Aiming at the problem of complicated structure

small torque range and low precision of the digital torque wrench in the market

a digital torque wrench based on the rotational speed difference method is researched and fabricated. The system implements torque control by using the linear relationship between torque and rotational speed difference of series motor

the pulse signal outputted by the speed sensor is captured by the capture compare PWM（ CCP） module of the programmable interrupt controller（ PIC） microcontroller unit（ MCU）

and use the sliding weighted average filtering algorithm to eliminate random interference noise and the speed difference is obtained. Finally

the calibration data is processed using the least squares method

and the relationship between the torque and the rotational speed difference is obtained. The experimental results show that torque output range of the digital torque wrench is 1 200 ～ 4 000 N·m

and the system error is less than 3 %

which verifies the feasibility of the design scheme.