Microprocessor based position and velocity control of a pneumatic actuator for low cost automation applications.

Babu, M. Vijaya Sekhar

Introduction

The old industry standard for motion control consists of various configurations of translated motors. While they exhibit the capability of positioning accurately within a micron, these electro-mechanical solutions are often overkill and they ignore the critical cost benefit relationship of the majority applications. Now the present system is an inexpensive, reliable and accurate, alternative for applications which do not require such high precision. The system here designed, fabricated and demonstrated the concept called a servopneumatic system.

Servopneumatics is an electro pneumatic solution for accurate and fast control. It is an alternative to "electric only" positioning. It eliminates electric motors with relatively expensive maintenance procedures and power supplies. It also offers the often crucial "clean air zero sparks" solution to control [1].

One problem in servopneumatic system is compressible nature of its medium i.e., air. This holds control over accuracy level, up to which we can control the system and affects the stability of the system. This is not a major constraint for industry, since it needs only 30 percent of its automation requirements to be high at precision level. The remaining part of the automation requirements can be fulfilled using Servopneumatics, which is a low cost solution and recognized technology now a day [1]. With sophisticated sensors and actuator technology the remaining 30 percent (up to some extent) can also be fulfilled.

For example, let us consider the application of Servopneumatics in robotics: Now a days 85% of all sold screw machines are equipped with electric drives which is due to higher torque accuracy ( electric: 2% error, conventional pneumatic: 10%) this advantage, however, involves a 10 times slower tool speed. When using proportionally controlled pneumatic servo valve, the error in torque is decreased to an amount of 1.9% [1]. Though optimization of the mechanisms and suitable controllers the maximum accuracy should be increased at a speed as high as possible.

[FIGURE 1 OMITTED]

Experimental Description

The basic idea of the present servo pneumatic system is shown in Figure 1. It consists of 1. Servovalve 2. Actuator, Flywheel and encoder 3. Microprocessor and control 4. Stepper motor and electronics 5. Micrometer. Brief description of these components is follows.

Servovalve

Servovalve is a directional control valve, which may be infinitely positioned, thus providing the additional characteristics of controlling the amount as well as the direction of fluid flow. When coupled with proper feed back devices, the position, velocity or acceleration of an actuator maybe controlled very accurately.

Servovalves are generally distinguished from other types of electrically controlled valves by characteristics like low power electrical input signals, linear output/input, negligible dead band and high output dynamic response bandwidth [2]. The Servovalve considered here is a modified pneumatic 4-way valve with its spool being translated back and forth by a micrometer, driven by a stepper motor, which converts the rotary to linear motion.

An ideal servovalve produces zero output flow at zero current. In practice, this ideal null or zero flow condition seldom occurs. Shifting of the null may be due to changes in temperature, supply pressure, of load pressure. Null shift is expressed in terms of null bias or current changes required to restore zero output flow [3].

The present servovalve is modified servovalve. It is a proportional control valve with negligible dead band. Since its spool can be infinitesimally can be moved, we call it as modified servo valve. Experiments are conducted to determine the characteristics of the modified Servovalve. The following Figure.2 shows the characteristic curves. Liner zone of the curve is selected for low cost effective control of the actuator i.e., 2mm of stroke (or 1mm on either side of the null).

[FIGURE 2 OMITTED]

Actuator

The actuator considered here is a conventional pneumatic motor. An experiment was conducted to find the characteristics of the motor with and without load at 3Kgf/[cm.sup.2] (2.942 bar) pressure of the air. Figure 3 shows the characteristics. A small flywheel of weight 160gms is used to simulate the load. Figure 3(a) shows the characteristics of the motor in terms of stroke of spool vs speed of motor at 3Kgf/sq.cm without flywheel (unloaded condition) and Figure 3(b) shows the same with flywheel (loaded condition).

[FIGURE 3 OMITTED]

Each curve is having mirrored shapes about the null of the spool. The proportional regions of the curves have been chosen for effective low cost control of the system. From the graphs it is decided to consider the operational zone of the spool [+ or -] 0.25mm (stroke length). So, the maximum speed at which the unloaded actuator can be operated is 3800 RPM. The second curve also shows the same nature and is almost same as the first. So, same analogy can be used to control the actuator with load i.e max speed of the loaded actuator is 2200 RPM.

An incremental encoder is connected at the end of the actuator to take the feedback. Appropriate circuit was designed and fabricated to further reduce the cost. The feed back is directly given to the microprocessor for subsequent action.

Microprocessor and control

The Microprocessor based control is flexible. It can be utilized to achieve different types of control. The changes can be very easily introduced by slightly modifying the software of the microprocessor. Microprocessor can perform not only the generation of pulse timings but also logic sequencing and the role of the input controller [4]. The processor used here for controlling is the Z-80 microprocessor which is a low cost flexible device.

Stepper motor and Electronics

The spool of servovalve is moved by stepper motor. The stepper motor differs from conventional electric motor, because even, when the motor is switched on, the shaft remains stationary, until a step pulse is sent to motor. When the stepping motor drive circuit receives a step pulse it drives a rotor through a precise angle or step then stops until the nest pulse is received. This is the essential feature from a motor to drive the spool. The pulses can be easily being generated in microprocessor [5].

The stepper motor is used to get the linear motion of the spool by connecting micrometer head in between. The stepper motor is a 2-phase bipolar winding motor which runs with 12V DC 0.5A with a maximum torque of 2Kg-cm. An interfacing circuit was designed and fabricated to interface stepper motor with Z80 processor. This further reduces the cost of automation.

Micrometer

To convert the rotation of stepper motor in to linear motion of the spool, a micrometer head is used. In micrometer, the rotary motion of the thimble is converted into linear motion of the head. Mechanical couplings and fixtures are properly chosen and used to achieve task.

Control Strategy

The objectives are i) Rotate the shaft by a particular number of rotations only (Position control) and ii) to make the shaft to rotate at an RPM (Velocity control)

Principle

Initially the spool is at null position. The stepper motor is rotated to some angle, there by moving the spool of servo valve. When the spool moves in the valve, the air enters into the pneumatic motor in the corresponding direction and shaft rotates. The encoder reads the rotation and feeds back to the microprocessor. Depending on the task to be performed and the pulses, microprocessor send pulses accordingly to stepper motor. After completing the task, the spool will be brought back to null position causing the shaft to not to rotate.

Procedure

Initially the spool is at null position i.e., there will not be any rotation of shaft at this position. Initially the task to be performed is to be selected and corresponding program need to be executed. Now, microprocessor starts giving pulses. These pulses are amplified by the interfacing circuit. The stepper motor starts to rotate and rotary motion of the stepper motor is converted in to linear motion of the spool by the micrometer head.

When the spool starts to move the air enters in to the motor in the corresponding direction. Now, air enters the motor at a flow rate. Proper opening of the port (by moving the spool) can control the flow rate to the motor, which is done by microprocessor through stepper motor.

Now the encoder reads the number of pulses and feeds back to the microprocessor. The number of pulses will be compared with the reference pulses and error signal is generated. Depending on logic from the error signal, the pulses will be given to the stepper motor to bring change in the position of spool.

When the time finished, or the rotations are completed, the microprocessor immediately gives the pulses continuously in the reverse direction, to bring back the spool to null position. Now, there is no rotation of the shaft.

Overshoot

Due to the compressibility of air, there is a possibility of overshooting. A considerable amount of the air has to be passed to the motor. So that, the pressure changes enough to get the loads moving. But this movement will not be smooth. It will rotate to a certain rotation and will stop. After sometime, when the pressure is sufficient, again it will shoot up i.e., uniform rotation will not be there in the beginning of the rotation. So, actuator is aimed to control in the range of 500-3800 RPM and 500-2200RPM in unloaded and loaded conditions respectively.

Software

Two assembly level programs are written, each for controlling position and velocity. With the aid of the program, it is possible to read the encoder, compare with the set value and take necessary action to control the actuator.

Results and Conclusions

A pneumatic position and velocity control system was developed successfully for low cost automation applications. Its accessibility depends on industrial/commercial application.

The characteristics of the pneumatic motor, at 3Kgf/[cm.sup.2] pressure in unloaded and loaded conditions have been studied. The shaft was successfully controlled with the software both in unloaded and loaded conditions and characteristics were plotted. The above Figure. 4 shows the plots both for unloaded and loaded conditions. From the curves it is clear that linear servo control of the actuator is achieved. The position and velo flywheel has been controlled successfully within the desired range i.e. within the following tolerances.

Position: [+ or -] 4 Revolutions.

Velocity: [+ or -] 30 RPM.

Target was to control the position with tolerances [+ or -] 5 and velocity or speed [+ or -] 30 RPM. Both are well within the targeted values. The present investigations envisage both position and velocity control within the tolerances of desired value using accuracy can be significantly improved.

[FIGURE 4 OMITTED]

Acknowledgements

Since this is a multidisciplinary task. I have taken help from others who are experts in their respective area. Here I want to convey my sincere thanks to Dr. T. Nagarajan, Professor, Department of Mechanical Engineering, IIT Madras, India who guided me to carryout this task. Further I want to convey my sincere thanks to Mr. Kumarvelu, Center for electronics, IIT Madras, my friends Mr. Seshu Kumar, Wipro Technologies and Mr. P. Srinivas, PHILIPS India Pvt. Ltd. and others who helped directly and indirectly in carrying out this task.

References

[1] S.Gordan,, July 1991, "Servo control of pneumatic systems is here" Journal of Hydraulics and pneumatics, P.No. 49-52.

[2] April 1985, Article on "Servovalves"--Journal of Hydraulics and pneumatics, P.No.49.

[3] Jan 1985, Article on "Experimental Air servo may improve robot performance"--Ideas and applications, Journal of Hydraulics and Pneumatics, P.No. 20.

[4] Ramesh S. Gaonkar "Microprocessor architecture, programming and applications with the 8085/8085A" Pearson education.

[5] Kenjo, Oxford University Press, 1994 "Stepping motors and their Microprocessor control".

[6] James E. Johnson "Electro hydraulic servo systems" published in 1978.

[7] Clarence W. Desilva, "Control sensors and Actuators" Prentice Hall PTR Upper Saddle River, NJ, USA, 1988

[8] May 1986, Article on "Differences between a proportional valve and a servovalve"--Journal of Hydraulics and Pneumatics, P.No. 90.

[9] April 2000, J. Pu, P. R. Moore, C. B. Woney "Smart components based servo pneumatic actuation system", Microprocessors and Microsystems, P. No. 113119.

[10] K. Araki, Saitama University, Japan "Effects of valve configuration of on a pneumatic servo" In 6th International Fluid power symposium, Cambridge, England, April 8th-10th, 1981.

M. Vijaya Sekhar Babu

Assistant Professor, Department of Mechanical Engineering, GMR Institute of Technology, Rajam, AP-532127. E-mail: sekhar359@yahoo.com