I think that I will not use stepper motors as they use more power than a standard dc motor from what I have read. I think they suck full amperage at o rpm as they need to "hold" the motor shaft lock in that position.
So having said that, what type of motor controller would I need? And what particular type of motor would I need?
I know that I would have a maximum voltage of 18v to play with, as I am hoping to use the cordless drill lithium battery packs. I would like to have the machine drive as smoothly as possible. So how do you do this? Do you just make sure that you have enough motor torque so that the motor is able to overcome any change in friction or drag. Or do you add a PID control system to it to keep it at a constant travel speed.
I would also need to use an encoder wouldn't I?
Operation of the machine:
1. turn the robot on
2. enter a total distance to drive. via a numerical keypad, or even a rotary knob having the values count from 0mm up to whatever.
3. enter a speed at which to drive that distance- again either via a keypad or rotary knob
4. hit button and go
And then on another mode;
1. turn the robot on
2. enter a total distance to drive
3. enter length of welding distance (or distance of of a relay turned on)
4. enter gap between welds (or distance of a relay turned off)
5. hit button and go
So I guess I would need an encoder to run the second mode where I would want it to turn a relay on and off at set distance intervals. I want to do it this way rather than use time to turn it on, ie on for 5 seconds off for 10 seconds etc.
So having said that what type of encoder do I need. I see the quadrature encoders are talked about, don't know what they are exactly though. How many count encoder would I need for this project?
With the motor selection, I roughly worked out that with a drive wheel diameter of 50mm I would need that wheel to spin at between say 1rpm and 10rpm to give me a forward motion speed of between 250 and 1500mm/min.
50mm dia wheel C=pie x diameter, C=3.142 x 50mm, = 157.10mm traveled per 1 rpm of motor.
How many rpm needed to travel 1000mm. distance / my wheel circumference, 1000mm/157.10, =6.36 rpm will move the robot 1000mm
Then I think I go, 6.36rpm / 1000mm = 0.00636 (what are these units called then?)
Then I can multiply my desired travel speed of 250mm/min x 0.00636 = gives me 1.59rpm, so 250mm/min travel speed, means I need a drive wheel rpm of 1.59rpm
and for my top speed of 1500mm/min, 1500 x 0.00636 = 9.54rpm for drive wheel speed.
So that is why I say I need the final drive wheel speed to be between 1rpm and 10rpms.
Now I see dc gear motors online, but now how much power do I need.
Here is a motor, http://www.servocity.com/html/10_rpm_precision_gear_motor.html
12v, 750oz, 1amps stall current, gear train damage can occur if motor is stalled???, also 10rpm no load free speed.
I will take a figure out of the blue and say that I want to have the motor powerful enough to pull a weighted load of say 10kg.
750oz/in torque rating on that motor means that it can lift a weight of 750 ounces at the point that is exactly one inch from the centerline of the motors shaft, right?
So If I am working in kg, I want to know how much 750oz is in kg or grams. So I found this online "divide the oz value by 35.275 to get kg"
750oz / 35.275 = 21.26kg
okay, is that right?? that seems like a lot.
Well if that is right, that motor can lift 21.26kg at 1" which is 25.4mm. My wheel is 50mm or say 2".
That would mean that that motor on my 50mm diameter wheel should be able to lift half of 21.26, which is 10.63kg.
Okay, but I do not need to actually lift 10.63kg of weight, that is just how heavy the moving robot and combined weight is.
It's would probably have enough torque from that motor, right?
What about the issue of "gear train damage might occur if the motor is stalled?" I don't like the sounds of that.
Can I find a better or stronger motor?
Can I have some kind of current detecting circuit that will trip out the motor drive power when it reached a predetermined amperage draw. This motor is rated at 1Amp, would you then make it stop at say 0.8amps? If so, then, I would need a bigger motor as I only get full torque at the 1 amp draw, right?