How to choose the right servo for me?

[ben2264]

Hi!
How can I know which servo I need for my robot?
(what I need to consider)
Thank you!

[newInRobotics]

You need to consider required torque, velocity, available voltage and current.

[ben2264]

Thank you for you reply, what about what I need to consider in the sense of what the servos need to "move" like - car weight,length and such.
My budget is around 20$ each.The car is going to weight 1-2 kilograms(or 2.5-5 pounds) and I want it to go fast enough to outrun an averege human while he is running.Can you help me with the properties of the servos I need?
Thank you!

[newInRobotics]

If servos are to run car, then we need to know diameter of driving wheels You're planing to use. Also, what is the average running speed of human being? Talking numbers makes everything much easier in engineering 

[Soeren]

Hi,

My budget is around 20$ each.The car is going to weight 1-2 kilograms(or 2.5-5 pounds) and I want it to go fast enough to outrun an averege human while he is running.Can you help me with the properties of the servos I need?

First off, it won't happen with R/C servos!

Average running speed (for a reasonably normal built Joe Random with no particular running  history) is around 15km/h or 4.17m/s.

Since you didn't mention wheel diameter, I'll use 100mm (close to 4") and that would need to spin at 796 RPM to go the 15km/h - since average means that some runs faster and you want your vehicle to be still faster, 1100 RPM is a more realistic number.

Fast servos go through 60° at around 0.1s which equates to 100 RPM and you need 11 times that!

The torque requirement of 2kg shared between 2 wheels of 100mm in diameter, to barely move it is 200gf-cm (19.6134mNm) so you'll need at least double that for just a modest acceleration (no use in going fast. if it takes forever to reach the top speed )

Conclusion:
you need DC-motors, either gearheads or with a separate gearbox - something around the size of what's used in cordless drills.

Comparison. servo motor vs. cordless drill motor

These motors are many times heavier than an entire servo with box, gears and electronics and the battery needs to be able to drive them (so is heavier as well).

Weight is bad when you want speed! Each time you double the weight, you need four times as much power to reach the same speed and accel. figures, while halving the weight cuts the power requirement down to 25% so it's worth making the chassis (and everything else) as light as practically possibel.

[ben2264]

Thank you!
Can a DC motor turn both directions?
The wheel's diameter will be something like 10-12mm.
I think I'll change it a bit.What is a good speed for a (robot) car, size 300X210mm?
Thank you so much for you patience and help.

[newInRobotics]

Can a DC motor turn both directions?

Yes, they can, for that You use H-bridge.

The wheel's diameter will be something like 10-12mm.

OK, so with rotation You bot will travel 12*pi(mm) ~= 12 * 3.14 = 37.68mm. To be able of going at 22.5km/h (say it's an average running speed) wheels of robot have to spin at 9952.23rpm.

What is a good speed for a (robot) car, size 300X210mm?

How long is a piece of string? It's only for You to decide.

[ben2264]

Can you explain the calculations you made?Or give me a guide or something,I didn't understand the 12mm*pi calculation(and what its purpose).
Thank you

[newInRobotics]

Can you explain the calculations you made?Or give me a guide or something,I didn't understand the 12mm*pi calculation(and what its purpose).

To find out how far wheel goes in one rotation You must find its circumference.
c = 2 * r * pi
r(radius) = 6mm
pi ~= 3.14
c = 2 * 6mm * 3.14 = 37.68mm

To get RPM (revolutions per minute) required to achieve certain velocity (You said it has to be around average human running speed (lets say it's ~22.5km/h)) You convert Your 22.5km/h to mm/min, which is 375000mm/min and then divide it by circumference of the wheel, it all comes to 9952.229299363057 RPM.

[Soeren]

Hi,

Can a DC motor turn both directions?

Yes, just think of a R/C car (you've probably driven at least one of them).

The wheel's diameter will be something like 10-12mm.

Eh?
I'd guess that you meant 10..12cm (100..120mm)?

I think I'll change it a bit.What is a good speed for a (robot) car, size 300X210mm?

That depends a lot on what you want it to do, how it's cobnstructed, where it will run (plain area/rough terrain/etc.).

It shuldn't run faster than its still sensors allow it to react and only a very well behaved robot of development maturity should be made able to run faster than you, as you might need to physically stop it from going somewhere dangerous.

A line follower going around 1m/s would be considered fast in most circles.
What do you plan to make your 'bot do?

[ben2264]

Yes,I meant 10-12 cm.
The 'bot will drive on a plain area.
It is a Spy Bot - a robot controlled from a computer with a camera on it, sending the feed to the computer. 1-3m/s sounds great,I realised I went too far on that 22 kmh speed. I got to 3184.71 rpm, isn't that crazy?a DC motor with 3200 rpm costs about 200$,i thought something like 20$ each. Am I doing something wrong here?
I cant thank you guys enough.