Test #9 Flashcards
Draw and correctly label the block diagram of a Digital Control System.
Name the four types of A/D converters. Explain, in three or four sentences, the operation of three of them
Counter or Servo Type A/D Converter : The counter starts at zero and the D/A produces an equivalent analog voltage (dig equiv) which is compared to the Analog input. The dig equiv will continue to increase (on successive clock cycles) until it gets higher than the analog input . At this point the counter is stopped and the conversion is done. The output of the counter is the digital output.
Dual Slope Integrating A/D Converter : In the operation of the dual-slope integrating A/D converter there are two separate periods. In the first period, the sample time, the analog input is applied to an integrator for a constant period of time generating a voltage proportional to the input. In the second period, the convert time, the integrator is connected to a negative reference which causes it to discharge. The time it takes to discharge is counted and used as a measure of the original analog input.
Successive Approximation A/D Converter : This converter takes only as many tests as it has bits in its converted. Its first comparison will use an analog equivalent value that is ½ of the full scale range to compare to the analog input. Each successive test will use the midscale of the remaining range by adjusting the bit pattern of the shift register until all tests have been completed. When finished, the conversion will generate a digital value that gives an analog value below the analog input.
Parallel or Flash A/D Converter : This converter uses a precision voltage divider network to establish the individual voltage levels for a series of comparators. It then compares the analog input against all the levels simultaneously. The result is encoded into a binary value. The number of comparators is 2N – 1.
Explain the difference in the Weighted Resistor and Binary Ladder network.
The difference between them is Weighted resistor needs a different precision resistor for each bit whereas the binary ladder uses only 2 precision resistor no matter how many bits are involved.
Describe the error incurred when analog values are converted to a digital form.
The error incurred when we convert an analog signal into a digital form is called Quantization error. It happens when a limited number of digital codes are used to describe an infinite number of analog input.
Comment on the effect, on the received signal, of sampling below the Nyquist criterion, and just above the Nyquist criterion.
If the received signal is sampled below the Nyquist criterion the frequency of the input signal will be misinterpreted and viewed as having a lower frequency than it actually has. If it is sampled at just above the criterion the frequency of the input will be accurately measured, though the shape of the waveform in time will be rather crude.
a) Name the two different numerical integration methods we discussed in class.
b) Which the above methods is preferred?
c) Why is the method stated in part b preferred?
a) Rectangular, trapezoidal
b) The trapezoidal method is preferred
c) It is more accurate as a result of approximating the curve by a straight line, not a horizontal one.
a) Given three error samples e0, e1, e2 show how the rate of change of error at the time of sample 1 can be found three ways.
b) Which the above methods is preferred in a real-time control system?
c) Why is the method stated in part b preferred?
Method A=((e_1-e_0 ))/∆T
Method B=((e_2-e_1 ))/∆T
Method C=((e_2-e_0 ))/∆T
Method A is preferred
This real-time control action does not know at the time of sample e1 what is the value of e2. Thus method B and C are not possible as they both require the knowledge of e2
a) Sketch a simple digital Low-pass filter.
a) Which of the A/D converters is considered to be resistant to temperature variations?
b) Which of the A/D converters requires the most comparators?
c) Which of the A/D converters is the fastest?
d) What would be the effect of bypassing the built-in Sample& Hold circuitry in the A/D and connecting it directly to the analog signal?
a) The dual slope
b) The parallel or flash requires the most comparators
c) The parallel or flash is the fastest
d) Bypassing the sample/hold circuitry would cause the input signal to vary in the middle of the conversion process and thus yield erroneous results.
a) A digital control system with sampling interval DELTA_T = 50ms is tuned using the Ultimate cycle method. Comment on the effect changing the sampling interval to DELTA_T =200ms will likely have on the operation of the control system with the previous tuned parameters.
b) How the problem in part a) be rectified without changing the sampling interval from DELTA_T =200ms?
a) The system’s step response is likely to be less than optimum. If the system’s response was critically damped, the new response will likely be underdamped.
b) This problem could be resolved by retuning the system.
What advantage does digital filtering offer over a physical filter made from discrete components?
Digital filtering offers the advantage of being able to reprogram the filter without changing any of the components.
Based on the results shown in figure 18 what is the potential result of decreasing the number of bits used in a Digital PID controller?
The potential result of decreasing the number of bits used in a digital PID control is overshoots and discontinuous response.
Sketch a simple high pass filter
Sketch a simple bandpass filter
