scope:

Introduction

The term "guidance" is used in a similar manner both in handling technology and in mechanism design. Handling technology primarily defines the term "guidance" with respect to the movement required for a task and without a direct relation to the solution or the device that is actually performing the operation, such as for example industrial robots, coupler mechanisms and vibration conveyors. VDI 2860 defines guidance as causing bodies to move from a specified position in space, as defined in the three-dimensional coordinate reference system, by at least three points of the body and along a specified path. Here the orientation - in other words, the position of the axes in the body's own coordinate system with respect to the reference system - is predetermined for every point on that path. "Guidance" is thus defined as a composite function of the subfunction "move" as can be seen in Figure 1. It can be described by the elementary functions of "rotate" and "translate". In certain applications, it can be broken down into the elementary function "hold" of the subfunction "secure" and into a further composite function of the subfunction "move", such as "swivel", "orient", "position", "order" or "pass on", for example. In this context the term "hold" is to be understood as a limit on a maximum of five possible movements (freedoms) effected by reducing the amount of freedom of a body. "Holding" or restricting the amount of freedom can also be effected here by means of guide elements, such as an inclined plane or guideways, for example.

In handling technology the body being guided is considered to be an element of the flow of material and not a direct part of the motion generating mechanism. One example is a cylinder being guided down an inclined plane or a ball sent down a ball race while exploiting the force of gravity.

In the field of linkages and mechanisms, however, "guiding" generally means the simultaneous positioning and orienting in space of a mechanism element belonging to a mechanism with a constant or varying speed ratio. Such a three-dimensional movement of the guided mechanism link is momentarily a helical motion with respect to a reference link. In other words, the element is executing a rotation about a screw axis and a shift along the same screw axis [1]. This means that it can also be described here, in accordance with VDI 2127, by the elementary functions "rotate" and "shift". If the general three-dimensional movement, which is characterised by three translational and three rotational degrees of freedom, is restricted or the degree of freedom reduced, this results in a spherical or a planar movement of the mechanism link. Spherical movement occurs when the mechanism link moves about a fixed point with three rotational degrees of freedom. A planar motion results if the individual mechanism link is guided parallel to a plane with two translational degrees of freedom and one rotational degree of freedom. Depending on the task to be performed, three-dimensional, spherical or planar coupler, cam or combined mechanisms with varying speed ratios are used as guidance mechanisms.

A comparison of the term "guidance" as used in handling technology and in mechanism design clearly reveals that in handling technology the definition focuses on the motion task and thus on the handling object while in the field of mechanism design it is primarily concerned with the mechanism which creates the movement. It is the definition of "guidance" which is used in mechanism theory which will be used predominantly in this guideline.