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
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.