Basic Programming Concepts millimeters of belt travel. 4. An encoder offset, which is used to adjust the origin of the belt frame of reference. 5. Window parameters, which define the working range of the robot along the belt. These components of belt variables are described in detail in the following sections. Unlike other eV+ data types, belt variables cannot be stored in a disk file for later loading. However, the location and real-valued data used to define a belt variable can be stored and loaded in the normal ways. After the data is loaded from disk, DEFBELT and WINDOW instructions must be executed to define the belt variable. For details, see Belt Variable Definitions on page 199.
(The file LOADBELT.V2 on the Utility Disk contains a subroutine that will read belt data from a disk file and execute the appropriate DEFBELT and WINDOW instructions.) Nominal Belt Transformation The position, orientation, and direction of motion of a belt are defined by a transformation called the nominal belt transformation. This transformation defines a reference frame aligned with the belt as follows: its X-Y plane coincides with the plane of the belt, its X axis is parallel to the direction of belt motion, and its origin is located at a point (fixed in space) chosen by the user.
Since the direction of the X axis of the nominal belt transformation is taken to be the direction along which the belt moves, this component of the transformation must be determined with great care. Furthermore, while the point defined by this transformation (the origin of the frame) can be selected arbitrarily, it normally should be approximately at the middle of the robot's working range on the belt. This transformation is usually defined using the FRAME location-valued function with recorded robot locations on the belt.
(The easiest way to define the nominal belt transformation is with the conveyor belt calibration program provided by Omron Adept.) The instantaneous location described by the belt variable will almost always be different from that specified by the nominal transformation. However, since the belt is constrained to move in a straight line in the working area, the instantaneous orientation of a belt variable is constant and equal to that defined by the nominal belt transformation.
To determine the instantaneous location defined by a belt variable, the eV+ system performs a computation that is equivalent to multiplying a unit vector in the X direction of the nominal transformation by a distance (which is a function of the belt encoder reading) and adding the result to the position vector of the nominal belt transformation. Symbolically, this can be represented as instantaneous_XYZ = nominal_XYZ + (belt_distance * X_direction_of_nominal_transform) where belt_distance = (encoder_count - encoder_offset) * encoder_scaling_factor eV+Language User's Guide, v2.x, 18318-000 Rev A Page 193