Control Systems/Glossary

The following is a listing of some of the most important terms from the book, along with a short definition or description. A, B, C Acceleration ErrorThe amount of steady state error of the system when stimulated by a unit parabolic input. Acceleration Error ConstantA system metric that determines that amount of acceleration error in the system. Adaptive ControlA branch of control theory where controller systems are able to change their response characteristics over time, as the input characteristics to the system change. Adaptive Gain when control gain is varied depending on system state or condition, such as a disturbance AdditivityA system is additive if a sum of inputs results in a sum of outputs. Analog SystemA system that is continuous in time and magnitude. ARMA Autoregressive Moving Average, see ATO Analog Timed Output. Control loop output is correlated to a timed contact closure. A/M Auto-Manual. Control modes, where auto typically means output is computer-driven, calculated while manual can be field-driven or merely using a static setpoint. Bilinear Transform a variant of the Z-transform, see Block DiagramA visual way to represent a system that displays individual system components as boxes, and connections between systems as arrows. Bode PlotsA set of two graphs, a "magnitude" and a "phase" graph, that are both plotted on log scale paper. The magnitude graph is plotted in decibels versus frequency, and the phase graph is plotted in degrees versus frequency. Used to analyze the frequency characteristics of the system. Bounded Input, Bounded OutputBIBO. If the input to the system is finite, then the output must also be finite. A condition for stability. Cascade When the output of a control loop is fed to/from another loop. CausalA system whose output does not depend on future inputs. All physical systems must be causal. Classical ApproachSee Classical Controls. Classical ControlsA control methodology that uses the transform domain to analyze and manipulate the Input-Output characteristics of a system. Closed Loopa controlled system using feedback or feedforward CompensatorA Control System that augments the shortcomings of another system. Condition Number Conditional StabilityA system with variable gain is conditionally stable if it is BIBO stable for certain values of gain, but not BIBO stable for other values of gain. Continuous-TimeA system or signal that is defined at all points t. Control Rate the rate at which control is computed and any appropriate output sent. Lower bound is sample rate. Control SystemA system or device that manages the behavior of another system or device. ControllerSee Control System. ConvolutionA complex operation on functions defined by the integral of the two functions multiplied together, and time-shifted. Convolution IntegralThe integral form of the convolution operation. CQI Control Quality Index, , 1 being ideal. CV Controlled variable D, E, F Damping RatioA constant that determines the damping properties of a system. Deadtime time shift between the output change and the related effect (typ. at least one control sample). One sees "Lag" used for this action sometimes. DigitalA system that is both discrete-time, and quantized. Direct action target output increase is required to bring the process variable (PV) to setpoint (SP) when PV is below SP. Thus, PV increases with output increase directly. Discrete magnitudeSee quantized. Discrete timeA system or signal that is only defined at specific points in time. DistributedA system is distributed if it has both an infinite number of states, and an infinite number of state variables. See Lumped. DynamicA system is called dynamic if it doesn't have memory. See Instantaneous, Memory. EigenvaluesSolutions to the characteristic equation of a matrix. If the matrix is itself a function of time, the eigenvalues might be functions of time. In this case, they are frequently called eigenfunctions. EigenvectorsThe nullspace vectors of the characteristic equation for particular eigenvalues. Used to determine state-transitions, among other things. See Euler's FormulaAn equation that relates complex exponentials to complex sinusoids. Exponential Weighted Average (EWA) Apportions fractional weight to new and existing data to form a working average. Example EWA=0.70*EWA+0.30*latest, see Filtering. External DescriptionA description of a system that relates the input of the system to the output, without explicitly accounting for the internal states of the system. FeedbackThe output of the system is passed through some sort of processing unit H, and that result is fed into the plant as an input. Feedforward When a priori knowledge is used to forecast at least part of the control response. Filtering (noise) Use of signal smoothing techniques to reject undesirable components like noise. Can be as simple as using exponential weighted averaging on the input. Final Value TheoremA theorem that allows the steady-state value of a system to be determined from the transfer function. FOHFirst order hold Frequency ResponseThe response of a system to sinusoids of different frequencies. The Fourier Transform of the impulse response. Fourier TransformAn integral transform, similar to the Laplace Transform, that analyzes the frequency characteristics of a system. See G, H, I Game TheoryA branch of study that is related to control engineering, and especially optimal control. Multiple competing entities, or "players" attempt to minimize their own cost, and maximize the cost of the opponents. GainA constant multiplier in a system that is typically implemented as an amplifier or attenuator. Gain can be changed, but is typically not a function of time. Adaptive control can use time-adaptive gains that change with time. General DescriptionAn external description of a system that relates the system output to the system input, the system response, and a time constant through integration. Hendrik Wade BodeElectrical Engineer, did work in control theory and communications. Is primarily remembered in control engineering for his introduction of the bode plot. Harry NyquistElectrical Engineer, did extensive work in controls and information theory. Is remembered in this book primarily for his introduction of the Nyquist Stability Criterion. HomogenietyProperty of a system whose scaled input results in an equally scaled output. Hybrid SystemsSystems which have both analog and digital components. ImpulseA function denoted δ(t), that is the derivative of the unit step. Impulse ResponseThe system output when the system is stimulated by an impulse input. The Inverse Laplace Transform of the transfer function of the system. Initial ConditionsThe conditions of the system at time , where t0 is the first time the system is stimulated. Initial Value TheoremA theorem that allows the initial conditions of the system to be determined from the Transfer function. Input-Output DescriptionSee external description. InstantaneousA system is instantaneous if the system doesn't have memory, and if the current output of the system is only dependent on the current input. See Dynamic, Memory. Integrated Absolute Error (IAE)absolute error (ideal vs actual performance) is integrated over the analysis period. Integrated Squared Error (ISE)squared error (ideal vs actual performance) is integrated over the analysis period. IntegratorsA system pole at the origin of the S-plane. Has the effect of integrating the system input. Inverse Fourier TransformAn integral transform that converts a function from the frequency domain into the time-domain. Inverse Laplace TransformAn integral transform that converts a function from the S-domain into the time-domain. Inverse Z-TransformAn integral transform that converts a function from the Z-domain into the discrete time domain. J, K, L Lag The observed process impact from an output is slower than the control rate. Laplace TransformAn integral transform that converts a function from the time domain into a complex frequency domain. Laplace Transform DomainA complex domain where the Laplace Transform of a function is graphed. The imaginary part of s is plotted along the vertical axis, and the real part of s is plotted along the horizontal axis. Left EigenvectorsLeft-hand nullspace solutions to the characteristic equation of a matrix for given eigenvalues. The rows of the inverse transition matrix. LinearA system that satisfies the superposition principle. See Additive and Homogeneous. Linear Time-Invariant LTI. See Linear, and Time-Invariant. Low Clamp User-applied lower bound on control output signal. L/R Local/Remote operation. LQR Linear Quadratic Regulator. LumpedA system with a finite number of states, or a finite number of state variables. M, N, O Magnitude the gain component of frequency response. This is often all that is considered in saying a discrete filter's response is well matched to the analog's. It is the DC gain at 0 frequency. Marginal StabilityA system has an oscillatory response, as determined by having imaginary poles or imaginary eigenvalues. Mason's Rule see MATLAB Commercial software having a Control Systems toolbox. Also see Octave. MemoryA system has memory if its current output is dependent on previous and current inputs. MFACModel Free Adaptive Control. MIMOA system with multiple inputs and multiple outputs. Modern Approachsee modern controls Modern ControlsA control methodology that uses the state-space representation to analyze and manipulate the Internal Description of a system. Modified Z-TransformA version of the Z-Transform, expanded to allow for an arbitrary processing delay. MPC Model Predictive Control. MRAC Model Reference Adaptive Control. MV can denote Manipulated variable or Measured variable (not the same) Natural FrequencyThe fundamental frequency of the system, the frequency for which the system's frequency response is largest. Negative FeedbackA feedback system where the output signal is subtracted from the input signal, and the difference is input to the plant. The Nyquist CriteriaA necessary and sufficient condition of stability that can be derived from Bode plots. Nonlinear ControlA branch of control engineering that deals exclusively with non-linear systems. We do not cover nonlinear systems in this book. OCTAVE Open-source software having a Control Systems toolbox. Also see MATLAB. Offset The discrepancy between desired and actual value after settling. P-only control can give offset. Oliver HeavisideElectrical Engineer, Introduced the Laplace Transform as a tool for control engineering. Open Loop when the system is not closed, its behavior has a free-running component rather than controlled Optimal ControlA branch of control engineering that deals with the minimization of system cost, or maximization of system performance. OrderThe order of a polynomial is the highest exponent of the independent variable in that exponent. The order of a system is the order of the Transfer Function's denominator polynomial. Output equationAn equation that relates the current system input, and the current system state to the current system output. Overshootmeasures the extent of system response against desired (setpoint tracking). P, Q, R ParabolicA parabolic input is defined by the equation. Partial Fraction ExpansionA method by which a complex fraction is decomposed into a sum of simple fractions. Percent OvershootPO, the amount by which the step response overshoots the reference value, in percentage of the reference value. Phase the directional component of frequency response, not typically well-matched between a discrete filter equivalent to the analog version, especially as frequency approaches the Nyquist limit. The final value in the limit drives system stability, and stems from the poles and zeros of the characteristic equation. PIDProportional-Integral-Derivative PlantA central system which has been provided, and must be analyzed or controlled. PLCProgrammable Logic Controller PoleA value for s that causes the denominator of the transfer function to become zero, and therefore causes the transfer function itself to approach infinity. Pole-Zero FormThe transfer function is factored so that the locations of all the poles and zeros are clearly evident. Position ErrorThe amount of steady-state error of a system stimulated by a unit step input. Position Error ConstantA constant that determines the position error of a system. Positive FeedbackA feedback system where the system output is added to the system input, and the sum is input into the plant. PSDThe power spectral density which shows the distribution of power in the spectrum of a particular signal. Pulse ResponseThe response of a digital system to a unit step input, in terms of the transfer matrix. PV Process variable QuantizedA system is quantized if it can only output certain discrete values. Quarter-decay the time or number of control rates required for process overshoot to be limited to within 1/4 of the maximum peak overshoot (PO) after a SP change. If the PO is 25% at sample time N, this would be time N+k when subsequent PV remains < SP*1.0625, presuming the process is settling. Raise-Lower Output type that works from present position rather than as a completely new computed spanned output. For R/L, the % change should be applied to the working clamps i.e. 5%(hi clamp-lo clamp). RampA ramp is defined by the function . ReconstructorsA system that converts a digital signal into an analog signal. Reference ValueThe target input value of a feedback system. RelaxedA system is relaxed if the initial conditions are zero. Reverse action target output decrease is required to bring the process variable (PV) to setpoint (SP) when PV is below SP. Thus, PV decreases with output increase. Rise TimeThe amount of time it takes for the step response of the system to reach within a certain range of the reference value. Typically, this range is 80%. Robust ControlA branch of control engineering that deals with systems subject to external and internal noise and disruptions. S, T, U, V SamplersA system that converts an analog signal into a digital signal. Sampled-Data SystemsSee Hybrid Systems. Sampling TimeIn a discrete system, the sampling time is the amount of time between samples. Reflects the lower bound for Control rate. SCADA Supervisory Control and Data Acquisition. S-DomainThe domain of the Laplace Transform of a signal or system. Second-order System; Settling TimeThe amount of time it takes for the system's oscillatory response to be damped to within a certain band of the steady-state value. That band is typically 10%. Signal Flow DiagramA method of visually representing a system, using arrows to represent the direction of signals in the system. SISO Single input, single output. Span the designed operation region of the item,=high range-low range. Working span can be smaller if output clamps are used. StabilityTypically "BIBO Stability", a system with a well-behaved input will result in a well-behaved output. "Well-behaved" in this sense is arbitrary. Star TransformA version of the Laplace Transform that acts on discrete signals. This transform is implemented as an infinite sum. State EquationAn equation that relates the future states of a system with the current state and the current system input. State Transition MatrixA coefficient matrix, or a matrix function that relates how the system state changes in response to the system input. In time-invariant systems, the state-transition matrix is the matrix exponential of the system matrix. State-Space EquationsA set of equations, typically written in matrix form, that relates the input, the system state, and the output. Consists of the state equation and the output equation. See State-VariableA vector that describes the internal state of the system. StabilityThe system output cannot approach infinity as time approaches infinity. See BIBO, Lyapunov Stability'. Step ResponseThe response of a system when stimulated by a unit-step input. A unit step is a setpoint change for setpoint tracking. Steady StateThe output value of the system as time approaches infinity. Steady State ErrorAt steady state, the amount by which the system output differs from the reference value. SuperpositionA system satisfies the condition of superposition if it is both additive and homogeneous. System Identification method of trying to identify the system characterization , typically through least squares analysis of input,output and noise data vectors. May use ARMA type framework. System TypeThe number of ideal integrators in the system. Time-InvariantA system is time-invariant if an input time-shifted by an arbitrary delay produces an output shifted by that same delay. Transfer FunctionThe ratio of the system output to its input, in the S-domain. The Laplace Transform of the function's impulse response. Transfer Function MatrixThe Laplace transform of the state-space equations of a system, that provides an external description of a MIMO system. Uniform StabilityAlso "Uniform BIBO Stability", a system where an input signal in the range [0, 1] results in a finite output from the initial time until infinite time. See . Unit StepAn input defined by . Practically, a setpoint change. Unity FeedbackA feedback system where the feedback loop element H has a transfer function of 1. Velocity ErrorThe amount of steady-state error when the system is stimulated by a ramp input. Velocity Error ConstantA constant that determines that amount of velocity error in a system. W, X, Y, Z W-plane Reference plane used in the bilinear transform. Wind-up when the numerics of computed control adjustment can "wind-up", yielding control correction with an inappropriate component unless prevented. An example is the "I" contribution of PID if output has been disconnected during PID calculation ZeroA value for s that causes the numerator of the transfer function to become zero, and therefore causes the transfer function itself to become zero. Zero Input ResponseThe response of a system with zero external input. Relies only on the value of the system state to produce output. Zero State ResponseThe response of the system with zero system state. The output of the system depends only on the system input. ZOH Zero order hold. Z-TransformAn integral transform that is related to the Laplace transform through a change of variables. The Z-Transform is used primarily with digital systems. See