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Author Topic: 1st sumo robot. require some advice (engines & sensors)  (Read 935 times)

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Offline thedude92Topic starter

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1st sumo robot. require some advice (engines & sensors)
« on: July 19, 2012, 04:07:15 AM »
Ok.. it goes like this.
Info: This will be my first mini sumo robot. I have a basic understanding on how everything will hopefull work but we would require some advice from a more experienced person.
We`re aiming at not spending very much money on it while also having a go at a decent performance :)
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Rules:
mass : 500g
dimensions : 100x100 base (milimeters), unrestricted height

Terrain:
Wodden disk like area. (magnets are forbidden and useless in this case)
77cm diameter . entirely black except a 2.5 cm circular white line on it`s edge.
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We have decided that the robot will be (90% certainty) 2 X wheeled with one wedge at the front and possibly additional wedges on the sides.
Picking the motor:  We were stuck between pololu`s Micro Metal and 25D mm categories.
http://www.pololu.com/catalog/category/60   <= micro      . 0.3 oz weight(~9 grams) , 24mm length
http://www.pololu.com/catalog/category/115 <= 25D mm . 3 oz weight(~ 90 grams) , 48 mm or larger length

Given the weight restriction I believe the best option would be the micro class since 2x25D would be about 200g and the accumulators would be about 200g as well .. not much left for ..anything.+25D would be hard to fit on a 100m width robot.

Major issue : we cannot decide how to balance out the Torque - RPM of these engines.
*RPM is at free-run , Torque at stall.
#1 : RPM:100, Torque: 70oz*in == 0.49 N*m
#2 : RPM:120, Torque: 60oz*in == 0.42 N*m
#3 : RPM:140, Torque: 50oz*in == 0.35 N*m
#4 : RPM:200, Torque: 40oz*in == 0.28 N*m

Wheels will be most likely custom made (we have the equipment for that) minimum diameter 30mm, maximum 40mm.

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Sensors:
4 x digital sensors : for detecting the enemy robot. we decided on digital because the distance to the enemy won`t help us very much and it would be a bit simpler to implement.
2 mounted on the wedge 1 on the left side 1 on the right side
sensor model : SHARP GP2Y0D340K. 40 cm range . we sacrificed a bit of range for the very good response time is has in comparison with the longer range models

4x floor sensors ( white/black).  here we are in .. trouble . we dont really know a decent model found this one
Pololu QTR-1RC digital distance sensor . no idea how good it is for the task.
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 So could anyone come up with a bit of advice :).. most importantly a good floor sensor :D we really don`t want to fly off the ring without being pushed at least. would be completely embarrassing.

Offline Soeren

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Re: 1st sumo robot. require some advice (engines & sensors)
« Reply #1 on: July 20, 2012, 07:40:36 PM »
Hi,

You may get better results building the reflex sensors yourself, as the Pololu sensors you linked are for very close distance work only.
A photo transistor, an IR-LED and two resistors is what's needed for a basic sensor, add an op-amp if you want it extra sensitive.

How many white line sensors do you plan?
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline thedude92Topic starter

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Re: 1st sumo robot. require some advice (engines & sensors)
« Reply #2 on: July 21, 2012, 01:39:57 PM »
Hi there Soeren. Thank you for your reply.
The robots base will be square with 1 sensor in each corner ( or very close to each corner) so 4 sensors.

I know they are designed for very short range :). I will mount them very close to the floor :D don`t worry!
If I have the time I`ll try building one too :)

I would like to ask for your opinion on a more critical aspect though.

I am afraid of picking a motor with too much torque that would sacrifice speed.
Therefore I tried making a few calculations to determine a respectable torque

Total down force of the robot will be ~ 5N
Exaggeration : 3N downforce per wheel ( in reality about 1.5..2 )
Exaggeration : 0.9 friction coeficient on the wheels
=> Torque = Force * radius of wheel.
suppose i have a 30oz*in (0.21 Nm) motor with 0.021m wheel radius
 => approximately 10N force per wheel (maximum)

//other values for motors would be 50 .. 40.. 22 oz in ( 0.35 .. 0.28..0.15 Nm respectively)

What do you think. Is it overkill? ..

I dont know what to expect in a REAL situation .. maybe you have more experience :)
I really dont know when you lose traction and wheels slip what matters more.. I thiink speed might matter then more than torque.. or neither.. dunno
« Last Edit: July 21, 2012, 01:44:04 PM by thedude92 »

Offline Soeren

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Re: 1st sumo robot. require some advice (engines & sensors)
« Reply #3 on: July 23, 2012, 11:44:54 PM »
Hi,

I know they are designed for very short range :). I will mount them very close to the floor :D don`t worry!
Why would I worry? ;)
And why would you not, given the 3mm working distance which make it harder to keep ambient light away from it? :o


If I have the time I`ll try building one too :)
Building four of them is faster, cheaper and way more healthy than swallowing a single MacD with fries, so I'd imagine that you've got time ;D


I am afraid of picking a motor with too much torque that would sacrifice speed.
There's no such thing as too much torque.


Therefore I tried making a few calculations to determine a respectable torque
[...]
What do you think. Is it overkill? ..
You didn't include the situation where you have to push another robot (with high friction wheels) sideways out of the ring.


I dont know what to expect in a REAL situation ..
Exactly, so you need to test things out. Make a dummy opponent with a realistic weight and wheel friction, similar to what you expect to meet in the ring (no need for motors and such, just a blank with some scrap or pebbles to weigh it in).

The best motor for your 'bot will depend on the exterior design and your strategy and as such, will be as personal as your tooth brush ;)
What's your strategy?
Got none?
Usually, you select your hardware to fit your strategy, it is possible to reverse the process of course, but then you have less freedom of choice.

If you think the motors you have chosen are too weak, get more powerful ones - if you can't find them in one place, look elsewhere.


maybe you have more experience :)
Nah, sumo isn't really my thing.
 

I really dont know when you lose traction and wheels slip what matters more.. I thiink speed might matter then more than torque.. or neither.. dunno
When you loose traction, only regaining it matters.
Zero traction equals zero torque and zero speed.

A weaker bot with a better exterior design and fighting strategy can easily mop the floor with a more powerful 'bot.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

 


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