A Review of Three-dimensional Vision for Robotics

Phase monitoring is a technique which can be used in several aspects of automated manu-facturing: inspection, assembly and positioning. In phase monitored inspection, acoustic wavesreflected off a master part «re picked up by an array of microphones. The phase shift of the re-flected wave in each aiicrophone is compared to the subsequently measured values of a part to beinspected; differences in these phase signals can be interpreted as geometry differences between«aster and part. Similarly, the phase monitoring technique can be used for automated assembly byattaching the sound source and microphone array to a manipulator hand; the phase signals determinethe relative position between the manipulator and the object being assembled. Automated position-ing, on the other hand, uses similar phase signals to position objects in the proper orientationbefore further machining or assembly. Phase monitoring is suitable for low production runs (where economics eliminate specializedtransfer line equipment) since it can be easily recalibrated for different part shapes. PM is ex-pected to be low cost--est unated at between S2.000 and $5,000 per system--because its componentsare all off-the-shelf speakers, microphones and microprocessors. Other advantages are high speed(several hundred measurements a second), no moving parts, and reasonable stand-off distance (sev-eral inches). Accuracy is high: changes in geometry of as small as a mil ( 0 3mm) can be detectedwith present equipment. INTRODUCTION. Inspection and assembly todayare the least automated of manufacturing oper-ations, though some automation of these opera-tions does exist.* Automated inspection canbe as sophisticated as Bendix's Cordax inspec-tion machine operating under computer control;?automated assembly can be as sophisticated asKawasaki's assembly of motorcycle engines.However, most shop practice is with hand-oper-ated inspection and assembly equipment: verylittle auromation has worked its way into theseprocesses. Consider automated inspection as an illus-tration. It has been applied most successfullyto large production runs. For example in themanufacture of automobile connecting rods,precision inspection of the machined surfacesis automatic. Because of the large number ofnearly identical parts, economics dictate aninspection 'transfer line". The inspectionmachine is specially designed for each partshape. While effective for large productionruns (a million parts or more) the transferline methods are not economical for low-volumeinspection. An inspection system must be flex-ible to inspect parts of many different shapes. Phase monitoring is a technique which caneliminate much of the labor involved in handInspecting and assembly for the smaller pro-duction runs where transfer line equipment isnot suitable. Phase monitoring (PM) is flexi-ble enough to be easily recalibrated for eachnew part shape. It is expected to be a low-cost method—estimated costs are between $2,000and $5,000 per inspection station. PM tech-niques have other advantages of high speed(several hundred measurements a second), nomoving parts and reasonable stand off distances(several inches) . Automated inspection will be used to illus-trate the PM technique; assembly and position-ing will be shown later to be variations ofinspection. Figure 1 shows a schematic of aPM automated inspection system. An array ofwave emitters and wave receivers are locatedaround the object to be inspected; for simpli-city, only a single emitter is shown. Theemitter sends out acoustic waves of a constantwavelength which are reflected from the objectand then picked up by the receivers. As a re-sult of the constant wavelength emission, theoutput of the receivers will be a ainusoidalsignal of the same frequency as the emittedwave but differing in both amplitude and phase.Inspection of the part is made possible by moni-toring the phase differences between emittedand received waves. First, a master workpiece is positioned atthe inspecting station. The plurality of thephase measurements monitored at each receiverconstitutes the "phase vector" ot the masterworkpiece: (», ,e 2' ' •V-(1)A subsequent object to be inspected is po-sitioned at the same point and orientation, anda similar phase vector:• ' •t«V'2.(2) is monitored for the object. The difference inphase between the master and the object: At (6j-e j.«2 -e-(3) can be used to determine whether a part is with-in tolerance, where it isn't, and by how much.Assume for the time being that the object andmaster can be placed in the same orientation andposition; this positioning can be either by con-ventional methods or by PM positioning (to bediscussed later). For many gross errors the object can be re-jected if the phase difference vector is simplyunrecognizable. Smaller errors require moresophistication. A sensitivity matrix, S, canbe determined which gives the sensitivity ofeach phase difference, ABJ • for each dimensiontolerance of the object; each element of S isthe partial derivative of a particular dimen-sion to the phase change at a particular micro-phone. Multiplication of the phase difference °vector times the inverse of the sensitivity ma-trix results in a deviation vector: -lad • S *ee(4) The deviation vector is the amount thateach dimension of the object has changed or'deviated* from that of the master workpiece: d • (d,•*»>(5) If any change la outside the tolerance forthat dimension the object is rejected.