Posted tagged ‘variation’

Governor

September 8, 2011
Diesel engine speed is controlled solely by the amount of fuel injected into the engine by the injectors. Because a diesel engine is not self-speed-limiting, it requires not only a means of changing engine speed (throttle control) but also a means of maintaining the desired speed. The governor provides the engine with the feedback mechanism to change speed as needed and to maintain a speed once reached.

A governor is essentially a speed-sensitive device, designed to maintain a constant engine speed regardless of load variation. Since all governors used on diesel engines control engine speed through the regulation of the quantity of fuel delivered to the cylinders, these governors may be classified as speed-regulating governors. As with the engines themselves there are many types and variations of governors. In this module, only the common mechanical-hydraulic type governor will be reviewed.
The major function of the governor is determined by the application of the engine. In an engine that is required to come up and run at only a single speed regardless of load, the governor is called a constant-speed type governor. If the engine is manually controlled, or controlled by an outside device with engine speed being controlled over a range, the governor is called a variable speed type governor. If the engine governor is designed to keep the engine speed above a minimum and below a maximum, then the governor is a speed-limiting type. The last category of governor is the load limiting type. This type of governor limits fuel to ensure that the engine is not loaded above a specified limit. Note that many governors act to perform several of these functions simultaneously.

Operation of a Governor
The following is an explanation of the operation of a constant speed, hydraulically compensated governor using the Woodward brand governor as an example. The principles involved are common in any mechanical and hydraulic governor.

The Woodward speed governor operates the diesel engine fuel racks to ensure a constant engine speed is maintained at any load. The governor is a mechanical-hydraulic type governor and receives its supply of oil from the engine lubricating system. This means that a loss of lube oil pressure will cut off the supply of oil to the governor and cause the governor to shut down the engine. This provides the engine with a built-in shutdown device to protect the engine in the event of loss of lubricating oil pressure.

Simplified Operation of the Governor
The governor controls the fuel rack position through a combined action of the hydraulic piston and a set of mechanical flyweights, which are driven by the engine blower shaft.

Figure 28 provides an illustration of a functional diagram of a mechanical-hydraulic
governor. The position of the flyweights is determined by the speed of the engine. As
the engine speeds up or down, the weights move in or out. The movement of the
flyweights, due to a change in engine speed, moves a small piston (pilot valve) in the
governor’s hydraulic system. This motion adjusts flow of hydraulic fluid to a large
hydraulic piston (servo-motor piston). The large hydraulic piston is linked to the fuel
rack and its motion resets the fuel rack for increased/decreased fuel.


Fig 28 simplified Mechanical-Hydraulic Governor

Detailed Operation of the Governor
With the engine operating, oil from the engine lubrication system is supplied to the
governor pump gears, as illustrated in Figure 29. The pump gears raise the oil pressure to a value determined by the spring relief valve. The oil pressure is maintained in the annular space between the undercut portion of the pilot valve plunger and the bore in the pilot valve bushing. For any given speed setting, the spring speeder exerts a force that is opposed by the centrifugal force of the revolving flyweights. When the two forces are equal, the control land on the pilot valve plunger covers the lower ports in the pilot valve bushing.


Fig 29 Cutway of Woodward Governor

Under these conditions, equal oil pressures are maintained on both sides of the buffer piston and tension on the two buffer springs is equal. Also, the oil pressure is equal on both sides of the receiving compensating land of the pilot valve plunger due to oil passing through the compensating needle valve. Thus, the hydraulic system is in balance, and the engine speed remains constant.

When the engine load increases, the engine starts to slow down in speed. The reduction in engine speed will be sensed by the governor flyweights. The flyweights are forced inward (by the spring), thus lowering the pilot valve plunger (again, due to the downward spring force). Oil under pressure will be admitted under the servo-motor piston (topside of the buffer piston) causing it to rise. This upward motion of the servo-motor piston will be transmitted through the terminal lever to the fuel racks, thus increasing the amount o f fuel injected into the engine. The oil that forces the servo-motor piston upward also forces the buffer piston upward because the oil pressure on each side of the piston is unequal.

This upward motion of the piston compresses the upper buffer spring and relieves the pressure on the lower buffer spring.

The oil cavities above and below the buffer piston are common to the receiving
compensating land on the pilot valve plunger. Because the higher pressure is below the compensating land, the pilot valve plunger is forced upward, recentering the flyweights and causing the control land of the pilot valve to close off the regulating port. Thus, the upward movement of the servo-motor piston stops when it has moved far enough to make the necessary fuel correction.

Oil passing through the compensating needle valve slowly equalizes the pressures above and below the buffer piston, thus allowing the buffer piston to return to the center position, which in turn equalizes the pressure above and below the receiving
compensating land. The pilot valve plunger then moves to its central position and the
engine speed returns to its original setting because there is no longer any excessive
outward force on the flyweights.

The action of the flyweights and the hydraulic feedback mechanism produces stable
engine operation by permitting the governor to move instantaneously in response to the load change and to make the necessary fuel adjustment to maintain the initial engine speed.

ERRORS

August 23, 2011

Errors in Measurement :

Error = Measured Value – True Value

E= V– Vt

1. Absolute Error :

01-errors in measurement-absolute error-greater accuracy and precision


            True absolute error :

= Result of measurement – True Value

            Apparent Absolute error :

= Result of measurement – Arithmetic Value

2. Relative error :


It is defined as the results of the absolute error and the value of comparison used for 450

calculation of that absolute error.

01-relative error-percentage relative error-absolute error

Causes of Errors :

1. Calibration Error:

These are caused due to the variation in the calibrated scale from it’s normal value.

01-error-calibration test-calibration of gauges-calibrate a instrument by a standard gauge-Panametrics-NDT_Calibration_Test_Blocks

2. Environmental Error :

These are caused due to humidity condition,Temperature and altitude.

3. Assembly Error:

i. Displaced Scale (incorrect Fitting)

ii. Non –uniform division of the scale.

iii. Due to bent /distorted pointer.

01-parallex error-eye fault defect by human-bias error

4. Random Error:

Naturally Occurred

No specific reasons

5. Systematic errors (or) Bias errors:

These are caused due to repeated readings.

01-errors-types of error-systematic error-random error-bias error

6. Chaotic errors :

These are caused due to vibrations,noises and shocks.

MEASUREMENT

August 23, 2011

Calibration:

01-the weighing scale-weighing machines-balance-calibration example

If a known input is given to the measurement system the output deviates from the given input, the corrections are made in the instrument and then the output is measured. This process is called “Calibration”.

Sensitivity:

Sensitivity is the ratio of change in the output signal to the change in the input signal.

Readability:

01-electroniccaliper-VERNIER CALIPER-DIGITAL VERNIER CALIPER-DIRECT MEASUREMENTS-ACCURATE-PRECISION MEASUREMENTS-CALIBRATED INSTRUMENTS-readability

Refers to the ease with which the readings of a measuring instrument can be read.

True size:

Theoretical size of a dimension which is free from errors.

Actual size:

Size obtained through measurement with permissible error.


01-true size-actual size-feet size-example-shoe-footwear

Hysteresis:

All the energy put into the stressed component when loaded is not recovered upon unloading. so the output of measurement partially depends on input called Hysteresis.

01-tachometer-digital tachometer-hysteresis due to pressure of force

Range:

The physical variables that are measured between two values. One is the higher calibration value Hc and the other is Lower value Lc.

01-range - read values from 0 to 11000 rpm - bezel meter - tachometer

Span:

The algebraic difference between higher calibration values to lower calibration values.

Resolution:

The minimum value of the input signal is required to cause an appreciable change in the output known as resolution.

Dead Zone:

It is the largest change in the physical variable to which the measuring instrument does not respond.

Threshold:

The minimum value of input signal that is required to make a change or start from zero.

01-threshold-minimum input given to start the engine-bike kick start action

Backlash:

The maximum distance through which one part of the instrument is moved without disturbing the other part.

01-backlash - continuous rotation possible without applying brake-SINGLE 3-PHASE AC ASYNCHRONOUS ELECTRIC MOTOR

Response Time:

The time at which the instrument begins its response for a change in the measured quantity.

Repeatability:

The ability of the measuring instrument to repeat the same results during the act measurements for the same quantity is known as repeatability.

Bias:

It is a characteristic of a measure or measuring instruments to give indications of the value of a measured quantity for which the average value differs from true value.

Magnification:

It means the magnitude of output signal of measuring instrument many times increases to make it more readable.

01-magnification-objective lens-magnify-loupe-ring

Drift:

If an instrument does not reproduce the same reading at different times of measurement for the same input signal, it is said to be measurement drift.

Reproducibility:

It is the consistency of pattern of variation in measurement. When individual measurements are carried out the closeness of the agreement between the results of measurements of the same quantity.

Uncertainty:

The range about the measured value within the true value of the measured quantity is likely to lie at the stated level of confidence.

Traceability:

It is nothing establishing a calibration by step by step comparison with better standards.

01-traceability-calibration step by step-vacuum calibration

Parallax:

An apparent change in the position of the index relative is to the scale marks.

 

 

01-parallax-error-measurement of length-eye view

MEASUREMENT

August 23, 2011

Calibration:

01-the weighing scale-weighing machines-balance-calibration example

If a known input is given to the measurement system the output deviates from the given input, the corrections are made in the instrument and then the output is measured. This process is called “Calibration”.

Sensitivity:

Sensitivity is the ratio of change in the output signal to the change in the input signal.

Readability:

01-electroniccaliper-VERNIER CALIPER-DIGITAL VERNIER CALIPER-DIRECT MEASUREMENTS-ACCURATE-PRECISION MEASUREMENTS-CALIBRATED INSTRUMENTS-readability

Refers to the ease with which the readings of a measuring instrument can be read.

True size:

Theoretical size of a dimension which is free from errors.

Actual size:

Size obtained through measurement with permissible error.


01-true size-actual size-feet size-example-shoe-footwear

Hysteresis:

All the energy put into the stressed component when loaded is not recovered upon unloading. so the output of measurement partially depends on input called Hysteresis.

01-tachometer-digital tachometer-hysteresis due to pressure of force

Range:

The physical variables that are measured between two values. One is the higher calibration value Hc and the other is Lower value Lc.

01-range - read values from 0 to 11000 rpm - bezel meter - tachometer

Span:

The algebraic difference between higher calibration values to lower calibration values.

Resolution:

The minimum value of the input signal is required to cause an appreciable change in the output known as resolution.

Dead Zone:

It is the largest change in the physical variable to which the measuring instrument does not respond.

Threshold:

The minimum value of input signal that is required to make a change or start from zero.

01-threshold-minimum input given to start the engine-bike kick start action

Backlash:

The maximum distance through which one part of the instrument is moved without disturbing the other part.

01-backlash - continuous rotation possible without applying brake-SINGLE 3-PHASE AC ASYNCHRONOUS ELECTRIC MOTOR

Response Time:

The time at which the instrument begins its response for a change in the measured quantity.

Repeatability:

The ability of the measuring instrument to repeat the same results during the act measurements for the same quantity is known as repeatability.

Bias:

It is a characteristic of a measure or measuring instruments to give indications of the value of a measured quantity for which the average value differs from true value.

Magnification:

It means the magnitude of output signal of measuring instrument many times increases to make it more readable.

01-magnification-objective lens-magnify-loupe-ring

Drift:

If an instrument does not reproduce the same reading at different times of measurement for the same input signal, it is said to be measurement drift.

Reproducibility:

It is the consistency of pattern of variation in measurement. When individual measurements are carried out the closeness of the agreement between the results of measurements of the same quantity.

Uncertainty:

The range about the measured value within the true value of the measured quantity is likely to lie at the stated level of confidence.

Traceability:

It is nothing establishing a calibration by step by step comparison with better standards.

01-traceability-calibration step by step-vacuum calibration

Parallax:

An apparent change in the position of the index relative is to the scale marks.

 

 

01-parallax-error-measurement of length-eye view

INDUCTIVE CHARGING

August 23, 2011

02-powermat-iphone-4-wireless-battery-charger-wireless charging mat-wireless receiver case-new wireless technology

In the future all electronic devices will be wirelessly powered. Small, battery-powered gadgets make powerful computing portable.

The battery charger should be capable of charging the most common battery types found in portable  devices today.  In addition, the charging  should be  controlled from the base station and a bidirectional communication system between  the pickups  and base  station  should be developed.


Inductive Power Systems:

Inductive Power Transfer (IPT)  refers to the concept of transferring electrical power between two isolated circuits across an air gap.  While based on the work and concepts developed by pioneers such as  Faraday and Ampere, it  is  only recently that IPT has been developed into working systems.

Essentially, an IPT system can be divided into two parts;

  • Primary and
  • Secondary.

The primary side of the system is made up of a resonant power supply and a coil. This power supply produces a high frequency sinusoidal current in the coil.  The secondary side (or ‘pickup’) has a smaller coil, and a converter to produce a DC voltage.

01-ecoupled wirelss charging technology-inductive coupling-keep battery life higher-concept-illustration

Working of Inductive Power Transfer:

In this system communications signals are encoded onto the waveform that provides power to the air gap. Communication from the primary side to the secondary is implemented by switching the power signal at the output of the resonant converter between its normal level  and a lower level which is detectable by the pickup but still provides enough power to control the pickup microcontroller. This process is called Amplitude Shift Keying (ASK). This is achieved by varying the output voltage of the buck converter which provides an input DC voltage to the resonant converter.

01-electric vehicles-charging-batteries-wireless charging of electric cars-Delphi_Witricity_Wireless

Communication from the secondary to the primary is achieved by a process called Load Shift Keying (LSK).  This involves varying the loading on the pickup.   Any load on the pickup will reflect a voltage on the primary circuit proportional to the load.  Therefore a variation in the load on the pickup can be detected by the charging station.

The communications system must provide two discrete levels of voltage reflected onto the primary side,  to represent the on and off states for digital communications. The difference must be easily detected on the primary side to provide a robust communications channel. Signals are decoded by simple filters and comparators which feed a  digital signal to the microcontrollers.

Advantages:

EV wireless charging parking 9-29

IPT has a number of advantages over other power transfer methods  – it is unaffected by dirt, dust, water, or chemicals.  In situations such as coal mining IPT prevents sparks and other hazards.  As the coupling is magnetic, there is no risk of electrocution even when used in high power systems.  This makes IPT very suitable for  transport  systems where vehicles follow a fixed track,  such as  in factory materials handling.

MAGNETIC BEARING TECHNOLOGY

August 22, 2011

01-Magnetic_Bearing-magnetic bearing technology-active non contact position sensors

Magnetic bearings have been utilized by a variety of industries for over a decade with benefits that include non-contact rotor support, no lubrication and no friction.

Conventional mechanical bearings, the kind that physically interface with the shaft and require some form of lubrication, can be replaced by a technology that suspends a rotor in a magnetic field, which eliminates friction losses.

01-floating rotors-magnetic bearing technologies-SKF compressor drive-advanced drive system

There are two types of magnetic bearing technologies in use today – passive and active.  Passive bearings are similar to mechanical bearings in that no active control is necessary for operation. In active systems, non-contact position sensors continually monitor shaft position and feed this information to a control system.  This in turn, based on the response commanded by the system, flows to the actuator via current amplifiers.  These currents are converted to magnetic forces by the actuator and act on the rotor to adjust position and provide damping.

Additional benefits of magnetic bearings include:

  • No friction
  • No lubrication
  • No oil contamination
  • Low energy consumption
  • Capacity to operate within a wide temperature range
  • No need for pumps, seals, filters, piping, coolers or tanks
  • Environmentally friendly workplace
  • Impressive cost savings

In practice, these attractions are balanced in order to maintain a gap between the shaft (rotor) and static parts (stator). The function of the magnetic bearing is to locate the shaft’s rotation axis in the center, reacting to any load variation (external disturbance forces),


01-typical examples for Floating rotors to run a heavy machineries-magnetic bearing systems to run shaft without friction

Floating rotors could boost compressor efficiencies

Traditional centrifugal compressors are based on low-speed drives, mechanical gears and oil-film bearings, resulting in high running costs because of their high losses, wear, and need for maintenance.

This new compressor drive (above) uses a permanent magnet motor, operating at an efficiency of around 97%, to drive a rotor “floating” on magnetic bearings, which spins the compressor impeller at speeds of around 60,000 rpm. These drives experience almost no friction or wear, and need little maintenance. They also minimize the risk of oil contamination, and result in compressors that are about half the size of traditional designs.


How they work

 

01-general-magnetic-principles-monitoring the air gap of shaft and bearings contact and position-position sensor-closed loop system-controlling of shafts in center position-position controller
Magnetic bearings are basically a system of bearings which provide non-contact operation, virtually eliminating friction from rotating mechanical systems. Magnetic bearing systems have several components. The mechanical components consist of the electromagnets, position sensors and the rotor. The electronics consist of a set of power amplifiers that supply current to electromagnets. A controller works with the position sensors which provide feedback to control the position of the rotor within the gap.

01-magnetic-5 axis shaft control-radial bearings-air gap- advanced bearing technologies

The position sensor registers a change in position of the shaft (rotor). This change in position is communicated back to the processor where the signal is processed and the controller decides what the necessary response should be, then initiates a response to the amplifier. This response should then increase the magnetic force in the corresponding electromagnet in order to bring the shaft back to center. In a typical system, the radial clearance can range from 0.5 to 1 mm.

This process repeats itself over and over again. For most applications, the sample rate is 10,000 times per second, or 10 kHz. The sample rate is high because the loop is inherently unstable. As the rotor gets closer to the magnet, the force increases. The system needs to continuously adjust the magnetic strength coming from the electromagnets in order to hold the rotor in the desired position.