SERVO VALVES - Frequently Asked Questions (FAQ''s) Notes | EduRev

: SERVO VALVES - Frequently Asked Questions (FAQ''s) Notes | EduRev

 Page 1


LECTURE 27 – SERVO VALVES 
FREQUENTLY ASKED QUESTIONS 
 
1. Define a servo valve 
Servo valve is a programmable orifice. Servo valve is an automatic device for controlling large 
amount of power by means of very small amount of power and automatically and continuously 
correcting the performance of a mechanism 
 
2. How do servo valves differ from proportional control valves 
A comparison of proportional valves and servo valves is given in the Table . Proportional valves 
are less accurate, but they cost less. They are not used for closed loops control, there is no 
feedback of the output to change the setting of the valve. 
 
Parameter 
 
Proportional Hydraulic valve 
 
Electro hydraulic servo valve 
 
Valve lap 
 
 
 
Overlap spool, causing a „dead 
zone? On either side of the null 
position  
 
Zero or underlap valve spool. 
No dead zone 
 
 
Response time for valve 
spool to move fully over 
 
 
40-60 ms 
 
 
 
 
5-10 ms 
 
 
 
 
Maximum operating 
frequency 
 
 
Approx. 10 Hz 
 
 
 
Approx. 100 Hz 
 
 
 
Hysteresis 
Without armature feedback 
approx. 5% 
With armature position feedback 
approx. 1% 
Approx. 0.1% 
 
3. Explain the operation of torque motor 
A torque motor is illustrated in Figure1given below. Torque motor is a simple electromagnetic 
device consisting of one or two permanent magnets, two pole pieces, a ferromagnetic armature, 
and two coils. The permanent magnet polarize the upper and lower pole pieces, so that they 
Page 2


LECTURE 27 – SERVO VALVES 
FREQUENTLY ASKED QUESTIONS 
 
1. Define a servo valve 
Servo valve is a programmable orifice. Servo valve is an automatic device for controlling large 
amount of power by means of very small amount of power and automatically and continuously 
correcting the performance of a mechanism 
 
2. How do servo valves differ from proportional control valves 
A comparison of proportional valves and servo valves is given in the Table . Proportional valves 
are less accurate, but they cost less. They are not used for closed loops control, there is no 
feedback of the output to change the setting of the valve. 
 
Parameter 
 
Proportional Hydraulic valve 
 
Electro hydraulic servo valve 
 
Valve lap 
 
 
 
Overlap spool, causing a „dead 
zone? On either side of the null 
position  
 
Zero or underlap valve spool. 
No dead zone 
 
 
Response time for valve 
spool to move fully over 
 
 
40-60 ms 
 
 
 
 
5-10 ms 
 
 
 
 
Maximum operating 
frequency 
 
 
Approx. 10 Hz 
 
 
 
Approx. 100 Hz 
 
 
 
Hysteresis 
Without armature feedback 
approx. 5% 
With armature position feedback 
approx. 1% 
Approx. 0.1% 
 
3. Explain the operation of torque motor 
A torque motor is illustrated in Figure1given below. Torque motor is a simple electromagnetic 
device consisting of one or two permanent magnets, two pole pieces, a ferromagnetic armature, 
and two coils. The permanent magnet polarize the upper and lower pole pieces, so that they 
present equal and opposite magnetic fields. Torque motors are very low power devices operated 
on low-voltage DC power. The armature is mounted at its midpoint so that it is free to rotate 
through a very limited arc either clockwise or counterclockwise. The ends of the armature are 
extended into the gaps between the pole pieces. The magnetic field holds the armature in a 
neutral position. The two coils surround the arms of the armature to form two small 
electromagnets. 
 
 
 
When a current is passed through the coils, a magnetic field is generated. The polarity of the 
field depends on the direction of the current flow. In Figure 2 the current flow has caused the left 
end to become the South Pole and right end to become the North Pole, resulting in counter 
clockwise rotation of the armature.  
Page 3


LECTURE 27 – SERVO VALVES 
FREQUENTLY ASKED QUESTIONS 
 
1. Define a servo valve 
Servo valve is a programmable orifice. Servo valve is an automatic device for controlling large 
amount of power by means of very small amount of power and automatically and continuously 
correcting the performance of a mechanism 
 
2. How do servo valves differ from proportional control valves 
A comparison of proportional valves and servo valves is given in the Table . Proportional valves 
are less accurate, but they cost less. They are not used for closed loops control, there is no 
feedback of the output to change the setting of the valve. 
 
Parameter 
 
Proportional Hydraulic valve 
 
Electro hydraulic servo valve 
 
Valve lap 
 
 
 
Overlap spool, causing a „dead 
zone? On either side of the null 
position  
 
Zero or underlap valve spool. 
No dead zone 
 
 
Response time for valve 
spool to move fully over 
 
 
40-60 ms 
 
 
 
 
5-10 ms 
 
 
 
 
Maximum operating 
frequency 
 
 
Approx. 10 Hz 
 
 
 
Approx. 100 Hz 
 
 
 
Hysteresis 
Without armature feedback 
approx. 5% 
With armature position feedback 
approx. 1% 
Approx. 0.1% 
 
3. Explain the operation of torque motor 
A torque motor is illustrated in Figure1given below. Torque motor is a simple electromagnetic 
device consisting of one or two permanent magnets, two pole pieces, a ferromagnetic armature, 
and two coils. The permanent magnet polarize the upper and lower pole pieces, so that they 
present equal and opposite magnetic fields. Torque motors are very low power devices operated 
on low-voltage DC power. The armature is mounted at its midpoint so that it is free to rotate 
through a very limited arc either clockwise or counterclockwise. The ends of the armature are 
extended into the gaps between the pole pieces. The magnetic field holds the armature in a 
neutral position. The two coils surround the arms of the armature to form two small 
electromagnets. 
 
 
 
When a current is passed through the coils, a magnetic field is generated. The polarity of the 
field depends on the direction of the current flow. In Figure 2 the current flow has caused the left 
end to become the South Pole and right end to become the North Pole, resulting in counter 
clockwise rotation of the armature.  
 
4. Define underlap, overlap, and line to line in the context  of servo valve spools. 
The most common condition is the line-to-line (or zero overlap) spool. Here , the bandwidth 
exactly matches the port width.   Overlapped spool have lands that are 0.5 to 5% wider than the 
ports.  An underlaped spool has lands that are 0.5 to 1.5% narrower than the ports. This design is 
often referred to as “open center” although there really are no open-center servo valves.  
 
 
  
 
5. Define deadband 
It is defined as the current required to move the spool from the exact centered position to the 
position where the first flow output is seen. It is usually expressed in milliamps or percent rated 
current. Deadband is the result of the spool inertia,  overlap, static friction, and any other forces 
that might impede the initial motion. 
Page 4


LECTURE 27 – SERVO VALVES 
FREQUENTLY ASKED QUESTIONS 
 
1. Define a servo valve 
Servo valve is a programmable orifice. Servo valve is an automatic device for controlling large 
amount of power by means of very small amount of power and automatically and continuously 
correcting the performance of a mechanism 
 
2. How do servo valves differ from proportional control valves 
A comparison of proportional valves and servo valves is given in the Table . Proportional valves 
are less accurate, but they cost less. They are not used for closed loops control, there is no 
feedback of the output to change the setting of the valve. 
 
Parameter 
 
Proportional Hydraulic valve 
 
Electro hydraulic servo valve 
 
Valve lap 
 
 
 
Overlap spool, causing a „dead 
zone? On either side of the null 
position  
 
Zero or underlap valve spool. 
No dead zone 
 
 
Response time for valve 
spool to move fully over 
 
 
40-60 ms 
 
 
 
 
5-10 ms 
 
 
 
 
Maximum operating 
frequency 
 
 
Approx. 10 Hz 
 
 
 
Approx. 100 Hz 
 
 
 
Hysteresis 
Without armature feedback 
approx. 5% 
With armature position feedback 
approx. 1% 
Approx. 0.1% 
 
3. Explain the operation of torque motor 
A torque motor is illustrated in Figure1given below. Torque motor is a simple electromagnetic 
device consisting of one or two permanent magnets, two pole pieces, a ferromagnetic armature, 
and two coils. The permanent magnet polarize the upper and lower pole pieces, so that they 
present equal and opposite magnetic fields. Torque motors are very low power devices operated 
on low-voltage DC power. The armature is mounted at its midpoint so that it is free to rotate 
through a very limited arc either clockwise or counterclockwise. The ends of the armature are 
extended into the gaps between the pole pieces. The magnetic field holds the armature in a 
neutral position. The two coils surround the arms of the armature to form two small 
electromagnets. 
 
 
 
When a current is passed through the coils, a magnetic field is generated. The polarity of the 
field depends on the direction of the current flow. In Figure 2 the current flow has caused the left 
end to become the South Pole and right end to become the North Pole, resulting in counter 
clockwise rotation of the armature.  
 
4. Define underlap, overlap, and line to line in the context  of servo valve spools. 
The most common condition is the line-to-line (or zero overlap) spool. Here , the bandwidth 
exactly matches the port width.   Overlapped spool have lands that are 0.5 to 5% wider than the 
ports.  An underlaped spool has lands that are 0.5 to 1.5% narrower than the ports. This design is 
often referred to as “open center” although there really are no open-center servo valves.  
 
 
  
 
5. Define deadband 
It is defined as the current required to move the spool from the exact centered position to the 
position where the first flow output is seen. It is usually expressed in milliamps or percent rated 
current. Deadband is the result of the spool inertia,  overlap, static friction, and any other forces 
that might impede the initial motion. 
 
 
 
 
 
 
6. Define threshold 
Threshold is the current that must be applied before a response is detected. Good quality two 
stage valves have a threshold less than 0.5% of the rated current. 
 
7. Define hysteresis 
In an ideal world, a unit  of current input would produce a unit of valve output. This is not 
possible to achieve practically.  Hysteresis is expressed as the percent difference in the rated 
current required to give the same output when approached from higher and lower inputs. For 
servo valves it is typically 1 to 2%. To overcome the problem of hysteresis, some controllers are 
designed so that the set point is always approached from the lower side. This requires a 
deliberate undershoot when approaching from the high side 
 
8. List and define the types of hydraulic amplifiers 
Figure below shows a single stage servo valve. The mechanical connection between the torque 
motor armature and the spool is a stiff wire. When there is no command input to the torque 
motor, the armature is in the neutral (nulled) position, which, in turn, causes the spool to be in 
the nulled position, and there is no flow through the valve. A clockwise deflection of the 
armature pushes the spool to the left, opening up flow path from P to B and A to T. a 
counterclockwise deflection opens P to A and B to T. 
+1  -1  
Deadband 
+ 
Flo
w 
-  
Flo
w 
Page 5


LECTURE 27 – SERVO VALVES 
FREQUENTLY ASKED QUESTIONS 
 
1. Define a servo valve 
Servo valve is a programmable orifice. Servo valve is an automatic device for controlling large 
amount of power by means of very small amount of power and automatically and continuously 
correcting the performance of a mechanism 
 
2. How do servo valves differ from proportional control valves 
A comparison of proportional valves and servo valves is given in the Table . Proportional valves 
are less accurate, but they cost less. They are not used for closed loops control, there is no 
feedback of the output to change the setting of the valve. 
 
Parameter 
 
Proportional Hydraulic valve 
 
Electro hydraulic servo valve 
 
Valve lap 
 
 
 
Overlap spool, causing a „dead 
zone? On either side of the null 
position  
 
Zero or underlap valve spool. 
No dead zone 
 
 
Response time for valve 
spool to move fully over 
 
 
40-60 ms 
 
 
 
 
5-10 ms 
 
 
 
 
Maximum operating 
frequency 
 
 
Approx. 10 Hz 
 
 
 
Approx. 100 Hz 
 
 
 
Hysteresis 
Without armature feedback 
approx. 5% 
With armature position feedback 
approx. 1% 
Approx. 0.1% 
 
3. Explain the operation of torque motor 
A torque motor is illustrated in Figure1given below. Torque motor is a simple electromagnetic 
device consisting of one or two permanent magnets, two pole pieces, a ferromagnetic armature, 
and two coils. The permanent magnet polarize the upper and lower pole pieces, so that they 
present equal and opposite magnetic fields. Torque motors are very low power devices operated 
on low-voltage DC power. The armature is mounted at its midpoint so that it is free to rotate 
through a very limited arc either clockwise or counterclockwise. The ends of the armature are 
extended into the gaps between the pole pieces. The magnetic field holds the armature in a 
neutral position. The two coils surround the arms of the armature to form two small 
electromagnets. 
 
 
 
When a current is passed through the coils, a magnetic field is generated. The polarity of the 
field depends on the direction of the current flow. In Figure 2 the current flow has caused the left 
end to become the South Pole and right end to become the North Pole, resulting in counter 
clockwise rotation of the armature.  
 
4. Define underlap, overlap, and line to line in the context  of servo valve spools. 
The most common condition is the line-to-line (or zero overlap) spool. Here , the bandwidth 
exactly matches the port width.   Overlapped spool have lands that are 0.5 to 5% wider than the 
ports.  An underlaped spool has lands that are 0.5 to 1.5% narrower than the ports. This design is 
often referred to as “open center” although there really are no open-center servo valves.  
 
 
  
 
5. Define deadband 
It is defined as the current required to move the spool from the exact centered position to the 
position where the first flow output is seen. It is usually expressed in milliamps or percent rated 
current. Deadband is the result of the spool inertia,  overlap, static friction, and any other forces 
that might impede the initial motion. 
 
 
 
 
 
 
6. Define threshold 
Threshold is the current that must be applied before a response is detected. Good quality two 
stage valves have a threshold less than 0.5% of the rated current. 
 
7. Define hysteresis 
In an ideal world, a unit  of current input would produce a unit of valve output. This is not 
possible to achieve practically.  Hysteresis is expressed as the percent difference in the rated 
current required to give the same output when approached from higher and lower inputs. For 
servo valves it is typically 1 to 2%. To overcome the problem of hysteresis, some controllers are 
designed so that the set point is always approached from the lower side. This requires a 
deliberate undershoot when approaching from the high side 
 
8. List and define the types of hydraulic amplifiers 
Figure below shows a single stage servo valve. The mechanical connection between the torque 
motor armature and the spool is a stiff wire. When there is no command input to the torque 
motor, the armature is in the neutral (nulled) position, which, in turn, causes the spool to be in 
the nulled position, and there is no flow through the valve. A clockwise deflection of the 
armature pushes the spool to the left, opening up flow path from P to B and A to T. a 
counterclockwise deflection opens P to A and B to T. 
+1  -1  
Deadband 
+ 
Flo
w 
-  
Flo
w 
 
Single stage spool type servo 
For higher flow rates, two-or even three-stage valve must be used. In this valves second and third 
stages are always sliding spools that are pilot operated from the previous stages. The first stage 
may use the sliding spool, but there are other designs, also.  
9. Servo valves are usually rated at what pressure drop 
Servo valves are generally considered to be high pressure devices. They are pressure rated at 
3000 PSI. , although most are capable of operating at 5000 PSI or even higher. 
10. Define gain. 
Gain is defined as = output/input. Two gains are defined for servo valves. Flow gain and pressure 
gain.  
11. Define Flow gain 
It  is defined as flow /input current. Flow gain is determined by measuring control flow versus 
input current. Flow gain in the null region may range from 50 % to 200 % of the expected gain 
based on the slope of the linear line.  
 
12. Define pressure gain. 
Pressure gain is defined as pressure/input current.  
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