Research of Model Sting Support Control System of Icing Wind Tunnel

Abstract: The sting support device of icing wind tunnel is a four-axis position closed-loop servo system consisting of a variable angle mechanism and a Y-direction mechanism. Due to motion coupling among the four axes, synchronous control is required. To ensure that the support device rotates uniformly at the specified angular velocity while accurately positioning the attack angle α and the sideslip angle β, and that the model center will not deviate from the axis of the wind tunnel during the rotation, this paper provides a detailed solution of complicated motion based on comprehensive mathematical analysis of the kinematic relationship between velocity and positions of the attack angle α, sideslip angle β and Y-direction lifting mechanism. In addition, the method used for controlling the multi-axis motion velocity and position is further discussed. At last, this paper sets forth the unique method for controlling virtual principal axis and α, β and y mechanisms position following. By use of this method, the velocity and positions of attack angle α mechanism sideslip angle β mechanism and Y-direction mechanism can be controlled in a highly-accurate manner.


Introduction
Control precision of wind tunnel model attitude angle is critical to acquisition of excellent model test data.Shown by relevant documents [1], most model test errors may be traced to model attitude error.The elevation angle α and yaw angle β are the most important model attitude angles.Research fellows have designed different types of mechanical structures and design approaches to control α and β angles of the model; however, only single-degree-of-freedom of α and β angles is controlled respectively by use of the conventional control method.It is difficult to implement synchronous control due to arduous arithmetical operation and unsatisfactory angle positioning precision; furthermore, the most important model security issue has not considered duly [2].
The new sting control system, specially developed for the icing wind tunnel, provides a unique multi-degree of freedom model support mechanism synchronous motion control approach, through which the elevation angle and yaw angle of the model can be controlled in a precise, safe and highly-effective manner.

Basic requirements for sting control system
The sting support system is composed of the angle of attack and sideslip angle of mechanism and Y mechanism, which lies in high speed test section.The overall structure is shown in Figure 1.The System is a four-axis closed loop servo system.Due to motion coupling among the four axes, synchronous control is required.See Table 1 for the design technical specifications.

System composition
As shown in Figure 2, the sting support control system is composed of a control cabinet (containing PLC, HMI, motion control unit, etc.), 4 sets of AC servo amplifiers, servo motor, etc.An upper control computer, as the user's operation interface, is used to implement remote control upon the attack angle, sideslip angle and lifting mechanism, and to display the motion status of these mechanisms and operating situation of servo drive in real time.PLC is used to transmit the data (like the motion status of these mechanisms, operating situation of servo drive) to the upper computer via industrial Ethernet, and the upper computer forwards control commands to PLC via the industrial Ethernet.Upon receipt of the control commands, PLC will implement velocity control or position control through motion control unit as per corresponding control strategy [3,4].Given that the clockwise moving direction of the screw is positive and α augmentation positive, then the relational expression of α and l is as shown in Figure 4:

Analysis of kinematic relationship between attack angle α mechanism and Y-direction mechanism
(1)  By taking derivative with respect to the two sides of the Expression (1), the relational expression (2) between the screw moving velocity and attack angle velocity is solved, as shown in Figure 5: (2)

Analysis of kinematic relationship between forward/backward sideslip angle mechanisms
During the changing process of forward sideslip angle β1 from -60° to 60°, the backward sideslip angle β2 changes from -30° to 30°.The β1 is dependent upon the screw displacement l1, the displacement relationship between the forward sideslip angle and screw is as shown in Figure 6: By taking derivative respectively, the velocity relationship between forward sideslip angle and screw is obtained, as shown in Figure 7: Displacement relationship between the backward sideslip angle and screw is as shown in Figure 8.The β2 is dependent upon the screw displacement l2, the displacement relationship between the backward sideslip angle and screw is as shown below: By taking derivative respectively, the velocity relationship between the backward sideslip angle and screw is obtained, as shown in Figure 9: The relationship between the sideslip angel and forward/backward sideslip angles: β=β1-β2 When dβ1=0.02°,α=0, to satisfy the precision requirement that the sideslip angle shall be 0.05°, the positioning precision of forward sideslip electric cylinder must be up to 0.055mm under limiting case.According to dβ2=0.02°, the positioning precision of backward sideslip electric cylinder can be solved; i.e., about 0.065mm.

Control approach and control strategy
According to the analysis above, a mountain of arithmetical operation is required for conversion between straight line displacement at different degrees of freedom and the target angle if control is made based on the aforesaid kinematic relationship; in addition, mechanism machining error, installation error and calculation error may be enlarged inevitably and control precision decreased accordingly.Furthermore, it is difficult to implement synchronous control due to improper synchronicity.However, the synchronous motion control approach of the sting support mechanism developed by us presents a sound resolution.This approach involves establishing a mapping relationship between the target position and the velocity of elevation angle/yaw angle of support device model and the motion parameters of actual mechanism at different DOF; simplifying mathematical derivation process and optimizing the motion control precision of the mechanism at different DOF by way of data fitting with the measured value; implementing positioning control of the mechanism support model by use of synchronous coordinated motion of DOF axis of the mechanisms; implementing motion protection of support model by use of the parameters concerning the DOF axis of the mechanisms, thus achieving synchronous coordinated control upon the elevation angle and yaw angle of the model, and improving control precision, operating efficiency and reliability.

Constant-speed variable attack angle control approach with virtual axis as principal axis (sideslip angle β=0°)
During the multi-axis motion process, the relationship between the position and motion velocity of the attack angle α screw and Y-direction lifting mechanism varies with the attack angle α.The attack angle α is the control core and control principal axis in the entire support system and the primary control mission is to achieve constant-speed variation and precise positioning of attack angle α.However, positioning of the attack angle α cannot be achieved by driving rotation axis with electric motor but by screw driving.Since non-linear relationship exists between the attack angle α and screw velocity & position, so the precise positioning and constant-speed variation of attack angle α can be implemented only by controlling the screw as per Expression (1) and function (2) above.It is difficult to implement such non-linear velocity control.By deep analysis, this paper provides an approach of building virtual α rotation axis.In using control algorithm, a virtual α rotation axis is built.Meanwhile, camshaft curve relationship is also built between α screw mechanism and α angle, and Y-direction mechanism and α angle.By controlling the virtual axis to rotate at the α target angular velocity, α screw mechanism and Y-direction mechanism will implement position follow-up in strict accordance with the camshaft relationship, thus achieving non-linear velocity and position linage during the multi-axis motion process.In this way, non-linear velocity control becomes position follow-up control while the operation process is simplified greatly and control precision improved remarkably.
During the test, the distance of multi-axis motion test model center deviating from the wind tunnel axis when the attach angel α motions from -15° to +35° is as shown in Figure 10.

Multi-axis follow-up control approach with virtual α rotation axis as principal axis (sideslip angle β≠0°, working condition of constant β and variable α)
To keep β be a constant angle other than 0°, the forward/backward β shall made follow-up compensatory motion and the four axes make combined motion in addition to variation of α angle by way of linkage of α mechanism and Y-direction mechanism.Firstly, the virtual α rotation axis rotates at the ideal angular velocity required by the attack angle and is located precisely at the target position.During the rotation of the virtual axis, the dynamic target position of all mechanisms can be worked out according to the position and motion velocity relationship among the attack angle α screw mechanism, Y-direction lifting mechanism and forward/backward β mechanism; in this way, the position follow-up of attack angle α, Y-direction lifting mechanism and forward/backward β mechanism can be implemented by control the axes.See Figure 11 for the calculation block diagram of multi-axis position follow-up.

Test result and analysis
As shown in Figure 12, the precision error of attach angle of sting support is less than 0.01°, superior to the design requirements.

Figure 12. Test data of attack precision
As shown in Figure 13, the precision error of the forward sideslip angle is less than 0.015°, satisfying the design requirements.As shown in Figure 14, the precision error of the backward sideslip angle is less than 0.01°, superior to the design requirements.As shown in Figure 15, most of linkage test data of the sideslip angle is better than 0.02°; most of linkage test data of the sideslip angle is better than 0.02°.

Conclusion
For a sting support control system, a mountain of mathematical operation as required by conversion between straight-line motion displacement at different DOF and target angles is omitted by way of building a virtual axis and data fitting with measured data, and the operation process is simplified greatly.By measuring with highlyprecise test apparatus, a mapping relationship is built between the actual straight line displacement value at different DOF and the attitude angle variable of the actual model, and corresponding function relationship is obtained by interpolation and data fitting; in this way, the mechanism machining error, installation error and calculation error may be reduced greatly and control precision improved.Introduction of new control approaches (including virtual axis, electronic camshaft) may facilitate implementation of synchronous control and bring about proper synchronicity.During the operation of mechanisms, the occurrence of accidents such as uneven stress, undue vibration and tunnel wall touching of mechanism will be minimized by monitoring the parameters and specifications in real time, like operation synchronicity of the axes, angle variation uniformity, and deviation of model center from the wind tunnel axis.According to the test results of function, safety and technical specifications of sting support system, the System is proved to be comprehensive, safe and reliable, and most of its specifications are up to even superior to the design requirements.

Figure 1 .
Figure 1.Schematic diagram of support device

Figure 2 .
Figure 2. Composition block diagram of control system See the Figure below for the geometrical relationship after simplification of attack angle mechanism.

Figure 3 .
Figure 3. Theoretical dimensions of attack angle mechanism

Figure 4 .
Figure 4. Displacement relationship between attack angle and screw

Figure 5 .
Figure 5. Velocity relationship between attack angle and screw

Figure 6 .
Figure 6.Displacement relationship between β1 and screw

Figure 7 .
Figure 7. Velocity relationship between β1 and screw

Figure 10 .
Figure 10.Distance away from axis of wind tunnel

Figure 11 .
Figure 11.Calculation block diagram of multi-axis motion follow-up

Figure 13 .
Figure 13.Test data of β1 precision

Figure 14 .
Figure 14.Test data of β2 precision

Figure 15 .
Figure 15.Test data of β1 and β2 precision

Figure 16 .
Figure 16.Test data of Y axis precision

Table 1 .
Main technical specifications