 Axis Error Compensation Mapping Explained If your machine is controlled by an NEE AMC controller it can probably be made significantly more accurate by Error Compensation Mapping, a new service available from NEE Controls. We recently performed Error Compensation Mapping on a 12 x 6ft dual Y drive, High gantry, Water Jet cutting machine. this brief report describes the process and the results achieved. Heart Of Georgia Metalcrafters have had an NEE AMC4 controlled Waterjet Cutter for some 4 years. The accuracy of parts produced on this machine was less than they required because of the inbuilt inaccuracies of the axes lead screws. When high accuracy parts were required they had to be bought in. In order to compensate for the inaccuracies the mapping process involved successively incrementing the machine position by a preset distance and comparing the machine positions derived from the motor encoders for the 3 machine axes, with an accurate linear encoder supplied by NEE and temporarily mounted to the machine cutting head. The operation was performed by a PC running NEE 'ECM' software, results obtained are stored to generate an error map. The error map was then transferred and permanently stored in the AMC controller where it is used to automatically correct machine positions as the machine operates. For these tests it was assumed that the linear encoder used was perfectly accurate. In reality this is not the case so the mapping errors obtained will have an additional error, the error in the linear encoder but the inaccuracies are far less than those inherent in the leadscrew. The linear encoder specification indicates that it is accurate to + (10+10L)µm where L is in meters. Over 12 ft, the length of the X axis, this equates to +0.046mm or 0.0018inches. Over 6ft, the length of the Y axis, it equates to +0.0009inches. The better the measuring device - the better the end result. The results of the mapping process is shown in graphs indicating: a) the machine errors in each axis of the machine without error compensation, and b) the machine errors in each axis of the machine with error compensation. Analysis of results Graph 1  The X axis graph shows that the machine error without compensation ranges from 0.074inches to -0.052inches. The Y axis graph shows that the uncompensated error range is from +0.001 to -0.044 inches. Y axis measurements were taken every 2mm. The Z axis graph shows the uncompensated error range is from 0 to -0.030 inches. Measurements were taken every 2mm. Graph 2  The X axis error after compensation is shown on the second graph as Axis X, the error, after error compensation, ranges from +0.0012 to -0.002. The Y axis error at compensated points, when error compensation is used, ranges from +0.001 to -0.002 inches. The Z axis errors at compensated points, when error correction is used, ranges from +0.002 to -0.002 inches. Graph 3  As an additional check the errors associated with each motor rotation were checked. The ball screw pitch is 20mm (0.7877ins). Readings were taken at 2.5mm points, so there are 8 measurement points per motor rotation. Close inspection of a range of 40 consecutive readings (5 motor rotations), indicated that there were errors of up to 0.005 inches during each motor rotation, these are shown in the graph 'X axis Errors from 500 to 600mm'. Errors were also present on the Y (0.006 inches during rotation) and Z axes (0.01inches during rotation). After investigation it was found that these errors emanated from an axial alignment problem between the ball screw and motor shafts. The type of coupling used produces errors as the motor rotates if there is an alignment problem. A different type of coupling is to be fitted, this will effectively resolve this problem. After error compensation mapping had been completed parts were cut at many positions on the machine then measured on a Computerised Measuring Machine. Part accuracy was within 0.005 inches 'ECM' software is also able to indicate other mechanical errors existing in the machine that impact part accuracy, including axis backlash. |