Stingers, sometimes also called quills, are required to perform modal testing with shakers. The shaker head should never be directly attached to the structure for modal testing. This would provide very poor frequency response measurements. If the shaker were to be directly attached to the structure, there would be significant dynamic effects of the shaker imposed onto the structure, resulting in a dramatically altered frequency response function.
Basically, the stinger decouples the shaker system from the structure and applies force to the structure. The stinger is designed to be rigid in the axial direction and flexible in the lateral direction. Force transducers measure axial force but still transmit forces into the structure through the transducers stiff casing. Therefore, any sideloads transmitted to the structure by the stinger through the force transducer are unmeasured and contribute noise on the measurement. A stinger that is properly designed, selected and aligned will reduce or eliminate this potential problem.
Of course the shakers dynamic subsystem will never be perfectly decoupled and there will practically always be some slight cross-axis force input to the structure. The intent of the stinger design is to be very stiff in the axial direction and extremely compliant to lateral loads to minimize this situation. The best stinger for minimizing any affects of cross-axis force input is a piano wire stinger, such at The Modal Shop model K2160G, which utilizes the through-hole armature design of a modal shaker allowing the wire to be pretensioned to push force into the test structure. The piano wire is completely flexible in the lateral direction, making it the optimal choice. Another alternative is a thin rod stinger design, such as The Modal Shop 2150G12, which also utilizes the through-hole armature design. Since this design is a stiff rod (rather than a wire) it does transmit some amount of force laterally. However, this style of stinger does not need to be pretensioned, and thus greatly simplifies setup. As a result it is more commonly used as an acceptable compromise of performance and ease-of-use.
当然，激振器动力子系统永远不能完美地解耦，并且实际上总会有某些轻微的横轴力输入到结构上。推力杆设计的意图是在轴向非常刚硬，并且对于横向载荷极端的柔顺，来减少这种情况。减少任何横轴力输入影响的最好推力杆是钢琴丝，例如Modal Shop的K2160G型，它利用模态激振器的通孔动圈设计，允许钢琴丝进行预紧来“推动”力到测试结构上。钢琴丝在横向完全地柔顺，使它成了最好的选择。另一种替代方案是一种细长杆的推力杆设计，例如Modal Shop的2150G12，它也利用了通孔动圈的设计。因为这种设计是一种刚硬的杆（跟钢琴丝比），它确实传递一定程度的横向力。但是，这种类型的推力杆不需要预紧，这样就大大简化了设置。因此，作为一种性能和易用性的可以接受的折中，这很常用。
The affects of the stinger assemblys lateral stiffness on the overall system is very dependent on the stiffness of the structure being tested. If the structure itself is stiff, then this is often not a serious concern. However, when the structure is flimsy or has a significant amount of rotational effect at the attachment point of the stinger then these lateral loads can become very important and a source of large measurement error. In addition, these rotational effects generally become more important at higher frequencies so it is always difficult to determine the actual impact on the overall results. One easy way to determine the effects of the stinger lateral and rotational effects is to make several test runs with the length of the stinger varying by +/- 10% and observe the change in the measured drive point frequency response.