A method and apparatus for adjusting torque in an electric impact torque wrench. A current sensing circuit passes current through a power relay for powering an electric impact torque wrench. The power relay is normally in a conducting state. The current sensing circuit includes a threshold current indicator which is activated when current flowing through the current sensing circuit reaches a predetermined threshold amount. The threshold current indicator is provided to a first timer which counts an activation-off time since the threshold indicator is activated, the first timer presuming that the current has remained above the predetermined threshold amount during this time. At the conclusion of the activation-off time, the first timer signals a time-off extender circuit. On receipt of the signal, the time-off extender circuit (1) switches the power relay to a non-conducting state, to interrupt the flow of current therethrough, (2) counts a predetermined time-off time, at the conclusion of which the time-off extender circuit switches the power relay back to the conducting state, and (3) resets the first timer for repeat of the aforedescribed cycle.

The present invention is directed to a motor rotation speed controlling apparatus including a comparator, a detecting circuit for detecting a feedback signal of a motor, a motor driving transistor, and a circuit for changing the level of a constant voltage fed to the comparator. The comparator compares a saw-tooth voltage with a constant voltage and outputs a signal which has a duty cycle corresponding to the result of the comparison. The motor driving transistor drives the motor in response to the output signal from the comparator. To change the duty cycle of the output signal from the comparator, a circuit is used that changes the level of the constant voltage in response to a feedback signal detected by the detecting circuit.

It is foreseen to measure the rotational speed (V) of a motor (4) by measuring the electromotive force (e.m.f.) This e.m.f. is calculated as the difference between the terminal voltage (Va) and the voltage drop at a reference resistor (S), through which the armature current flows, then this value is used as a velocity feedback signal for motor regulation. Particularly, the terminal voltage (Va) is cyclically measured at two successive close instants (t1, t2), during which the armature resistance (Ri) and the motor e.m.f. can be considered constant; these terminal voltage values (V.sub.at1, V.sub.at2) are stored, then elaborated to perform the actual armature resistance (Ri) during that interval (t1-t2), then this value is multiplied for the instant current (I). The result is then substracted from the terminal voltage (Va) and only at that time the resulting e.m.f. is used as a velocity feedback signal for the velocity feedback circuit. The apparatus for carrying out this process is also disclosed.

A first capacitor is discharged with every pulse of a train of pulses of a frequency proportional to the actual speed of a rotary machine under control and is charged up during intervals between these pulses. The voltage of the first capacitor provides a signal to a threshold circuit which operates to provide a second signal whenever the voltage of the first capacitor exceeds a predetermined voltage which may be set in order to set the speed at which the machine is to be controlled. During the presence of the second signal a second capacitor is charged, beginning at a predetermined voltage, preferably zero. The regulating magnitude for the machine to be controlled is derived from the voltage of the second capacitor either directly or indirectly through a controller circuit and a power stage. The circuit is particularly useful for control of the speed of machines that run slowly or can conveniently provide only relatively few speed-indicating pulses per revolution.

A control circuit is used during slow or stop-motion playback operation of a video tape device to drive video tape intermittently. In order to avoid errors in the stopping position of the tape, a pulse width modulated (PWM) signal is provided to a motor controller circuit associated with the capstan motor of the video tape device. A frequency signal generator coupled to this motor generates a frequency signal that varies with the capstan motor speed, and this frequency signal is provided to a retrigger monostable multivibrator which generates a PWM signal whose duty ratio changes in proportion to the capstan motor speed. A drive pulse generator and a brake pulse generator provide drive start and brake start signals, respectively, when it is desired to commence and stop movement of the tape. A drive circuit, including the retrigger monostable multivibrator, receives the drive start signal, the brake start signal, and the frequency signal to generate the PWM signal which is supplied to the motor controller circuit. The frequency signal is selected to have a pulse rate several tens of times the ripple frequency of the motor's torque ripple characteristic so that inaccuracies in stopping position of the motor are eliminated.