"Homemade" servo

Hi

So I want to make my own servo to meet the demands of an upcoming project. I'm planning on using an RS395 from BaneBots(link in bottom of post). I can't seem to figure out, how fast it will travel 60 degrees(for comparison to commercially available servos). I know that I haven't said anything about gearing ratio, but that is where you guys come in. I really need a formula where I can insert an RPM value and get the travel-time in seconds(like 0.11/60 degrees) to compare speed to other servos. Do any of you have such a formula lying around? And if so, would you mind sharing it with me and the others?

Motor used
http://banebots.com/pc/MOTOR-BRUSH/M5-RS395-12

Thank you
ThatNinthGuy

No Load RPM : 15500

That is going to be very difficult to gear down to make a servo out of. What is your need to build you own servo? There are high-torque servos available, based on much slower motors.

ThatNinthGuy:
I really need a formula where I can insert an RPM value and get the travel-time in seconds(like 0.11/60 degrees) to compare speed to other servos.

In order to do that you would need to know the gear ratio between the motor shaft and the output shaft. Naturally, if this is a DIY servo and you haven't selected the motor yet you may not have chosen the gear ratio. To choose the ratio you would need to make a compromise between the torque provided by the motor, the torque and acceleration you need on the output shaft, the speed of the motor and the speed of the output shaft. I suggest that basing it on the stall current (rather than the speed) is probably easiest.

Trying to calculate this from the motor's no-load speed is probably going to be wildly inaccurate since the motor is not in a no-load condition, and since this ignores the acceleration time which will be significant for small duration movements of the sort you're describing. However, it would give you a ballpark figure to give you some idea what sort of speed you might be looking at for a given gearing.

I agree, it's going to be pretty-much impossible to calculate...
But, 15500 RPM = 15500 x 360 degrees / 60 seconds per minute = 93,000 degrees per second. (Of course, that's after it get's up to full-speed.)

Things we don't know.... Drag in the gears & resistance of the load, inertia in the motor/gears/load, acceleration of the motor under various loads, acceleration required... and probably some other unknown stuff. (Even if I did know that stuff, it's been a long time since I took a dynamics class or a motors class.)

The math:

15500 RPM / 60 sec = 258.3 RPS (revs per second)

So, that motor takes 1/258.3 (or 0.00387) seconds to do one revolution (360 degrees).

60 degrees is 1/6 of 360 degrees, so 1/258.3 * 1/6 = 0.000645 seconds to go 60 degrees.

Yeah I think you'll need some serious gearing down on that motor to make it into a useful servo motor, especially at full speed.

To figure out the speed with gearing... say you gear it 125:1... multiply the above by 125... 0.081 seconds to go 60 degrees.

Like stated before, under no load & perfect voltage/current conditions. So you'll have to know what conditions the servo you are comparing it to were tested under to make a fair comparison.