Home Important Questions Anna University Important Questions for DIGITAL CONTROL SYSTEM Nov/Dec Exams 2012

Anna University Important Questions for DIGITAL CONTROL SYSTEM Nov/Dec Exams 2012

Anna University Important Questions

Anna University Important Questions for Digital Control System ,

  1. 1.       
Distinguish between analog and discrete signalsAnalog signals are continuously vary with time, frequency and space but discrete signals do not have continuous signal. It produces output only at different instants.
  1. 2.       
What are the advantages of digital control over conventional control?Flexibility, Wide selection of control algorithm, integrated control of industrial systems, Future generation control system.
  1. 3.       
What is sampling? How it is achieved?Sampling is a process of converting continuous signal into discrete signal with proper selection of sampling period / frequency. Sampling is achieved by sample and hold device.
  1. 4.       
How information loss happens due to sampling.In practice, signals in control system include low frequency components carrying useful information, as well as frequency noise components. The high frequency components appear as low frequency signals due the aliasing effect, causing a loss of information.
  1. 5.       
Obtain the Z transform of e-atsinωtZ { e-at x(t) } = X ( z e-at) Z { sinωt } =  Z { e-atsinωt } =
  1. 6.       
What is the use of modified Z transform?The Z transform method determine the transient responses of sampled –data control systems only at the sampling instant. To obtain the values of the response between sampling instants, modified Z transforms are useful. They are also helpful in analyzing sampled data control systems containing transportation lag.
  1. 7.       
Obtain the closed form solution of state difference equationThe state equation is

The solution for this equation in closed form is



  1. 8.       
When the losses occur in controllability and Observability?Losses of controllability and Observability due to sampling may occur only when the continuous time system has oscillary modes and the sampling interval is half the period of oscillation of an oscillatory mode or an integer multiple of that period.
  1. 9.       
Distinguish between velocity form and position form of Digital PID algorithm.

Position form

Velocity form

it requires initialization of controller outputit does not need initialization of controller output
it does not protect against integral windup as sum or error sequences are not computedit is protected against integral windup as sum or error sequences are not computed.
it does not protect the process against computer failureit protects the process against computer failure
it produces bump transferit produces bumpless transfer


  1. 10.
What is pole placement?Finding the elements of control gain matrix K so that the roots of the characteristic equation ( i.e., the poles of the closed loop system ) are in the desired location is called pole placement. The parameters are iterated in the compensators to find acceptable closed loop root locations with full state feedback.

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