Emergency head unload system for magnetic disk drive

In a disk drive including a disk having a magnetic surface and a motor having a positioner coil for radially moving a head support structure along said disk surface, apparatus responsive to an emergency condition for causing said coil to rapidly move said support structure radially outwardly at a controlled velocity, said apparatus comprising:

• switch means having first and second input terminals and including means capable of selectively defining a first state to connect said first input terminal to said positioner coil or a second state to connect said second input terminal to said positioner coil;
• brake circuit means electrically connected to said first input terminal and responsive to a signal indicative of said emergency condition for reducing the velocity of said positioner coil toward zero;
• means responsive to said emergency condition signal for switching said switch means from said first to said second state after a predetermined time delay;
• velocity control circuit means connected to said second input terminal for supplying a substantially constant current through said switch means to said positioner coil;
• emergency power supply means;
• and
• means for connecting said emergency power supply means to said brake and velocity control circuit means to supply power thereto during said emergency condition.