(a) Pneumatic lock up and fail safe system
- Pneumatic lock up and fail safe system serve to bring about particular valve position in event of power failure
- Lock up system used to hold valve in its last position in event of supply failure
- Fail safe system arranged to move valve disc to open or close position in event of supply failure
- If valve fully open position is fail safe condition valve move to this position in event of supply failure.
(b) Zieglar Nicolas tuning method (one method of PID controller tuning method)
- Close loop or on line tuning method proposed by zieglar and Nichols
- Set up system in closed loop mode ie with controller in automatic mode
- Remove integral (reduce proportional band) until controller just begin steady hunt.
- Small step change of desired value necessary to begin oscillation
- Record proportional band setting as value P when amplitude of oscillation is constant
- Record periodic time of sinusoidal hunt as value T
- Setting of PID parameters are done as per table shown below
- Small amount of trimming around these setting result in optimum performance
Q Draw a detail a Ward-Leonard system installation used onboard a ship. State where and why they are used onboard.
Ward-Leonard System
Consists
- A.C motor (Driving)
- D.C generator
- D.C motor (Control)
- Transformer and rectifier circuit for D.C generator and D.C motor
- Control potentiometer D.C generator
Operation
3 phase induction motor drive DC generator.
DC generator output voltage
- Applied as power supply to DC motor armature.
- Increase or decrease by potentiometer as magnetic field strength alter by changing field current to GE field winding.
- Varies, DC motor speed varies.
- DC motor speed and direction varies magnitude and direction of applied voltage.
- Potentiometer change current flow direction, through DC GE field poles.
- It will cause change in direction of generated current supply to DC motor and running direction of DC motor.
Uses
- Use for fine control of DC motor speed from zero to full in either direction.
- Easier speed control and robust torque characteristic.
- Ship with DC power widely use steering gear motor.
- Ship with AC power, other deck machinery such as windlass, mooring winches, deck crane, etc.
Write short notes for the following:
(a) Drag cup tachometer
(b) Pneumatic flapper nozzle
(a) Drag cup tachometer
- Uses an aluminum cup, rotated in a laminated iron electromagnet stator.
- Stator has two separate windings at right angles to one another.
- An alternating current is supplied to one winding, eddy currents are set up in the aluminum cup.
- Induced e.m.f in other stator winding which is proportional to speed of rotation
- Output voltage measured on a voltmeter calibrated to read in units of speed.
(b) Pneumatic nozzle flapper
- Used in many pneumatic devices, use as a transducer.
- Transduces from displacement to a pneumatic signal.
- The very small linear movement of flapper convert control pressure output.
- Compressed air about (1) bar supplied through orifice (about 0.25mm diameter) to nozzle (about 0.4mm) which larger than orifice.
- Position of flapper in relation to the nozzle determine amount of air escapes.
- If flapper is close to nozzle, high control pressure exit, if distance away, low control pressure.
How many steps do you know in troubles shooting procedure electrical Coil-in electrical System.
Explain about any TWO items;-
1. Symptom Identification
2. Failure Analysis
3. Listing of probable faulty unction
Six step of trouble-shooting are:,
1. Symptom identification
2. Symptom analysis
3. Listing of probable faulty function
4. Localising of faulty function
5. Localising trouble to circuit 6.Failure analysis
(1) Symptom identification
When handed a repair order with a electrical machine, three things to do:
1. Identify problem noted
2. Determine if it is a problem or not
3. If there is a problem, determine if it is intermittent or continuous
- look for information how system should operate are System Outline in the Manual
- When verifying the problem, determine if the problem is continuous or intermittent
- If the problem is continuous (or not intermittent), fairly obvious when operate the suspect system.
- Intermittent problems more difficult to find.
- If the conditions are repeated and problem does not re-occur, visual inspection of harness, connectors, and terminals
(2) Failure analysis
- After making the repair, always verify that the problem was actually fixed.
- Make sure all of the functions and features of circuit are workiing properly.
- Sometimes, a circuit has multiple problems which are causing it to be inoperative.
- Re-check of the circuit ensures that the owner will be satisfied
(a) What is close loop control?
(b) What are the advantage and disadvantage of close loop control?
(c) What are the Different types of Control actions?
(d) What are the fundamental problems with using simple proportional control?
(e) Desired value, Deviation, offset
(f) Cascade control
(g) Ratio control
(h) Split range control
(a) Closed loop control
- Depend output.
- Monitor output and generate an error signal, correct input, call close loop control
- When operator replaced by a controller, sensor provide feedback of the controlled condition, comparator to get the Deviation from measured value and the set value.
- Correcting signal to motor element to carry action on input, call automatic closed loop.
(b) Advantages of closed loop
1. Finer control, with less chances of deviation.
2. Suitable for systems having considerable load changes.
Disadvantages of closed loop
1. More expansive (than open loop).
2. Possibility of hunting (loss of stability).
3. For example, manual closed-loop water level control, an operator observes water level and accordingly opens or shuts feed-check valve to maintain desired level, so more care is needed.
4. In Automatic closed loop, same work can be done by a Feed-water Controller, but design complicated and more maintenance required.
(c) There are several types of Control actions
1. ON-OFF or Two step control action
2. Simple Proportional action
3. Integral action
4. Derivative action
(d) fundamental problems
1. System have a large Offset or Sustained deviation.
2. Too slow in response to Load changes.
3. System become unstable (hunting) if Gain is changed to reduce Offset.
(e) Desired Value
The value of controlled condition, operator desire to obtain.
Deviation
- Difference between desired value and measure value.
- Signal send to comparator, initiate correction action.
Offset
- Measure value stabilized (reaches equilibrium) at some point other than set value.
- Load condition change.
- Offset occur in simple proportional control.
(f) Cascade Control
- Two controller, Master and slave.
- Master controller sense tank 2, level provide desired value.
- Slave controller controls and measure tank 1 level.
- Slave controller loop determine correcting unit for master controller.
- Master controller- P+I and D, if time lags present.
- Slave controller- only P or possibly P+I action.
(g) Ratio Control
- Another form of cascade control.
- Relation between controlled variable (supply B) and controlled variable (supply A), maintain at desire ratio.
- Master controller provide desire value signal to slave controller, relate to measurement of flow A.
- Slave controller control flow B, maintain desire ratio between flow A and flow B.
(h) Split range control
- Another form of multi-loop control.
- Single controller split into two or more ranges to operates corresponding correcting units.
- Range of output signal determine; correcting unit operate to bring desired action.
