calculating the wheels, engine, gearbox and h-bridge

dear friends

I had the assignment to calculate al the specifications considering the engines, gearboxes, wheels and h-bridge of our robot. First we had to chose our wheels. Our chose went to the pololu wheels (60mm x 8mm).

Now we can calculate the speed that the engines need, to drive at a topspeed of 2 m/s.
circumference: 0.06*3.1415 = 0.18849 m
RPM / meter : 1 / 0.18849 = 5.305
so for 2 m/s we find: 5.305 * 2 * 60 = 636.6 RPM

Second we calculate the torque that the engines minimum need to drive at 2 m/s.
We want the robot to accerelate in 2s to the topspeed of 2 m/s and the weight of the robot is calculated about 0.3 kg.

a= v/t    => a= 2 m/s / 2s = 1 m/s²

F= m*a     => F= 0.3kg * 1m/s² = 0.3 N

T= F*r      => T= 0.3N * 0.03m = 0.009Nm = 0.9Ncm

So the engine we search needs al least a torque of 0.9Ncm and a speed of +- 630 RPM. These values are calculated in free-run. To make sure that the engines we order are strong en fast enough, we will order an engine that is faster and has more torque than calculated. We can chose out of 2 engines:

-> 30:1 Micro Metal Gearmotor HP  ( 1000 RPM and 9 oz-in, stall current 1600mA )
-> 10:1 Micro Metal Gearmotor        ( 1300 RPM and 2 oz-in, stall current 360mA )

The first engine ( 30:1 Micro Metal Gearmotor HP) has a good speed, but the torque that he can generate is very high. this causes that the stall current is (to) high. Also the wheels of the robot will spin with such a high torque.

The second engine (10:1 Micro Metal Gearmotor) has a rather high speed, but the torque of the engine is good for our robot (2 oz-in  ->   1.4123 Ncm). This is a bit more than calculated, but will have no significant causes for us.

We have chosen for the 10:1 Micro Metal Gearmotor. The decisive was the good torque and the much lower stall current of the engine. The speed of the engine can we regulate later with our microship.



 data about the motor:
- torque                                            1.4123 Ncm
- speed                                             1300 RPM
- stall current:                                    360 mA
- free-run current                               40 mA
- weight                                             9.63 g
- ratio                                                10:1
- size                                                 24*10/12 mm

The torque of 1.4123 Ncm gives us an accerelation of:

F= M/r  => F= 1.4123Ncm / 3cm = 0.470N

a= F / m  => a = 0.470N / 0.3kg = 1.567 m/s²

In 1.27s we go to our topspeed of 2 m/s

Now we have chosen our DC motor, we can further on by searching a good H-bridge. The stall current of our engine is 360 mA. This  value is important for the choise of our h-bridge. The output current of our h-bridge must be higher than 360 mA.
Our choise went to the l 293D h-bridge, it is al frequently used h-bridge witch we already worked with last year.

Summarized:

-> WHEELS: pololu (diameter 60mm x 8 mm)

-> ENGINE: pololu, 10:1 Micro Metal Gearmotor, ( 1300 RPM and 2 oz-in, stall current 360mA )

-> GEARBOX: pololu, ratio of 10:1, in 2 stages (comes together with the engine)

-> H-BRIDGE: L 293D, output current +- 1A

With these calculations guillaime can finish the part list and order the products we need. I hope that we will recieve them as soon as possible. So we can go on testing en designing our robot.

Greets

Stef Ingels