Q1. What is the difference between an incremental and absolute rotary encoder?
Incremental Encoder – measure change in position.
Absolute Encoder – determine absolute position of an object
Q2. How is the PPR of the encoder selected?
The minimum ppr should produce more than 25000Hz at rated speed & less than 300000Hz at 1.2 times rated speed. So if Rated RPM = 1000
PPRmin = 25000x 60/RatedRPM=1500ppr
PPRmax=300000x60/RatedRPMx1.2=15000ppr
Q3. What is the principle of Hall effect current sensor? How is the hall effect sensor different from the rotary encoder ?
The Hall effect sensor is based on the principle that a voltage (VH) is created when current (Ic) flows in a direction perpendicular to a magnetic field (B). Hall effect sensor is used to sense feedback from DC brushless motors. Encoders are used to sense feedback from ac brushless motors. If used to commutate a brushless DC motor an optical encoder must also be enhanced with supplementary commutation tracks.
Q4. State some important sensor characteristics
Sensitivity : Change in output signals to input signals
Span : Range of input physical signals that may be converted to electrical signals
Accuracy: Largest expected error between actual & ideal output signals
Hysteresis: Width of the expected error in terms of measured quantity
Nonlinearity: The max deviation from a linear TF over the specified dynamic range.
Resolution : Max detectable signal fluctuation.
Q5. Name the ways in which EMI/EMC is generated in circuits.
EMI/EMC can occur due to
1. Switching On/Off of currents in a circuit. V = Ldi/dt
2. Magnetic Fields : In AC circuits, currents flowing in & out of the conductor causes EMI in nearby conductors.
3. By conduction through the wires that the electronics uses.
Q6. In what ways can we shield from EMI/EMC?
Shielding – By grounding the conducting plates to the earth.
Metal plates shield against magnetic field. Nonferromagnetic metals have no effect on magnetic fields below 10MHz in frequency. Metals like Aluminium are transparent to these fields. The material must be ferromagnetic, i.e. with low magnetic reluctance, so it conducts the magnetic field.
Snubber networks –These are combinations of resistors & capacitors connected across the source of electrical noise or to be protected from the spikes.
It reduces the voltage generated across the MOSFET
It reduces the rate of voltage change.
Suppression : It is the process of adding components that dampen noise
To dampen high freq noise, inductance is added in series. Ferrite beads can be used.
Q7. What is the use of emissivity & susceptibility tests?
The emissions test (CE &RE) record any undesireable emissions from the test article.
The susceptibility test (CS &RS) record the article’s ability to operate in a typical operating environment.
Q8. What are the coupling mechanisms for EMI/EMC
They are
Conduction – Electric current
Radiation – Electromagnetic field
Capacitive coupling – By an Electric field
Inductive coupling – By Magnetic field
Electric Field coupling is caused by a voltage difference between conductors. The coupling may be modelled by a capacitor.
Magnetic Field coupling is caused by current flow in conductors. The coupling mechanism may be modelled by a transformer.
Most conducted coupling from external sources occurs through ac power lines.
Q9. Explain what are Differential mode interference & Common mode interference
Common mode interference occurs between all lines in a cable & reference potential , It occurs at high frequencies of 1MHz & up.
Differential mode interference occurs between 2 lines (L-L & L-N). It occurs at low frequencies upto several KHz.
Q10. What are optical fibres ? How much is the attenuation in OF? What is splicing?
They are wave guiding devices used to confine & guide light. They are made of silica glass cores surrounded by a cladding which is protected by a jacket. They work on the principle of total internal reflection. The composition of the cladding glass relative to the core glass determines the fibre’s ability to reflect light. The refractive index of the core is increased by doping it. The attenuation of signal strength is 0.35dB/Km at 1300nm wavelength of light. An optical signal can travel more than 100Km without regeneration or amplification. Attenuation is caused by scattering & absorption. Splicing objective is to match the core of one optical fibre with that of another in order to produce a smooth junction through which light signals can continue without alteration & interruption.
Q11. What are the advantages of OF over wired cable?
1) It has greater bandwidth & capacity
2) It provides electrical isolation
3) It has low error rate
4) Greater immunity to external influences
5) Greater immunity to interference & crosstalk
It is instructive to compare fiber to copper. Fiber has many advantages. To start with, it can handle much higher bandwidths than copper. This alone would require its use in high-end networks. Due to the low attenuation, repeaters are needed only about every 50 km on long lines, versus about every 5 km for copper, a substantial cost saving. Fiber also has the advantage of not being affected by power surges, electromagnetic interference, or power failures. Nor is it affected by corrosive chemicals in the air, making it ideal for harsh factory
environments. Oddly enough, telephone companies like fiber for a different reason: it is thin and lightweight.
Q12. Name some Optical detectors & Optical Sources used in FOC systems
2 types of photodiodes are used as optical detectors
a) P-I-N Photodiode
b) Avalanche Photodiode
Heterojunction LED’s and LASER’s are mostly used as optical sources in FOC communication. Heterojunction means that a p-n junction is formed by a single crystal such that the material on one side of the junction differs from that on the other side of the junction.
Q13. What is the use of multiplexing & name the techniques?
The info carrying capacity of a fibre is increased by multiplexing. There are 3 types of multiplexing techniques:
TDM –Time Division Multiplexing
FDM – Frequency Division Multiplexing
WDM – Wave Division Multiplexing
Q14. Explain the principle of operation of an optocoupler.
It protects the controller from high voltage transients, surge voltage or noise. It consists of an LED and a phototransistor (or darlington pair) in a single 8 pin dual in line package. The LED can be made to emit light by passing forward current. The collector base of the transistor can be used as a photodiode & it produces output current by detecting light.
Q15. What are relays? What are Solid State Relays?
A relay consists of a coil of electromagnet with 2 states – close & open. The current & voltage are higher than a TTL gate can provide. Transistor buffers may be used to drive the relay coils. The problems of electromechanical relays are corrosion of contacts, arcing , contact bounce & slow speed of operation.
A SSR (Solid State Relay) consists of a SCR triggered by infrared light source. They are capable of controlling only ac operated devices like motors or heaters.
IRED (Infra red emitting diode) is used to isolate the control circuit & to trigger the SBS(Silicon Bilateral Switch). IRED is activated by current of 1 -10mA. SBS controls current as high as 40A.
Q16. Name some devices used for isolation & advantages & disadvantages of each.
They are used to isolate a circuit from high voltages & to break ground loops
Exp – Transformer, High voltage capacitors & Opto Isolators
Transformers have analog accuracy of 12 to 16 bits, bandwidth of several KHz and max voltage upto 10KV
Capacitively coupled isolation amplifiers have lower accuracy of 12 bits, lower band width & voltage ratings, are cheap.
Optical isolators are fast & cheap 4-7KV but have poor analog linearity.
Q17. What is meant by resolution & digit count , sensitivity , accuracy & precision of an instrument?
Resolution -- the smallest amount of input signal change that the instrument can detect reliably. This term is determined by the instrument noise (either circuit or quantization noise). For example, if you have a noiseless voltmeter that has 5 1/2 digits displayed and is set to the 20 V input range, the resolution of this voltmeter is 100 µV. This can be determined by looking at the change associated with the least significant digit.
Digits Displayed and Overranging -- the number of digits displayed by the readout of a DMM. It is often specified as a certain number of full digits (i.e. digits that can display values from 0 to 9) and an additional overrange digit referred to as a 1/2 digit. That 1/2 digit typically shows only the values 0 or 1. For example, a 6 1/2 digit display has a 7-digit readout, but the most significant digit can read 0 or 1 while the other 6 digits can take any value from 0 to 9. Hence, the range of counts is ±1,999,999. This should not be confused with resolution; a DMM can have many more digits displayed than its effective resolution.
• Sensitivity -- a measure of the smallest signal the instrument can measure. Usually, this is defined at the lowest range setting of the instrument. For example, an AC meter with a lowest measurement range of 10 V may be able to measure signals with 1 mV resolution but the smallest detectable voltage it can measure may be 15 mV. In this case, the AC meter has a resolution of 1 mV but a sensitivity of 15 mV.
• Accuracy -- a measure of the capability of the instrument to faithfully indicate the value of the measured signal. This term is not related to resolution; however, it can never be better than the resolution of the instrument. The accuracy is often specified as:
• Accuracy -- a measure of the capability of the instrument to faithfully indicate the value of the measured signal. This term is not related to resolution; however, it can never be better than the resolution of the instrument. The accuracy is often specified as:
For example, a 5 1/2 digit voltmeter can have an accuracy of 0.0125% of reading + 24 µV on its 2.5 V range which results in an error of 149 µV when measuring a 1V signal. On the other hand, the resolution of this same voltmeter is 12 µV, 12 times better than the accuracy. Keep in mind that the accuracy of your measurement is affected by several factors and we will discuss these factors later in this paper.
• Precision -- a measure of the stability of the instrument and its capability of resulting in the same measurement over and over again for the same input signal. It is given by:
where Xn = the value of the nth measurement
and Av(Xn) = the average value of the set of n measurement.
Incremental Encoder – measure change in position.
Absolute Encoder – determine absolute position of an object
Q2. How is the PPR of the encoder selected?
The minimum ppr should produce more than 25000Hz at rated speed & less than 300000Hz at 1.2 times rated speed. So if Rated RPM = 1000
PPRmin = 25000x 60/RatedRPM=1500ppr
PPRmax=300000x60/RatedRPMx1.2=15000ppr
Q3. What is the principle of Hall effect current sensor? How is the hall effect sensor different from the rotary encoder ?
The Hall effect sensor is based on the principle that a voltage (VH) is created when current (Ic) flows in a direction perpendicular to a magnetic field (B). Hall effect sensor is used to sense feedback from DC brushless motors. Encoders are used to sense feedback from ac brushless motors. If used to commutate a brushless DC motor an optical encoder must also be enhanced with supplementary commutation tracks.
Q4. State some important sensor characteristics
Sensitivity : Change in output signals to input signals
Span : Range of input physical signals that may be converted to electrical signals
Accuracy: Largest expected error between actual & ideal output signals
Hysteresis: Width of the expected error in terms of measured quantity
Nonlinearity: The max deviation from a linear TF over the specified dynamic range.
Resolution : Max detectable signal fluctuation.
Q5. Name the ways in which EMI/EMC is generated in circuits.
EMI/EMC can occur due to
1. Switching On/Off of currents in a circuit. V = Ldi/dt
2. Magnetic Fields : In AC circuits, currents flowing in & out of the conductor causes EMI in nearby conductors.
3. By conduction through the wires that the electronics uses.
Q6. In what ways can we shield from EMI/EMC?
Shielding – By grounding the conducting plates to the earth.
Metal plates shield against magnetic field. Nonferromagnetic metals have no effect on magnetic fields below 10MHz in frequency. Metals like Aluminium are transparent to these fields. The material must be ferromagnetic, i.e. with low magnetic reluctance, so it conducts the magnetic field.
Snubber networks –These are combinations of resistors & capacitors connected across the source of electrical noise or to be protected from the spikes.
It reduces the voltage generated across the MOSFET
It reduces the rate of voltage change.
Suppression : It is the process of adding components that dampen noise
To dampen high freq noise, inductance is added in series. Ferrite beads can be used.
Q7. What is the use of emissivity & susceptibility tests?
The emissions test (CE &RE) record any undesireable emissions from the test article.
The susceptibility test (CS &RS) record the article’s ability to operate in a typical operating environment.
Q8. What are the coupling mechanisms for EMI/EMC
They are
Conduction – Electric current
Radiation – Electromagnetic field
Capacitive coupling – By an Electric field
Inductive coupling – By Magnetic field
Electric Field coupling is caused by a voltage difference between conductors. The coupling may be modelled by a capacitor.
Magnetic Field coupling is caused by current flow in conductors. The coupling mechanism may be modelled by a transformer.
Most conducted coupling from external sources occurs through ac power lines.
Q9. Explain what are Differential mode interference & Common mode interference
Common mode interference occurs between all lines in a cable & reference potential , It occurs at high frequencies of 1MHz & up.
Differential mode interference occurs between 2 lines (L-L & L-N). It occurs at low frequencies upto several KHz.
Q10. What are optical fibres ? How much is the attenuation in OF? What is splicing?
They are wave guiding devices used to confine & guide light. They are made of silica glass cores surrounded by a cladding which is protected by a jacket. They work on the principle of total internal reflection. The composition of the cladding glass relative to the core glass determines the fibre’s ability to reflect light. The refractive index of the core is increased by doping it. The attenuation of signal strength is 0.35dB/Km at 1300nm wavelength of light. An optical signal can travel more than 100Km without regeneration or amplification. Attenuation is caused by scattering & absorption. Splicing objective is to match the core of one optical fibre with that of another in order to produce a smooth junction through which light signals can continue without alteration & interruption.
Q11. What are the advantages of OF over wired cable?
1) It has greater bandwidth & capacity
2) It provides electrical isolation
3) It has low error rate
4) Greater immunity to external influences
5) Greater immunity to interference & crosstalk
It is instructive to compare fiber to copper. Fiber has many advantages. To start with, it can handle much higher bandwidths than copper. This alone would require its use in high-end networks. Due to the low attenuation, repeaters are needed only about every 50 km on long lines, versus about every 5 km for copper, a substantial cost saving. Fiber also has the advantage of not being affected by power surges, electromagnetic interference, or power failures. Nor is it affected by corrosive chemicals in the air, making it ideal for harsh factory
environments. Oddly enough, telephone companies like fiber for a different reason: it is thin and lightweight.
Q12. Name some Optical detectors & Optical Sources used in FOC systems
2 types of photodiodes are used as optical detectors
a) P-I-N Photodiode
b) Avalanche Photodiode
Heterojunction LED’s and LASER’s are mostly used as optical sources in FOC communication. Heterojunction means that a p-n junction is formed by a single crystal such that the material on one side of the junction differs from that on the other side of the junction.
Q13. What is the use of multiplexing & name the techniques?
The info carrying capacity of a fibre is increased by multiplexing. There are 3 types of multiplexing techniques:
TDM –Time Division Multiplexing
FDM – Frequency Division Multiplexing
WDM – Wave Division Multiplexing
Q14. Explain the principle of operation of an optocoupler.
It protects the controller from high voltage transients, surge voltage or noise. It consists of an LED and a phototransistor (or darlington pair) in a single 8 pin dual in line package. The LED can be made to emit light by passing forward current. The collector base of the transistor can be used as a photodiode & it produces output current by detecting light.
Q15. What are relays? What are Solid State Relays?
A relay consists of a coil of electromagnet with 2 states – close & open. The current & voltage are higher than a TTL gate can provide. Transistor buffers may be used to drive the relay coils. The problems of electromechanical relays are corrosion of contacts, arcing , contact bounce & slow speed of operation.
A SSR (Solid State Relay) consists of a SCR triggered by infrared light source. They are capable of controlling only ac operated devices like motors or heaters.
IRED (Infra red emitting diode) is used to isolate the control circuit & to trigger the SBS(Silicon Bilateral Switch). IRED is activated by current of 1 -10mA. SBS controls current as high as 40A.
Q16. Name some devices used for isolation & advantages & disadvantages of each.
They are used to isolate a circuit from high voltages & to break ground loops
Exp – Transformer, High voltage capacitors & Opto Isolators
Transformers have analog accuracy of 12 to 16 bits, bandwidth of several KHz and max voltage upto 10KV
Capacitively coupled isolation amplifiers have lower accuracy of 12 bits, lower band width & voltage ratings, are cheap.
Optical isolators are fast & cheap 4-7KV but have poor analog linearity.
Q17. What is meant by resolution & digit count , sensitivity , accuracy & precision of an instrument?
Resolution -- the smallest amount of input signal change that the instrument can detect reliably. This term is determined by the instrument noise (either circuit or quantization noise). For example, if you have a noiseless voltmeter that has 5 1/2 digits displayed and is set to the 20 V input range, the resolution of this voltmeter is 100 µV. This can be determined by looking at the change associated with the least significant digit.
Digits Displayed and Overranging -- the number of digits displayed by the readout of a DMM. It is often specified as a certain number of full digits (i.e. digits that can display values from 0 to 9) and an additional overrange digit referred to as a 1/2 digit. That 1/2 digit typically shows only the values 0 or 1. For example, a 6 1/2 digit display has a 7-digit readout, but the most significant digit can read 0 or 1 while the other 6 digits can take any value from 0 to 9. Hence, the range of counts is ±1,999,999. This should not be confused with resolution; a DMM can have many more digits displayed than its effective resolution.
• Sensitivity -- a measure of the smallest signal the instrument can measure. Usually, this is defined at the lowest range setting of the instrument. For example, an AC meter with a lowest measurement range of 10 V may be able to measure signals with 1 mV resolution but the smallest detectable voltage it can measure may be 15 mV. In this case, the AC meter has a resolution of 1 mV but a sensitivity of 15 mV.
• Accuracy -- a measure of the capability of the instrument to faithfully indicate the value of the measured signal. This term is not related to resolution; however, it can never be better than the resolution of the instrument. The accuracy is often specified as:
• Accuracy -- a measure of the capability of the instrument to faithfully indicate the value of the measured signal. This term is not related to resolution; however, it can never be better than the resolution of the instrument. The accuracy is often specified as:
For example, a 5 1/2 digit voltmeter can have an accuracy of 0.0125% of reading + 24 µV on its 2.5 V range which results in an error of 149 µV when measuring a 1V signal. On the other hand, the resolution of this same voltmeter is 12 µV, 12 times better than the accuracy. Keep in mind that the accuracy of your measurement is affected by several factors and we will discuss these factors later in this paper.
• Precision -- a measure of the stability of the instrument and its capability of resulting in the same measurement over and over again for the same input signal. It is given by:
where Xn = the value of the nth measurement
and Av(Xn) = the average value of the set of n measurement.
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