14-02-2013, 04:40 PM
Mini-Lab Projects in the Undergraduate Classical Controls Course
Mini-Lab Projects.pdf (Size: 477.13 KB / Downloads: 120)
Abstract:
To address a common complaint from students that the undergraduate controls lecture course in mechanical engineering is too abstract, an electromechanical mini-lab was developed. The term “mini-lab” is used here to emphasize the fact that the lab augments the lecture, but does not replace a full controls lab. This mini-lab consists of a simple DC motor and flywheel with either tachometer speed, or potentiometer position, feedback to implement speed or position control. The students were required to model the system, design controllers using root locus techniques, simulate the compensated system using MATLAB and Simulink, and implement their controllers using analog circuitry contained in a supplied breadboard kit. The students, placed into groups of three, then debuged and tested their controllers on the mini-lab to determine the actual performance in comparison to simulation. The outcomes over two trials will be presented along with recommended modifications.
Introduction
One of the main complaints of students in the mechanical engineering classical controls course at UMR (ME279) is that the material covered is too theoretical in nature, and the examples provided in the text are too abstract. ME279 is an introductory control systems design and analysis course that includes classical control system design topics. Topics presented in the course normally include classical feedback control system analysis and design of single-input single output feedback control systems, time domain performance specification and analysis, time domain control system design using root locus techniques, and frequency domain analysis and design. The classical control systems course follows a course in linear systems where students study linear ordinary differential equations for modeling, Laplace transforms applied to mechanical systems, circuits and electromechanical systems. Students in the control systems course are usually required to complete a control systems design project near the end of the semester using MATLAB and Simulink.
System Apparatus
The contents of this section describe the physical electromechanical system for which students to develop analog control systems. Velocity and position control were studied in successive semesters. In both cases all students were required to participate in the laboratory experience. The two configurations have some common components that we discuss first. Space limitations prohibit complete explanation of the implementation details, but these will be presented thoroughly in a subsequent full-length journal paper.
Common Components
Schematic and functional block diagrams of the system are shown in Figures 2 and 3 respectively. Components of the system are: (1) an Advanced Motion Controls 12A8K servo amplifier, (2) a permanent magnet DC drive motor labeled M1 and a sensing element to provide the feedback signal. For the velocity control case we used a second DC motor configured as a tachometer generator. For the position feedback configuration we used a series connection of three potentiometers connected between ±12 V control signal power supply. The front panel of the apparatus has binding posts for a 24 V drive motor power supply, ±12 V control system power supply, common ground, drive motor input and feedback voltage output. The front panel also has a potentiometer that can be used to reduce the input voltage seen by the servo amplifier. This allows us to change the transfer function open loop gain so that different lab groups may have a slightly different design problem to work. We use a toggle switch on the front panel to disable the control system if necessary during testing.
Angular Velocity Control
A schematic for the velocity control is shown in Figure 4. The reference voltage supplied to the servo amplifier is used to adjust the duty cycle of constant amplitude output pulses produced by the servo amplifier. A higher reference input voltage for the servo amplifier causes the “on time” for output pulses to be longer. Lower reference input voltage causes low “on time” for the output pulses. By changing the duty cycle (ratio of on-time to off-time) of the motor drive voltage we change the average current supplied to the motor being controlled. Higher motor current causes higher motor torque and in steady state this drive torque matches opposing torque due to system damping which is proportional to speed. So as the input voltage supplied to the servo amplifier is there is an increase in the steady state speed of the flywheel. Step changes in servo amplifier input voltage cause a speed transient response that is approximately first order and linear.
Angular Position Control
For the position control laboratory exercise the input side of the open loop system is essentially the same as above. A non-zero input voltage, either positive or negative, produces driving torque at the flywheel shaft, in either the clockwise or counter clockwise direction, which causes motion in the respective directions. To detect angular position of the flywheel we coupled it directly to a 100 KΩ, off-the-shelf, rotary potentiometer. Trimming potentiometers on either side of the feedback potentiometer provide a way to fine tune sensitivity and offset. Physical limitation of the potentiometer used limits the range of motion for the flywheel to ±120 degrees.
Due to the nature of the position control apparatus it is not possible to directly determine, experimentally, the open loop transfer function of the system. We chose to identify the open loop transfer function indirectly in this case and give this to students in order for them to complete their designs. To set this up, we created a proportional control system with (approximately) unity feedback gain and unity controller gain. We then adjusted the servo amplifier and apparatus front panel potentiometer to yield an under-damped, closed loop system with overshoot set at approximately 100 % and settling time approximately 0.75 seconds.
Conclusions and Further Work
We were unable to measure any significant improvement in learning during our first mini-lab, and found only modest enthusiasm for the mini-labs based on question one and two. However, based on their response to question three following the second mini-lab, the students seem to agree that some form of laboratory implementation should always accompany ME279. Student written comments seem to be on the whole rather positive towards having a lab experience – the most common complaint being that they would rather do the lab much earlier than the end of the semester. They were also bothered when the equipment failed to work exactly as expected, or when a component broke. We plan to continue to refine our mini-lab procedures, and hope to begin the project earlier in the semester.