Vision system for dial gage torque wrench calibration

ABSTRACT In this paper, we present the development ofa fast and robust vision system which, in conjunction with the Dial Gage Cal-ibration system developed by AKO Inc., will be used by the U.S. Army in calibrating dial gage torque wrenches. The visionsystem detects the change in the angular position of the dial pointer in a dial gage. The angular change is proportional to theapplied torque. The input to the system is a sequence of images of the torque wrench dial gage taken at different dial pointerpositions. The system thenreports the angulardifference between the differentpositions. Thepnmary components ofthis visionsystem include modulesfor imageacquisition, linearfeature extraction and angle measurement. For each ofthese modules, sev-eral techniques were evaluated and the most applicable one was selected. This system has numerous other applications like vi-sion systems to read and calibrate analog instruments. 1. INTRODUCTION Vision plays a very importantrole in providing useful information about the environment thatcan be used for objectrecog-nition, industrial inspection, automated navigation and many other applications. Visual inspection tasks are increasingly beingautomated using computer vision systems. Their primary advantages are speed, accuracy, reliability, robustness and ability tooperate in environments not suitable for humans. This documentpresents a fastandrobustautomated vision system for the test-ing and calibration ofdial gage torque wrenches. This vision system, in conjunction with the commercial torque wrench testingsystem developed by AKO Inc., will be used by the United States Army.A torque wrench is a hand tool used for applying a measured amount of turning force (torque). It provides leverage andindicates the amount oftorque being applied. Theirproper calibration is critical because nuts and bolts have to be tightened to aprescribedvalue. Toolittle ortoomuch torquecanresultin damage to thecomponents. Therearemany types oftorque wrenches.This system has been designed fora special kind oftorque wrench called dial gage torque wrench (Fig. 1). The torque applied isindicated by a dial pointer in the dial gage, mounted on top of the torque wrench.In calibrating torque wrenches, a specified setoftorques isappliedto thetorquewrench using acomputerconirolled, servo—actuated torque wrench test and calibration system. The controller and the torque sensing circuit drive the servo system to theprescribed torque value. The operator visually inspects the torque wrench dial gage reading and manually adjusts the appliedtorque to bring the reading to the prescribed torque value. The difference between the applied and the prescribed torque valuesconstitutes the calibration error. The vision system performs the task ofthe operator. Instead ofreadjusting theapplied torque toobtain an accurate match, this system detects the torquereading on thedial gage. This is accomplishedby measuring the changein the angular position of the dial pointer.The vision system primarily consists of three modules, viz. image acquisition, feature extraction and angle measurement.Image acquisition module acquires gray level images of the dial gage using a frame grabber. The frame grabber digitizes framesfrom a video camera and stores in the image buffer. The calibration process requires two images of the dial gage at differentpositions of the dial pointer. In the feature extraction module, the images are subtracted to remove the static background whichconsists of the dial gage. The subtracted image contains the dial pointers in the shape of thin triangles. This image is thresholdedto remove noise and skeletonizetho approximate the dial pointers by thin lines. The Hough Transform method of line detection is