CSC Final Exam

Discuss the general tasks of the controls engineer in the design of a control system. Which are emphasized in this course and which are emphasized in a classical controls course?

Understand the plant, choose a control approach, select and size components, model the system, and build and test it. This course emphasizes components and implementation. Classical controls emphasizes mathematical analysis and controller design.

Draw the typical control system that exemplifies the various components under study in this course. Explain each block.

The summing junction compares the command u with the feedback to produce an error. The amplifier boosts the signal, the actuator moves the plant, and the sensor measures the output and sends it back for comparison.

In the block diagram of a typical control system, discuss the power levels around each block. At which point is the minimum power level and the maximum power level?

The command, summing junction, and feedback use little power, with minimum power at the error signal before the amplifier. The amplifier gives maximum power at its output. The actuator converts this power into mechanical power to drive the plant or load, with some lost as heat and friction. The sensor measures the output and returns a low power feedback signal.

Which components in the block diagram have the most significant effect upon the dynamics of the control system? Which are generally fixed and of which do we have more selection? What parameters can we vary to improve the system dynamics?

The actuator and load usually dominate the dynamics. The load is usually fixed. We can select the amplifier, actuator, sensor, and compensation. We can change gain, damping, gearing, impedance, and available power.

What engineering technologies can be used to control a system? What typical components are used in each as amplifiers, actuators or effectors, and circuit components?

Electrical, fluid, mechanical, and thermal tech can be used.  

Electrical: An amplifier is a transistor or op-amp, an actuator is a motor or solenoid, and circuit components include resistors and capacitors.

Fluid: An amplifier is a servo valve or jet amplifier, an actuator is hydraulic motor, and circuit components include restrictions and storage chambers.

Mechanical: Levers and gears amplify force or motion not power, linkages transmit motion to the load, and circuit components include springs, masses, and dampers.

Thermal: An amplifier is a convection thermal amplifier, where a small temperature change controls a larger heat flow. An effector is a heater, and a circuit component is insulation, which provides thermal resistance.

Compare and contrast the bandwidth potential, the linearity and ease of modeling, control power levels obtainable, and physical size of control components in the three major control technologies.

The three major control technologies I will use are electrical, fluid, and mechanical.

Electrical: High bandwidth, easy linear modeling, and compact circuits. High power is limited by heat.

Fluid: Lower bandwidth and harder nonlinear modeling. Hydraulics give high power in small actuators, while pneumatics are less stiff.

Mechanical: Inertia limits bandwidth. Friction and backlash complicate modeling. High power is possible, but parts become large and heavy.

What are the general advantages and disadvantages of each of the engineering technologies used in control? When would we use each (if the performance were the same)?

Electrical: Clean and easy to control, but limited by heating.

Fluid: Hydraulics are powerful and compact, but need plumbing and leak control. Pneumatics are simple, but less stiff and precise.

Mechanical: Simple and direct, but have friction and wear.

Thermal: Useful for temperature control, but usually slow.

Choose based on cost, size, environment, maintenance, and available power.

Discuss the techniques and hardware used in amplification of signals and power in each of the control technologies.

An amplifier uses a small signal to control larger supply power.

Electrical: Transistors and op-amps control electrical power.

Fluid: Servo valves and control jets control fluid power.

Mechanical: Levers and gears increase force or motion, but not power.

Thermal: A small signal controls a larger heat flow.

Discuss some techniques of computation in each technology. Which computation function are easier to implement with hardware and which are more difficult?

Computation is when you process signals using operations such as addition, subtraction, multiplication, or division to produce the desired output signal.

Electrical: Op-amps and resistor networks add, subtract, and scale signals.

Fluid: Flow junctions add flows, and differential-pressure devices subtract pressure signals.

Mechanical: Levers and gears combine motions and change their ratios.

Thermal: Thermal-resistance networks combine temperature signals.

Easy: Addition, subtraction, and fixed gain.

Harder: Multiplication and division of two changing signals.

Name the different types of filters used in control systems. Discuss the components and hardware used in building filters and compensation circuits in each control technology? Are there any limitations and fundamentally different approaches used in the different technologies?

Common types are low-pass, high-pass, band-pass, notch, lead, lag, integrator, differentiator, PD, and PID.

Electrical: Resistors, capacitors, inductors, and op-amps.

Fluid: Restrictions, compliance, and fluid inertia.

Mechanical: Dampers, springs, and masses.

Thermal: Thermal resistance and heat capacity; no ordinary inductance equivalent.

Limitations: Different hardware can produce similar responses, but real components have limited bandwidth, and differentiation amplifies noise.

Define and contrast direct and inferential sensing techniques. Why might each be used?

Direct sensing measures the variable you want, like a ruler. Inferential sensing measures something related and uses it to determine the desired variable, like a thermometer. Direct sensing is useful when practical, while inferential sensing is useful when direct measurement is difficult, expensive, or inaccessible.

Discuss the difference between signal detection and signal conditioning in the measurement of a variable. Give some examples that identify detection and signal conditioning as separate things.

Signal detection turns the measured quantity into a signal, while signal conditioning makes that signal easier to use. For example, a microphone turns sound into an electrical signal, and an amplifier strengthens it. A light sensor turns light into an electrical signal, and a filter reduces noise in that signal.

What should the input and output impedances be in effort and flow type sensors or transducers? Discuss some common sensors or sensing approaches that are used in each technology.

An effort measurement needs high input impedance, while a flow measurement needs low input impedance. An effort output should have low output impedance, while a flow output should have high output impedance. Examples include electrical potentiometers and Hall sensors, fluid pressure sensors, mechanical springs or diaphragms, and thermal thermocouples or thermistors.

Mention several different actuators and approaches to actuation (mechanical output).

DC motors, stepper motors, and hydraulic motors produce rotation. Solenoids, hydraulic cylinders, and pneumatic cylinders produce linear motion. A motor connected to a lead screw converts rotation into linear motion.

Compare fluid actuators and electromechanical actuators. What are the limiting problems with electromechanical devices? What are the advantages of each?

Hydraulic actuators produce large forces in a small space, and their fluid carries heat back to the reservoir. Pneumatic actuators are simple and light, but compressibility reduces stiffness. Electromechanical actuators are clean and easy to control electronically, but heating, force or torque limits, and limited stroke in devices such as solenoids can restrict their use.

Discuss the concept of effort and flow variables. Mention the effort and flow variables in each technology. Discuss the concept of impedance in each technology.

Effort is the driving quantity, and flow is the associated movement or transfer. The pairs are voltage and current, pressure and volume flow rate, force and velocity, torque and angular velocity, and temperature difference and heat-flow rate. Impedance is effort divided by flow

What are some advantages to multiport modeling? How is a multiport diagram different from a conventional controls block diagram? Is multiport modeling limited to linear systems?

Multiport modeling shows power transfer, effort, flow, and how components load each other. A conventional control block diagram shows mathematical signal relationships. Multiport modeling works for both linear and nonlinear systems.

Discuss the configuration and concepts of general one-, two-, and three-port elements. What type of things are 1-ports? Mention the one-port elements in each technology.

A one port has one energy exchange connection, a two port transfers energy between two connections, and a three port uses a control input to regulate supply power delivered to an output. One port elements include electrical resistors, capacitors, and inductors; mechanical dampers, springs, and masses; fluid restrictions, compliant chambers, and fluid inertance; and thermal resistance and heat capacity.

What types of things are 2-ports in general. Mention several examples of 2-port devices.

Two port devices transfer or convert energy between an input and an output. Examples include gears, levers, transformers, ideal motors, hydraulic pistons, and lead screws. They can trade greater effort for less motion or flow.

What types of things are 3-ports in general. Mention several examples of 3-port devices.

Three port devices are amplifiers or power modulators with a control port, supply port, and output port. Examples include transistor amplifiers, powered op-amps, servo valves, and fluidic amplifiers.

What are the characteristics or graphical characteristics of multiports? From these characteristics, what are the important parameters that we can use to characterize the component?

The transfer characteristic gives gain, the input characteristic gives input impedance, and the output characteristic gives output impedance. The supply characteristic describes the supply-port behavior. These graphs also show operating points, limits, and linearity.

If two multiports are staged together, what must be the impedance ratio Zi2/Zo1 in order to achieve maximum effort gain or output? Maximum flow gain or output? Maximum power gain or maximum power transfer?

For maximum effort transfer, Zi2 / Zo1 should be much greater than one. For maximum flow transfer, it should be much less than one. For maximum power transfer between resistive impedances, it should equal one.

Describe the conditions under which a linear system is transferring a maximum power to the load component. Under these conditions, what is the operating effort variable relative to the maximum effort output? What is the operating flow variable? What are these effort and flow percentages for a nonlinear system?

For a straight line source characteristic, maximum power occurs when the load and source impedances match. The operating effort is half the maximum effort, and the operating flow is half the maximum flow. A nonlinear system has no fixed percentages; find the point where effort times flow is largest.

Define open-loop and closed-loop control. What advantages does closed-loop control have? Name various effects can we expect to achieve with closed-loop control.

Open loop control does not use measured output to correct its action. Closed loop control measures the output and feeds it back to reduce error. Properly designed feedback can improve accuracy, reject disturbances, reduce sensitivity to component changes, and improve response speed and damping.

Discuss the various performance factors that we use to describe the performance of a control system or a control system component.

Static gain describes the final output change per input change, and static error describes how far the final output is from the command. Disturbance sensitivity describes how much a disturbance changes the output, while stiffness describes resistance to load-caused displacement. Time constant, natural frequency, damping ratio, and bandwidth describe the response speed and oscillation. Range, linearity, and hysteresis describe other performance limits.

What general effect does increasing the forward gain of a closed-loop system have on the performance of a system? Would we increase or decrease the forward gain without bound?

Increasing forward gain often improves accuracy, disturbance rejection, stiffness, and response speed. We cannot increase it without limit because too much gain can cause oscillation, instability, saturation, and stronger response to noise. Too little gain gives weak correction and poor performance.

What general effect does the output impedance of the amplifier have on the performance of a closed-loop system? Do we usually want to increase or decrease the output impedance of a system with mechanical output?

Amplifier output impedance affects loading, damping, and disturbance response, with the exact effect depending on the system. For mechanical position control, we generally want high closed-loop output stiffness so load forces cause little movement. This mechanical stiffness is different from the amplifier’s electrical output resistance.

What are the different types of friction and what physical effect gives rise to each? Which are the most common?

Viscous friction comes from fluid shear and increases with velocity. Coulomb friction comes from sliding contact and has nearly constant magnitude, while stiction prevents motion from starting. Turbulent resistance comes from turbulent fluid motion and is approximately proportional to velocity squared. Coulomb and viscous friction are common in mechanical systems.

What effect does coulomb friction have on the static response of a closed-loop system? What steps can be taken to reduce this effect?

Coulomb friction causes hysteresis and a position offset. Reduce its effect by reducing friction, increasing static stiffness, or adding dither.

Justify the statement that an increase in bandwidth of a closed-loop system requires an increase in the power capability from the controller, or a decrease in the impedance of the load.

Higher bandwidth means following faster changes. Faster motion and acceleration require more force, torque, and power. Therefore, the controller needs greater power capability, or the load must require less effort for the desired motion.

What are some typical nonlinearities encountered in control systems? What is the most common nonlinearity?

Common nonlinearities include saturation, deadband, friction, hysteresis. Saturation is a standard answer for the most common because every real component has limits.

Discuss the concepts of pulse modulation. What is the general configuration of a pulse modulation system? Define the important characteristics of the shape of the pulse wave form.

Pulse modulation controls average output by changing pulses. The command goes into a pulse controller and switch, which sends pulses through a filter or smoothing load. Pulse height is the amplitude, width is the on-time, and period is the time between pulse starts. Frequency is 1/tau, duty cycle is w/tau.

Discuss several reasons why pulse modulation is so attractive.

Pulse modulation is efficient, produces little heating in the switch, and uses inexpensive hardware. It lets a small signal control large power and is easy to generate electronically. In some mechanical systems, the pulsing also helps overcome sticking.

Name and discuss several general pulse-modulation techniques. Draw the block diagrams for each.

PWM changes how long each pulse stays on while pulse height and frequency stay constant. Examples include motor speed control, LED brightness control, and servo motor.

PHM changes pulse height while duration and frequency stay constant. Examples include proportional amplifiers, Waterpik, and sampled audio signals

PFM changes how often pulses occur while height and duration stay constant. Examples include tachometers, flowmeter, and frequency meters.

PWFM changes both pulse duration and frequency while pulse height stays constant. Examples include thermostat controlled ovens, air conditioners, and air compressors.

Mention several examples of pulse modulation systems in use in control applications.

Examples include PWM motor speed control, LED brightness control, heater power control, and the motor drive in the idle speed controller. A pulse frequency tachometer circuit measures speed by converting pulse frequency into an average signal.

What are some of the most interesting things that you have learned about control systems this semester?

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