Method and apparatus for control of current in a motor winding

In a motor-drive circuit for causing the current in the motor winding to follow a certain command waveform having a rapidly decaying portion comprising

• current supply means for supplying a flow of current in a first direction;
• a motor winding connected across said current supply means for flow of current from said current supply means through said motor winding for driving said motor;
• first and second current modulator means connected in series with said motor winding and on opposite side of said winding for modulating the flow of current through said winding;
• a first diode means connected in parallel with the series connection of said motor winding and said second current modulator means, and in reverse polarity to said first direction of current flow, for allowing current to circulate through said motor winding and said second current modulator means in the first direction when the current flow to said motor winding from said power supply means is interrupted by said first current modulator means;
• a second diode means connected in parallel with the series connection of said motor winding and said first current modulator means and in reverse polarity to the first direction of current flow for allowing the current to circulate through said motor winding in the first direction, and through said power supply means and said first diode means in the opposite direction, when the current is interrupted by both said first and second current modulator means, whereby the circulating current in said motor winding is caused to decay rapidly;
• first sensing means for sensing the flow of current through said second current modulator means to derive a first sensed output;
• comparator means for comparing said first sensed current output with the command waveform to derive a first error signal output;
• said first current modulator means being responsive to said first error signal output, for causing the current flowing through said motor winding and said second current modulator means to follow the command waveform;
• control sensing means including means for sensing loss of control over the current in said motor winding and means for producing a loss of control output when said first current modulator means can no longer cause the current through said motor winding, to decay rapidly enough to follow the rapidly decaying portion of the command waveform;
• and
• said second current modulator means being responsive to said loss of control output, for modulating current flow therethrough and for causing current to circulate through said motor winding in the first direction, through said second diode, power supply means, and first diode means in the opposite direction, and to cause the circulating current in said motor winding to decay rapidly.