DC motor polarity changer. Spec in detail inside.

[jop_bal]

err.... I am a layman. I am trying to design the following.

A hand held battery operated simple blade cutting lawn mower for farm use.

I have a 20K rpm DC motor @ no load operating with a 9V rechargeable battery.
A PWM circuit that is used for changing the speed.

Now, I need this motor to reverse the direction when it is struck(i.e. when rpm falls to zero) due to the load.

Note: I am not eager to measure the rpm. All I need is, if the motor does not run in one direction change the polarity, so that it strikes grass from the other direction.

[rbtying]

Use a current sensor on the motor leads, and reverse when nearing the motor's stall current.

[jop_bal]

How do I do it?

[newInRobotics]

I'm not sure how to achieve it because I've never used one before; but have a look at this Wiki and this SoR page