1. Field of the Invention
The invention relates generally to a coordinate measuring machine and more particularly to a high accuracy, lightweight, and inexpensive coordinate measuring machine having a plurality of length-adjustable legs for tracing an object.
2. Description of the Related Art
To trace a measuring point in a cartesian coordinate system, for example, on a workpiece, 3-D coordinate measuring machines which typically consist of a stand, a tracing system, preferably a separate supply and control unit, and an evaluation computer, are used. The function of the stand is to move the tracing system relative to the workpiece in a measurable manner. According to the prior art, this function is performed by means of three linear axes each of which are perpendicular to one another. In the case of coordinate measuring machines with a movable measuring table, two of the three linear axes supplement one another in terms of measurement and design. In the case of coordinate measuring machines with a stationary measuring table, the three linear axes supplement one another in terms of measurement and design. Alternatively, as described in German Offenlegungsschrift 44 03 901 A1, the workpiece can be traced by means of an appliance which is derived from an articulated arm robot.
The conventional means of moving the 3-D coordinate measuring machine of the three linear axes type are complicated. This is because, to displace the tracing head in a coordinate direction, each axis contains a drive, a scale system, and a guide. The function of the guide is to prevent deviations in straightness and rotation during the displacement of the tracer. An additional function of the guide is to make the deviations reproducible and therefore capable of being corrected in a machine-related manner, specifically with very high accuracy and under all permissible temperature and load conditions. For example, in a coordinate measuring machine of the gantry type having a measuring length of one meter in all three axes, the rotational guide accuracy of each axis must be well below 0.5 seconds, so that a measuring accuracy in the range of 2 .mu.m can be achieved. Because of these requirements, most of the production costs for an axis of the coordinate measuring machine are incurred in the guide and not in the drive or the scale system.
With the measuring means available in the conventional coordinate measuring machines, it is possible to record rotational errors to an accuracy of about 0.3 seconds. On the other hand, position errors can be measured to an accuracy of about 0.5 .mu.m. In a coordinate measuring machine with a large measuring volume, therefore, the geometrical accuracy which can be achieved is often limited by the accuracy in recording the rotational deviations.
In conventional coordinate measuring machines having a stationary measuring table, all three axes, i.e., the X-, Y- and Z-axes, supplement one another during the displacement of the tracing head. In other words, all the elements of the third axis (Z-axis) must be moved along with the second axis (Y-axis) and all the elements of the second axis (Y-axis) and of the third axis (Z-axis) must be moved along with the first axis (X-axis). In measuring machines with a movable measuring table, only two axes, namely the Y- and Z-axes, supplement one another, but the workpiece has to be moved additionally in the X-axis by means of the table carrying the workpiece. In both instances, large masses have to be moved to displace the tracing head in the cartesian coordinate directions. This makes the coordinate measuring machines of conventional design expensive and slow.
However, the three linear axes of the conventional coordinate measuring machine make it possible to achieve a displacement of the tracing head parallel to itself by means of a relatively simple control. If the tracing head is to be rotated, however, such as during optical tracing or also in the case of specific mechanical measuring tasks, it is necessary to have a turn-and-pivot device which contributes to additional inaccuracies, costs and control outlay, even when only a small pivoting range is required. Further, each linear axis of the conventional coordinate measuring machine necessitates a careful parallel adjustment of the guide, drive and scale as well as auxiliary structures, for example, structure for protection of the guide surfaces against soiling, low-friction hose, and cable guides.
The articulated arm measuring machines according to German Offenlegungsschrift 44 03 901 A1 have no linear axes and therefore avoid some of the above-mentioned costs and control outlay associated with the conventional coordinate measuring system. However, in this type too, all the axes supplement one another, thus leading to relatively large masses to be moved, particularly when a complicated tracing head is employed. Warping of the arms caused by thermal influences or by alternating loads has an effect on the measuring accuracy. The resolution of available rotary encoders is not sufficient for an accuracy in the .mu.m range. Consequently, in terms of their performance, coordinate measuring machines of the articulated arm type are still far removed from the coordinate measuring machines of the conventional type previously described.
Coordinate measuring machines have to be checked regularly in order to prove that the specified accuracy is being adhered to. Calibrated test bodies, for example, gage blocks or ball plates, can be used for this purpose. It is also possible, however, to check the position of the tracing head by means of an independent measuring system.
To have the correctness of the coordinates of the traced measuring point confirmed, it is known from DD 141 061 to displace the tracing head according to the coordinates in a known way, but to provide, in addition to the tracing head, for checking purposes, rods which can be tilted about fixed points and the free ends of which are rotatably connected to the tracing head on all sides, and to design the rods in such a way that their length can be adjusted in a measurable manner. When the tracing head is displaced mechanically according to predetermined cartesian coordinates, it is possible, by measuring the lengths of the extendable rods of the checking system, to test whether there are measurement and/or displacement errors. Based on the changes in length of the rods, the position of the traced measuring point is calculated a second time and the set value and the measured value are compared with one another. In this design, it can only be presumed which of the two values is the correct value or an average value can be formed from the two values. This design is highly complicated, since it necessitates two measuring systems, the first to be used as the coordinate measuring machine and the second to be used as a checking system. In DE-C1 35 04 464, for example, a corresponding checking method is carried out to determine the positioning accuracy of a toolholder for a tool which is moved by a robot. This checking appliance is designed as a transportable appliance.
The technical problem on which the invention is based is to specify a coordinate measuring machine, which can be produced simply and inexpensively in terms of its basic design, in which the desired measuring accuracy and measuring speed are guaranteed, and with which measuring points which are difficult to reach can be traced easily.