1、 A Closed Loop Feedback Method for a Manual Bar Straightener Robert J. Miklosovic, Zhiqiang Gao Department of Electrical and Computer Engineering Cleveland State University Cleveland, Ohio, USA Abstract Automation of a unique manually controlled industrial bar straightener is proposed. A continuous-
2、time closed loop model is constructed in Simulink for an event-driven process through the use of asynchronous timers. The system is simulated with linear and nonlinear PD controllers. A nonlinear filter,called the tracking differentiator, is introduced as an alternative to a linear approximate means
3、 of providing accurate derivative feedback in the presence of noise. In both cases, the nonlinear techniques outperformed their linear counterparts while retaining tuning simplicity. I. BACKGROUND Precision straightening of a cylindrical metal bar is largely based on the ability to precisely measure
4、 its geometry. A few fundamental measurements and how each influences the tolerance specification on straightness should first be understood. Methods for measuring roundness and straightness are covered to lay the groundwork for the problem formulation. The basic operation of the machine is outlined
5、 in Section II, and its fundamental limitations and need for automation are discussed. Section III addresses the task of closing the loop through block diagrams and the role of new hardware in the process. Section IV contains descriptions of all of the blocks that are modeled in Simulink. The linear
6、 and nonlinear controller designs are discussed in Section V, the system is simulated in Section VI, and concluding remarks are made in Section VII. A. Measuring Roundness Roundness is a quantity derived from comparing the shape of a cross-sectional area at one distinct point along a cylinders lengt
7、h against a circle. A round metal bar that is arbitrarily long with respect to its diameter has to be checked for roundness in many locations lengthwise and averaged to insure overall consistency. Roundness is approximated by rotating the work piece one revolution in a Vee block while measuring the
8、surface with an indicator. Taking the difference between the minimum and maximum indicator readings in this case is referred to as the total indicator reading (TIR) 1. B. Measuring Straightness Straightness is a quantity derived from comparing the axial centerline of a specific section of a cylinder
9、s length against a straight line. A simple method for approximating straightness is by rotating the bar one revolution between two Vee blocks that are a fixed distance (d) apart, while measuring in the center with an indicator. The distance that the axial centerline of the part deviates from a theor
10、etically straight centerline directly below the indicator equals the extent to which the part is bowed, or warped, over length d. The maximum and minimum indicator readings (IX and IN) are physically represented in Fig. 1. From this, TIR is derived as: TIR= IX IN = (R + |Bow|)-(R |Bow|) =2*|Bow| (1)
11、 Deviations in roundness, outside diameter (OD) size, and finish can adversely affect the measurement. Figure 1. Max. and min. indicator readings of a bowed part C. Straightening The straightening process, which can be broken into steps, simply involves correcting any error while checking for straig
12、htness. First, the part is measured for straightness. Then, it is rotated so that the bow is oriented 180 degrees away from the Vee blocks with the maximum indicator reading facing upwards. Finally, a counter-bending force replaces the indicator and straightens the work piece against the Vee blocks.
13、 II. MACHINE OPERATION The straightener to be automated uses a non-contact ultrasonic sensor in place of the indicator and rollers in place of the Vee blocks in an effort to minimize contact wear. The part slowly spirals through the machine. The indicator reading becomes a continuous sinusoid at the
14、 sensors output, having a peak-to-peak value equal to the TIR each revolution. TIR is sampled from the sensor output and calculated each revolution, making the sample period of one revolution the minimum time between consecutive bends (YSP). TIR is the plant output (Y) to be controlled. When the par
15、t is straightened, the machine stops rotation with the bow facing upwards, but the part continues to feed lengthwise while an air cylinder counterbends the part over a period of time. This bend time (BT) is the control variable (U). Fig. 2 illustrates this operation. Figure 2. The straightener to be
16、 automated A. Process Limitations There are aspects of the process that can limit the controllers performance and slow it down by extending YSP. Each is observed and taken into consideration when producing an accurate simulation model: 1. The ultrasonic sensor introduces RFI noise into its. The use
17、of a feedback filter is essential. 2. A rough part surface finish adds distortion to the sensor output. 3. An out-of-round part superimposes harmonics on the sensor output sinusoid, placing a bound on the minimum steady state error that is achievable. 4. Inconsistent material density produces false
18、measurements. The measured focal length of a transducer is dependent on the density of the material that is being measured 2. The unit cannot measure accurately in the presence of a time-variant material density (i.e. hard spots). Although unavoidable, it can be detected, since TIR changes monotonic
19、ally. 5. An inconsistent OD causes vertical shifts in the sensor output. A differential TIR measurement cancels these affects. 6. A twisted part condition is detected when the angular position of the maximum indicator reading slowly moves with each revolution. This condition is created when the part
20、 is not straightened at the precise angular location and occurs because of the quantization affect of the digital readout used by the operator. The new controller will use a continuous signal and the part can be straightened 30 to 45 degrees ahead of the twist when encountered. B. The Need for Autom
21、ation Replacement of the operator with electronic hardware is beneficial in several ways. The cost of the electronics is much less than the ongoing hourly wage and schedule of an operator. The limitations associated with the digital readout are eliminated, which helps the machine to straighten faste
22、r with more precision. The process can be drastically sped up to produce more. Though the minimum sample time is one revolution, it does not need to be slow enough for human comprehension. A Programmable Logic Controller (PLC) can make several calculations and test the result against a set of rules
23、many times faster than a human being. C. Research Methodology The focus is split between modeling and control design, since this is a new control problem. The process is manually controlled rather than being strictly manual in operation, meaning the machine needs only a new controller. There is no n
24、eed for a complete mechanical overhaul, so the best method of straightening is not researched. Typical of a small company, time and money are limited. Gao and Huang 3 presented a new error-based control design framework including such innovations as a nonlinear tracking differentiator and a nonlinear proportional-integral-derivative (NPID) control method. These methods prove to be powerful and simple to tune, which make them ideal for use in an industrial environment.
