Note: Super minibot descent speed in brake mode

Using the Tetrix motors without a gear box  in the brake mode  can provide enough drag to slow the descent to keep the robot from self destructing when it hits the tower.   This can be done with a single pole, double throw (spdt)  typical micro switch.    A four way switch is not required.   The center pole of the spdt is wired to the plus sides of both motors.   The normally open pole of the spdt is connected to the negative sides of both motors and the normally closed pole  of the spdt is wired to one side of the battery through the robot start switch.

I’d like to review the equations as a check to my simulation results that show the descent speed to be about .27 m/s.

When the acceleration is zero, the steady state descent speed is reached.  Under this condition

tq_motor/r_wheel = W_lbs – drag

where :

tq_motor = tq_stall*(v/v_free) + tq_motor_drag

or solving for speed v/v_free

v/v_free =(( W_lbs – drag )*r_wheel  – tq_motor_drag)/tq_stall

tq_motor_drag = i_free*kt = i_free*tq_stall/(i_stall – i_free)

W_lbs = weight of robot

drag = u_drag*Normal_force

i_free = motor free speed current at 12 volts

i_stall = motor stall current

tq_stall = motor stall torq

v_free = motor free speed

The super minibot parameters are:

W_lbs = 2.7 , i_stall = 7.5a, i_free = .5a, v_free = 122 ips , r_wheel = .15,

tq_stall = 9.3oz in*2/16 = 1.2 in lb  (remember 2 motors)

drag = 1.5 lbs , tq_motor_drag = 1.2*(.5/(7.5-.5)) = .6/7 = .085 inlb

So plugging into the v/v_free equation

v/v_free = ((2.7 – 1.5)*.15 -.085)/1.2 = (.18-.085)/1.2 = .08

or v = 122*.08  = 9.8 in/sec or .24 m/s

This is equivalent to dropping the minibot from a height of   .24*.24/(2*9.8)=.003 m or about .1 ins.

So… things check out pretty well and we should not need a brake.

More minibot:

Note: Super minibot test run with PTC


Note: Minibot current calculation vs battery voltage

Minibot Model


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