The Effect of Input Shaping on Coordinate Measuring Machine Repeatability

Coordinate Measuring Machines (CMMs) are a strategic element in the manufacture of high-precision parts because their measurements of part geometries are used for quality control and feedback on the manufacturing process. Proper CMM operation requires accurate knowledge of the position of the CMM's part sensor. The performance of a CMM is limited by background vibration levels, environmental conditions (temperature changes, cleanliness of the environment, etc.), accuracy in the measurement equipment (encoders and part sensor), and structural deflections between the encoders and the part sensor. The most important limitation depends on the type of CMM and the operating conditions; however, structural deflection is always an important limitation because it introduces an error in the indicated position of the part sensor. The effect of input shaping, a method of reducing residual vibration, on the quality of CMM measurements has been investigated. Tests were performed that verified the reduction of structural deflections when input shaping is used. Standard diagnostic tests were used to evaluate the repeatability of the CMM. Non-standard tests aimed at evaluating the CMM under adverse conditions were also performed. Input shaping improved measurement repeatability over a large range of operating parameters. Introduction Coordinate Measuring Machines (CMMs) measure manufactured parts to determine if tolerance specifications have been achieved. A CMM consists of a workspace in which parts are fixtured, a sensor for detecting the part surfaces, a mechanical assembly for moving the part sensor around the workspace, and a computer for calculating the part dimensions based on the sensor measurements. A sketch of a typical CMM is shown in Figure 1. The CMM sketched in Figure 1 is shown with a touchtrigger probe part sensor This sensor uses a ruby-tipped stylus to sense the part. When the stylus is brought into contact with a part surface, the deflection of the stylus triggers the computer to record the position indicated by the x, y, and z encoders. By probing the part on critical surfaces and recording their locations, the critical dimensions of the part can be calculated. A single cycle of a CMM measurement consists of four phases. First, the CMM performs a gross motion to move the part sensor to the vicinity of the part geometry that is to be measured. Second, the probe is allowed to come to rest. Third, the probe is reaccelerated to a small constant velocity in the direction of the part. This constant velocity portion of the measurement is called the pre-hit region because it immediately precedes the contact between the probe and the part. Finally, the stylus contacts the part and the computer records the location of the contact. The position of a touch-trigger probe during a measurement with a 2 mm pre-hit distance is shown in Figure 2. x y z TouchTrigger Probe y Encoder