Effort modulation for process control of friction stir operations

Abstract

A system and method for making adjustments to output effort from a spindle driver using a multi-stage nested control loop of an active controller to provide constant power to a friction stir zone during a friction stir operation. Providing constant power facilitates temperature control within the friction stir zone and thereby improves the result of the operation such as a weld.

Claims

1. A method comprising: sensing a flow variable of a friction stir tool to a friction stir zone; and controlling, to a temperature setpoint and with an active controller, a temperature of the friction stir zone by adjusting a power provided by the friction stir tool to the friction stir zone, the controlling, to a power setpoint, the power provided by the friction stir tool to the friction stir zone being performed by controlling an output effort variable of a spindle driver that is used to drive the friction stir tool in response to the sensed flow variable feedback.

2. The method as defined in claim 1, wherein using the active controller to adjust power provided by the friction stir tool to a friction stir zone further comprises implementing a multi-stage nested control loop.

3. The method as defined in claim 2, wherein implementing the multi-stage nested control loop further comprises: 1) implementing an inner control loop to thereby maintain the power through sensing the flow variable of the friction stir tool and control of the output effort variable of the spindle driver; and 2) implementing an outer control loop that adjusts the power provided to thereby maintain the temperature for the friction stir zone.

4. The method as defined in claim 3, wherein controlling the output effort variable of the spindle driver further comprises controlling an input flow to the spindle driver.

5. The method as defined in claim 3, wherein controlling the output effort variable of the spindle driver further comprises controlling torque of the friction control tool.

6. The method as defined in claim 3, wherein controlling power provided by the friction stir tool to the friction stir zone comprises controlling the output effort variable of the spindle driver in response to changes in the flow variable of the friction stir tool, the flow variable comprising angular velocity of the friction stir tool.

7. The method as defined in claim 1, wherein controlling power provided by the friction stir tool to the friction stir zone comprises using spindle flow variable feedback, wherein power, the effort variable and the flow variable are related by the equation:
P=e*f, wherein: P is power provided by the friction stir tool to the friction stir zone; e is the output effort variable of the spindle driver; and f is the flow variable of the friction stir tool.

8. The method as defined in claim 7, wherein maintaining the power through control of the output effort of the spindle driver further comprises increasing the output effort variable of the spindle driver in response to a decrease in the spindle flow variable.

9. The method as defined in claim 7, wherein maintaining the power through control of the output effort of the spindle driver further comprises decreasing the output effort variable of the spindle driver in response to an increase in the spindle flow variable.

10. The method of claim 3, wherein adjusting the power provided to thereby maintain the temperature for the friction stir zone comprises adjusting a reference power level to the inner control loop.

11. A friction stir system comprised of: a friction stir tool; a spindle that is coupled to the friction stir tool; a spindle driver that is coupled to the spindle to thereby cause the friction stir tool to rotate; a sensor that senses a flow variable of the friction stir tool; an active controller for controlling operation of the friction stir tool, wherein the active controller controls a temperature, to a temperature setpoint, of a friction stir zone by adjusting a power provided by the friction stir tool to the friction stir zone, the active controller configured to control power, to a power setpoint, provided by the friction stir tool to the friction stir zone during friction stirring operations by controlling an output effort variable of the spindle driver that is used to drive the friction stir tool in response to the sensed flow variable feedback.

12. The system of claim 11 wherein using the active controller to adjust power provided by the friction stir tool to a friction stir zone further comprises implementing a multi-stage nested control loop.

13. The system of claim 12, wherein implementing the multi-stage nested control loop further comprises: 1) implementing an inner control loop to thereby maintain the power through sensing the flow variable of the friction stir tool and control of the output effort variable of the spindle driver; and 2) implementing an outer control loop that adjusts the power provided to thereby maintain the temperature for the friction stir zone.

14. The system of claim 13, wherein controlling the output effort variable of the spindle driver further comprises controlling an input flow to the spindle driver.

15. The system of claim 13, wherein controlling the output effort variable of the spindle driver further comprises controlling torque of the friction control tool.

16. The system of claim 13, wherein controlling power provided by the friction stir tool to the friction stir zone comprises controlling the output effort variable of the spindle driver in response to changes in the flow variable of the friction stir tool, the flow variable comprising angular velocity of the friction stir tool.

17. The system of claim 11, wherein controlling power provided by the friction stir tool to the friction stir zone comprises using spindle flow variable feedback, wherein power, the effort variable and the flow variable are related by the equation:
P=e*f, wherein: P is power provided by the friction stir tool to the friction stir zone; e is the output effort variable of the spindle driver; and f is the flow variable of the friction stir tool.

18. An active controller controlling operation of a friction stir tool to a friction stir zone, the active controller comprising: an inner control loop that controls power, to a power setpoint, provided by the friction stir tool to the friction stir zone by using sensed angular velocity of the friction stir tool to control torque of a spindle driver of the friction control tool, wherein power, the torque and the angular velocity are related by the equation:
P=co*M, wherein: P is power provided by the friction stir tool to the friction control zone; M is the torque of the spindle driver; and co is the angular velocity of the friction stir tool; and an outer control loop that maintain a desired temperature, to a temperature setpoint, for the friction stir zone by using a sensed temperature to adjust the power provided by the friction stir tool to the friction stir zone.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a perspective view of a tool as taught in the prior art for friction stir welding;

(2) FIG. 2 is a cut-away profile view of an FSW tip, a locking collar and a shank from the prior art;

(3) FIG. 3 is simplified block diagram for an active control system for temperature control as taught in the prior art;

(4) FIG. 4 is a block diagram for a spindle speed power control system as taught in FIG. 3;

(5) FIG. 5 is a block diagram for a close-up of a plant for a control model of the prior art;

(6) FIGS. 6A and 6B are system block diagrams depicting power control via modulation of effort;

(7) FIG. 6C is a graph showing power control by adjusting spindle speed;

(8) FIG. 7 shows that constant spindle speed welds contain variation in power and temperature;

(9) FIG. 8 is a block diagram of the inner loop of an active controller;

(10) FIG. 9 is a graph showing the results when torque (i.e. effort) is adjusted to keep power constant;

(11) FIG. 10 is a graph showing that constant spindle speed welds contain variation in power and temperature;

(12) FIG. 11 is a graph of temperature response;

(13) FIG. 12 is a graph showing that poor reported torque resolution results in unreported torque dynamics;

(14) FIG. 13 is a graph showing linear torque-spindle speed assumption used for comparing spindle speed control to torque (i.e. effort) control;

(15) FIG. 14a is a graph showing a step increase in power adjusting spindle speed;

(16) FIG. 14b is a graph showing a step increase in power adjusting torque; and

(17) FIG. 15 is a block diagram showing the inner loop and the outer loop of an active controller.

DETAILED DESCRIPTION OF THE INVENTION

(18) Reference will now be made to the drawings in which the various elements of the present invention will be given numerical designations and in which the invention will be discussed so as to enable one skilled in the art to make and use the invention. It is to be understood that the following description is only exemplary of the principles of the present invention, and should not be viewed as narrowing the claims which follow.

(19) As disclosed herein, the effort of a friction stir system is modulated based on flow feedback. Based on the principle of system similarity in system dynamics a power, P, is defined as an effort, e, multiplied by a flow, f. This relationship is shown in Equation (2) as:
P=e*f(2)

(20) For example, power in the mechanical rotation domain is defined by torque (effort) multiplied by angular velocity (flow). Examples of various power domains, efforts and flows are shown in Table 1 below.

(21) TABLE-US-00001 TABLE 1 Power domains Domain Effort (e) Flow (f) Mechanical Force F [N] Velocity v [m/s] (translation) Mechanical (rotation) Torque M [Nm] Angular velocity [rad/s] Pneumatic Pressure p [Pa] Volume Flow Q [m.sup.3/s] Hydraulic Pressure p [Pa] Volume Flow Q [m.sup.3/s] Electric Voltage u [V] Current i [A] Magnetic Magnetomotive Flux rate force

(22) One method of modulating output effort of the spindle driver includes modulating the input flow or effort of the spindle driver. If the spindle driver is a transformer, output effort can be effectively modulated by modulating input effort. If the spindle driver is a gyrator, output effort can be effectively modulated by modulating input flow. For example, an electric motor driver is a gyrator. The input flow (e.g. current) can be used to modulate the output effort (torque).

(23) This detailed description discloses specific examples that demonstrate controlling an electric spindle motor. Electric motors perform a transformation from the electrical domain, through the magnetic domain to the rotational mechanical domain. In particular, for an electric motor the output torque is controlled by means of adjusting the input current. A device that transforms effort in one domain to flow in another domain, or vice versa, is known as a gyrator. Many devices that provide spindle energy are gyrators. Although the electric spindle motor is a representative embodiment, the invention is not limited to such embodiments. For example, the power of a friction stir system could be controlled by monitoring any form of flow (including those presented in Table 1) and modulating any form of effort (including those presented in Table 1).

(24) As used herein, the temperature of the friction stir zone refers to any direct measurements or estimated measurements and may include any useful method of measuring or approximating weld (i.e. friction stir zone) temperature. For example the friction stir zone temperature may be approximated by measured tool temperature or backing plate temperature. Methods to measure the internal temperature of the friction stir operation zone, such as ultrasonic technology, may also be used.

(25) System diagrams depicting power control via modulation of effort by an active controller 610 are shown in FIG. 6A and FIG. 6B where P.sub.cmd is commanded power, f.sub.rep is reported flow, f.sub.real is real flow and e.sub.cmd is commanded effort. It is important to note the reported flow may be a measured value or an estimated value. FIG. 6A shows reported flow as a measured value while FIG. 6B shows reported flow as an estimated value. One of skill in the art will appreciate that there are a variety methods of providing reported flow and that FIG. 6A and FIG. 6B are examples for reference only.

(26) The present invention is an improved control system for friction stir operations. Research shows that power provided to a friction stir zone leads tool temperature. Due to the inertia associated with the spindle, power control is best achieved by commanding torque (i.e. effort) rather than spindle speed (i.e. flow) as will be explained.

(27) The active control system disclosed herein was initially developed for a dual loop control system for FSW where the inner loop maintains constant power and the outer loop adjusts power to maintain constant temperature. Although much of the operation of the present invention as described herein refers to a dual loop control system for controlling welding operations, the invention is not limited to a dual loop control system nor to welding operations.

(28) FIG. 7 is provided to show the hardware elements 700 of a friction stir welding system that are relevant to an understanding of the present invention. These elements include a spindle motor 710 that is part of a friction stir welding machine (not shown). The spindle motor 710 is coupled to a spindle 720. The spindle in turn is coupled to a friction stir welding tool 730 that is used to perform friction stir welding in all its various forms and on both low melting temperature and high melting temperature materials. There are various configurations of friction stir welding tools and this document should not be seen as limiting the variety that can be used with the present invention.

(29) The present invention uses the relationship between torque (i.e. effort) and power as expressed in Equation (2) to control the FSW process. In this first embodiment, the spindle motor (i.e. driver) 710 is a variable frequency AC induction motor. Although an induction motor is shown, any appropriate driving mechanism may be used including mechanical, pneumatic, hydraulic, electric and magnetic drivers. When running under torque control with an induction motor, the motor interface control software commands and maintains a constant torque using flux vector control as directed by an active controller (not shown). The desired torque is obtained from the spindle motor by controlling the current fed to the induction motor. This control diagram is shown in FIG. 8.

(30) It should be noted that torque (i.e. effort) control without spindle speed (i.e. flow) feedback is unstable. A constant torque can only be maintained for a short time. The torque can only be controlled when the load supports the torque. Torque that is greater than the natural process torque leads to greatly decreasing loads causing an exponential increase in spindle speed. This is because when spindle speed and torque are increasing, the power increases and the material softens. Conversely, if the torque is lower than the natural process torque, the spindle speed will decrease exponentially as the material cools and hardens. Process variation causes the commanded torque to be either too low or too high to maintain equilibrium causing the spindle to rapidly decelerate until it stops or accelerate until machine safety limits are triggered.

(31) It has been determined that controlling power provided to a friction stir zone by adjusting torque (i.e. effort) in response to changes in spindle speed (i.e. flow) is a stable process. Torque increases in response to decreasing spindle speed to maintain a constant power. Torque decreases in response to increasing spindle speed to maintain a constant power.

(32) FIG. 9 shows the results from a weld where torque (i.e. effort) is adjusted to maintain constant power. As the workpiece is heated, the material softens. The torque and RPM signals are mirror images of each other. The spindle speed increases as the material softens. The torque (i.e. effort) decreases with increasing spindle speed to maintain the constant power. FIG. 9 shows that power control achieved by torque control is a stable process.

(33) FIG. 10 shows that constant spindle speed welds contain variation in power and temperature. The measured power contains power spikes that persist throughout the weld. The filtered power value varies throughout the weld in response the changes in the process.

(34) Previously, it was shown in FIG. 6C that power control achieved by adjusting spindle speed results in a constant average power. However, the actual power contains torque spikes throughout the weld. The temperature response shows that the tool gradually rises in temperature until thermal equilibrium is reached and the temperature is constant.

(35) FIG. 9 showed that power control achieved a torque (i.e. effort) control to maintain constant power produces a constant power with negligible variation. The temperature response is linear. This indicates that the tool is still heating and will reach a steady state-temperature.

(36) A demonstration of the principles of the present invention was performed and the results are shown in FIG. 11. A power control weld adjusting torque to keep power constant was run for 406.4 mm (16 in) at 2.238 kW then 609.6 mm (24 in) at 2.536 kW in AA 7075. The temperature response is shown in FIG. 11. Before the change in power is commanded, temperature is constant. After the change in power is commanded, the temperature increases logarithmically until a new steady-state temperature is reached. These results show that changes in power lead changes in temperature.

(37) In the prior art when spindle speed is adjusted to control power, the commanded spindle speed is determined by the torque that is reported by the spindle controller. When torque is adjusted to control spindle speed, a command that is transmitted to control torque is sent to the spindle controller. The difference in resolution between reported and commanded torque affects the design of the power control loop.

(38) Torque measurements used in this embodiment are reported by the spindle motor controller. However, the reported torque fails to capture torque dynamics. FIG. 12 shows the torque reported by the controller and the torque calculated using load cells on the FSW machine in response to a sinusoidal torque command. The sinusoid is reported as a square wave due to limited resolution in reported torque.

(39) The sinusoid can be seen in the torque calculated from forces measured by load cells. Torque calculated using load cells has poor signal quality due to cross talk. Because changes in torque during temperature and power control are small spindle torque will be assumed to be equal to commanded torque.

(40) In developing the present invention, a dynamics analysis was performed to compare adjusting spindle speed and adjusting torque to maintain constant power. Returning to FIG. 7, M(mtr) is the motor torque, M(spn) is spindle torque, D(mtr) is the diameter of the motor pulley, D(spn) is the diameter of the spindle pulley, M(b) is torque lost due to bearings, (spn) is the rotational velocity of the spindle, and J is the mass moment of inertia of associated with the spindle. The gear ratio, R, is given by:
R=D(spn)/D(mtr)Equation (3)
and has a value of 2.5.

(41) The relationship between torque and spindle speed is derived using FIG. 7 and is given by:
RM(mtr)=M(spn)+M(b)+J(spn)Equation (4)
The effective motor torque, M(mtr_eff), is given by:
M(mtr_eff),=RM(mtr)Equation (5)
Reported torque in experimental data is M(mtr_eff). Substituting Equation (4) into Equation (5) yields:
M(mtr_eff)=M(spn)+M(b)+J(spn)Equation (6)
For the purpose of comparing spindle speed control to torque control, it is assumed that (spn) and M(spn) have a linear relationship as shown in FIG. 13 and given by:
(spn)=M(spn)/BEquation (7)
where it is assumed that J, M(b) and B are constant.

(42) Because the spindle motor's maximum torque is finite, changes in commanded spindle speed result in large torque spikes, shown in FIG. 14a, in attempt to instantaneously accelerate the spindle to the desired rotational velocity.

(43) The spindle speed response to a step change in effective motor torque was found to be exponential. However, the spindle speed response to changes in torque contains no discontinuities.

(44) Experimental results for step changes in spindle speed and torque (i.e. effort) validate that torque (i.e. effort) has a derivative relationship with spindle speed, and spindle speed has an integral relationship with torque (i.e. effort). FIG. 14a shows the effective motor torque response to a step change in RPM. At the instant the change in desired spindle speed is made, the motor attempts to instantaneously accelerate the spindle in order to achieve the desired spindle speed. This results in a large spike in motor torque.

(45) FIG. 14b shows the spindle speed response to a step increase in power where power control was obtained by adjusting torque (i.e. effort). When the change in power is commanded, the torque instantly increases slightly to obtain the desired power value. Due to the increased power the material begins to soften causing the spindle speed (i.e. flow) to increase and the motor torque or effort to decrease. The spindle speed response to torque contains no discontinuities.

(46) Experimental data shown in FIGS. 14a and 14b indicate that higher spindle speeds correspond to lower torques and lower spindle speeds correspond to higher torques. These results suggest that the slope shown in FIG. 13 should be negative.

(47) The control methods presented assume M(mtr_eff) is approximately equal to M(spn). Torque (i.e. effort) has a derivative relationship with spindle speed (i.e. flow). When a change is RPM is commanded, the spindle motor attempts to instantaneously accelerate the spindle to a new RPM causing a spike in motor torque. A near instantaneous acceleration of the spindle motor would cause a large difference between motor torque and spindle torque. The motor torque is not approximately equal to spindle torque when a change in RPM is commanded.

(48) Experimental data shows that when torque is adjusted to keep power constant, the difference between motor torque and spindle torque is much smaller than when spindle speed is used to keep power constant.

(49) When a step change in power is commanded as shown in FIG. 14a, the torque error during the spindle spike is 203.0 Nm (149.7 ft-lb) which corresponds to a 406% error in power at the high power level and a 565% error at the low power level. In contrast, for power control by adjusting torque as shown in FIG. 14b, the error associated with the spindle acceleration after the change in power, where acceleration is greatest, is 7.4 Nm (5.46 ft-lb) which corresponds to 10% error in power.

(50) Adjusting spindle speed to keep power constant is undesirable because reported torque is a poor control signal. Torque spikes cause the difference between motor and spindle torque to be as high or even higher than 400%. The reported torque signal has low resolution, adding to the error.

(51) The main advantage of using torque to control power is the avoidance of artificial torque spikes caused by attempting to change the RPM instantaneously. Using torque control to control power results in a smooth power signal with low uncertainty. The difference between the motor torque and actual spindle torque is proportional to the acceleration of the spindle. Low uncertainty exists because under torque control, power and spindle speed change in a controlled fashion.

(52) Having addressed the inner control loop, attention is now directed to the outer control loop. FIG. 15 is a block diagram showing both the inner and outer loops of the first embodiment of the present invention. Proportional-integral-derivative (PID) controllers are the most commonly used type of closed-loop feedback controller. The outer loop uses PID control to adjust power to maintain a commanded temperature. The implementation of PID control in the outer loop is shown in FIG. 15. P(init) is a constant power value. The PID controller provides a change in power that is added to the P(init) term. The P(init) term is the spindle power at the moment the controller switches from commanding a constant spindle speed (i.e. flow) to temperature control. Once temperature control is engaged P(init) is constant until temperature control is disengaged.

(53) It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention. Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the spirit and scope of the present invention. The appended claims are intended to cover such modifications and arrangements.