a wheeled robot, good torque means good acceleration. My personal rule is if you have 2 motors on your robot, make sure the stall torque on each is enough to lift the weight of your entire robot times your wheel radius. Always favor torque over velocity. Remember, as stated above, your torque ratings can change depending on the voltage applied. So if you need a little more torque to crush that cute kitten, going 20% above the rated motor voltage value is fairly safe (for you, not the kitten). Just remember that this is less efficient, and that you should heat sink your motor.
« Last Edit: November 25, 2011, 05:35:28 AM by newInRobotics »
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Offline RITESH29Topic starter

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Re: Question about motor torque, etc...............
« Reply #11 on: November 25, 2011, 06:34:13 AM »
Quote
hence multiply minimum required torque (770Kg.cm) by 2 (to be on the safe side)

OK, i think i am getting you  multiply it by 2???
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Offline newInRobotics

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Re: Question about motor torque, etc...............
« Reply #12 on: November 25, 2011, 07:16:46 AM »
Quote from: RITESH29 on November 25, 2011, 06:34:13 AM
OK, i think i am getting you  multiply it by 2???
That's if You want Your robot going up the hill with no (or very small) RPM loss (as a precausion). If Your bot goes on flat only, You can use smaller number to multiply it (say 1.2), however acceleration won't be as good.

All in all, You need at least 770Kg.cm combined torque (from all motors) to get Your bot going.

It would be also useful to know how fast You want Your bot to go. And, can You get datasheet for motors which contains RPM vs Torque chart?
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Offline RITESH29Topic starter

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Re: Question about motor torque, etc...............
« Reply #13 on: November 25, 2011, 07:40:16 AM »
Quote
That's if You want Your robot going up the hill with no (or very small) RPM loss (as a precausion). If Your bot goes on flat only, You can use smaller number to multiply it (say 1.2), however acceleration won't be as good.

How, please explain...................

and suggest any cheap and good motor for this purpose.........!!
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Offline newInRobotics

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Re: Question about motor torque, etc...............
« Reply #14 on: November 25, 2011, 07:54:32 AM »
Quote from: RITESH29 on November 25, 2011, 07:40:16 AM
How, please explain...................
How what? Sorry, I don't follow You.
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Offline RITESH29Topic starter

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Re: Question about motor torque, etc...............
« Reply #15 on: November 25, 2011, 09:51:15 AM »
Quote
How what? Sorry, I don't follow You


Actually I mean to say why and how there come 2 and 1.2 multiplication factor....exact condition.
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Offline newInRobotics

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Re: Question about motor torque, etc...............
« Reply #16 on: November 25, 2011, 06:49:12 PM »
It doesn't - it's sort of guesstimation. To know precisely how much torque is needed, You have to work it out using friction constants of tires and ground surface, bearings and other related moving parts, also desired acceleration and top velocity together with motors RPM vs Torque chart have to be taken into consideration. Calculations I gave You is a rule of thumb and should be regarded as one if high precision is required (in terms of not having motors that are too powerful).
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Offline RITESH29Topic starter

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Re: Question about motor torque, etc...............
« Reply #17 on: November 25, 2011, 11:46:04 PM »
HI again,

Please suggest where to buy this motor, one i have seen in robot marketplace but it was very costly.
I have seen one here is info of it.
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Offline newInRobotics

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Re: Question about motor torque, etc...............
« Reply #18 on: November 26, 2011, 06:29:57 AM »
What speed do You need Your robot to be capable of?
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Offline RITESH29Topic starter

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Re: Question about motor torque, etc...............
« Reply #19 on: November 26, 2011, 06:44:21 AM »
Hi again,

As per video i think there should be 200RPM at least.
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Offline newInRobotics

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Re: Question about motor torque, etc...............
« Reply #20 on: November 26, 2011, 07:13:07 AM »
Quote from: RITESH29 on November 26, 2011, 06:44:21 AM
As per video i think there should be 200RPM at least.
Well, that gives You 5km/h speed. I believe Your best bet is to go for 2x Wheelchair motors.
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