Control method and control unit for impact type screw fastening device

A control unit for an impact type screw fastening device using an electric motor as a rotation driving source, the control unit comprising:

• a torque detecting portion that detects tightening torque of a screw by the motor;
• a setting portion that sets a first set value and a second set value with respect to the tightening torque;
• a pulse driving portion that drives the motor to rotate by supplying thereto pulse current having a rectangular waveform with a flat peak and a predetermined period so that a speed of the motor becomes an intermittent waveform corresponding to the pulse current in a time axis and, at the same time, torque exerted on a load by rotation of the motor becomes pulse torque corresponding to the pulse current and increases pulse by pulse in succession in the time axis;
• a pulse control portion that controls the pulse current, in a first stage until the pulse torque reaches a first set value so that the pulse torque increases pulse by pulse in succession in increments of a first torque value, and controls the pulse current, in a second stage starting from when the pulse torque reaches the first set value and until the pulse torque reaches a second set value, so that the pulse torque increases pulse by pulse in succession in increments of a second torque value that is smaller than the first torque value;
• and
• a stop control portion that stops supply of the pulse current to the motor when the tightening torque reaches the second set value.

A screw fastening device comprising:

• a screw fastening device main body that uses an electric motor as a rotation driving source;
• a torque detecting portion that detects tightening torque of a screw by the motor;
• a setting portion that sets a first set value and a second set value with respect to the tightening torque;
• a pulse driving portion that drives the motor to rotate by supplying thereto pulse current having a rectangular waveform with a flat peak and a predetermined period so that a speed of the motor becomes an intermittent waveform corresponding to the pulse current in a time axis and, at the same time, torque exerted on a load by rotation of the motor becomes pulse torque corresponding to the pulse current and increases pulse by pulse in succession in the time axis;
• a pulse control portion that controls the pulse current, in a first stage until the pulse torque reaches a first set value, so that the pulse torque increases pulse by pulse in succession in increments of a first torque value, and controls the pulse current, in a second stage starting from when the pulse torque reaches the first set value and until the pulse torque reaches a second set value, so that the pulse torque increases pulse by pulse in succession in increments of a second torque value that is smaller than the first torque value;
• and
• a stop control portion that stops supply of the pulse current to the motor when the tightening torque reaches the second set value.