Abstract: Because the machine tool is characterized by large mass and difficulty in moving， it is always a difficult problem to measure the position error of its vertical positioning surface on the spot. Therefore， a detection mechanism is designed which can measure the position relationship of the vertical parallel planes in the field. The detection mechanism can use the geometric relationship between itself and the parallel plane’s shape contour to separate the known datum plane’s shape profile， and the calculated shape profile of the measured plane can be thus obtained. According to the difference between the calculated shape profile and the known measured plane shape profile， the position relationship between the two parallel planes can be determined. Meanwhile， the principle of the detection mechanism is introduced， the detection scheme is given， the algorithm for describing the position relationship is developed， and the measurement test is carried out. The experimental results show that the detection mechanism can accurately obtain the position relationship between the two vertical parallel planes.

Key words: position relationship, on-line measurement, shape profile, detection mechanism, vertical parallel planes