crelesy
Newbie
I stubeled upon this when i was searching for some info on putting in a stronger servo on my LNC with stock elektronics.
I learned a lot from reading this..
Its quite long and based on rc hely's but still very useful I hope!
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Digital Servos and BEC's
This is an extremely important post, and I urge everyone to read it. It is quite long, but it contains VITAL information concerning motors, speed controller and servos
We have been having an above average rate of failures of ESC's lately, and as a result, I have taken a great deal of time investigating this issue to discover the root cause. From what I have been reading in other posts in other threads and on other sites, this is not an isolated incident, and is affecting all brands of speed controllers. These failures are occuring about 98% of the time in helicopters as compared to airplanes, so obviously, there is something inherent to helicopters that is the cause.
The conclusion that I have come up with is that the true cause of the ESC failures is the servos that are being used in models these days. With the advent of the newer Digital Servos, and the current availablity of these servos are reasonable prices, just about everyone has switched to them.
a bit of education is necessary so here it is. I will attempt to keep it as simple as I can, but there are some electronics terms that have to be used, so try to follow along as best you can.
For those of you that do not know how a servo actually works, here is a crash course. A servo consists of a motor, a set of gears that reduce the speed and increase the torque of the motor output, a feedback potentiometer, a feedback amplifier circuit and a drive circuit.
A servo receives a pulse from the radio receiver that tells the servo what position it should move to. In a typical radio system, the pulse has a width that varies from 1.0 milliseconds at one extreme to 2.0 milliseconds at the other extreme, with 1.5 milliseconds considered to be the center point.
The feedback potentiometer in the servo provide a variable resistance that is converted to a varying pulse signal inside the feedback amplifier. The feedback amplifier then compares the width of this signal to the one that is coming in from the radio receiver. If the width of the two pulses are the same, then the servo sits still at that position.
If you move the control stick a bit, the width of the pulse coming from the radio receiver will change and the feedback amplifier will now sense a difference between the two signals. The feedback amplifier will then send out a signal to the servo's drive circuit, and this causes the motor to spin in the proper direction to match the new signal input. As the motor turns, it spins the gears in the servo. These gears eventually attach to the output arm of the servo and to the feedback potentiometer. As the output arm turns the potentiometer, the resistance value changes until a point is reached where it matches the position of the control stick and the servo stops at the new position.
This process repeats itself over and over again, hundreds of times per minute as we fly our models around, constantly matching the servo outposition to match the control inpuuts that we give at the transmitter. Now that we know how the system works, we can take a look at the difference between the older analog servos versus the newer digital servos.
In analog servos, the transistors used in the driver circuit were normaly traditional NPN and PNP bi-polar transistors. When these servos are set up in an amplifier circuit, there is a small range of operation on either side of neutral where the servos operate in a linear mode. What this means is that if you move your stick a tiny bit, the servo would react slowly at a lower power level. This would pull less current that normal, and the servo would move a little slower than normal. However, if you made a large stick movement, the servo would quickly ramp up to full power and full speed and move to the new position.
Since we are talking about current, I want to clarify a few things here about the different types of servo current. There are basically 3 different current levels you need to wory about. First is the Idle current. This is how much current the servo pulls when it is sitting still doing no work. In most cases, this value is very small, somewhere in the 5mA to 20mA range, which is very negligible.
The second current is the Working Current. This is how much current the servo pulls when it is in the process of moving from one position to another, with normal flight loads applied to the output arm. Depending on the size of the servo, and the applied load, this value can range from around 200mA up to 1 amp or more.
The last current is the Stall Current. This is how much current the servo draws if you hold the output arm from moving and apply a command to make the servo move. It is called Stall Current because the motor is stalled and cannot move. In this condition, the motor acts almost like a dead short, and pulls a lot of current. Again, depending on the size of the servo, and primarily the size and quality of the motor in the servo, this value can be anywhere from 500mA to 2 amps or more.
Another current value that has become very important is the Start Current of the servos. When a servo is sitting still at a fixed position, it only pulls the Idle Current. However, whenever a control signal is given, the motor has to go from a dead stop and accelerate to full speed. At the instant that the control signal is given, the motor is not spinning, so for a very brief period of time, the motor draws the stall current, and then as the motor starts turning, this current level drops down to the Working Current value of the motor.
With the earlier analog type servos, this start-up was softened somewhat because of the slight linear region of the transistors, so it never really got up to the short circuit current. However, with the newer Digital Servos, this is not the case.
The new digital servos use FET type transistors in the drive circuit, and these have almost no linear range around neutral. They also sent command signals to the motor much more quickly that the analog servos do, so the respond much more quickly. This change is what makes Digital Servos so popular with helicopter pilots. If you move the stick the smallest amount, the servo instantly reacts with full power to provide the desired control input. Helicopter pilots see this as a God-send, and use this power to perform amazing stunts with their helicopters.
The bad news is that this speed and responsiveness does not come without a very high cost. Unfortunately, very few pilots are aware of this, and it is this fact that has been the root cause of speed controller failures all over the world. ( I am sure you were al wondering when I was going to get back to the speed controllers. )
Because of the insanely fast response of the new Digital Servos, and the fact that they instantly go to full power every time you move the stick, they pull HUGE amounts of current every time they move. The new digital servos basically pull the full stall current of the servo every single time you make any control movement on the sticks. Due to the fact that almost all of the helicopters made today use CCPM mixing, there are 3 servos attached directly to the swashplate.
Any time you make a collective pitch change, all 3 servos move together in unison, starting and stopping at exactly the same time. This means that every single time you move the collective stick, you are hitting full stall current on all three cyclic servos for a brief period of time. As I have said earlier, these new digital often pull 2 amps of current or more in a stall, so when you multiply that by 3 servos, you are pulling current spikes that are 6 amps or more every time the colective stick is moved.
As you know, any time you make a collective change, the torque from the head changes, and the gyro compensates with a rudder input to the tail rotor. This servo will also react, adding to the current. When you start adding all of this up, you can quickly see how the BEC circuit is getting constantly hammered with HUGE current surges.
Most of the on-board BEC circuits are rated for around 3 amps with a 4 amp surge. For a 400 or 450 size machine with 325mm blades, this is usually sufficient, even with the smaller digital servos. However, when you start getting into larger machines such as the Logo 400, Trex 500, and others with 400mm or larger blades, the current levels from the servos can quickly out-strip the ability of the BEC circuit to provide the required current without over-heating.
When the BEC circuit gets overloaded, they either go into an over-current or over temperature protection mode and shut down for a while, or just burn out all together. If you lose the BEC voltage, the microprocessor in the ESC can no longer function, and whatever phase was turned on in the ESC when the power goes out usually stays stuck on. This pulls full short circuit from the battery, through the ESC ind into the motor. This current can be several hundred amps for a brief period of time, depending on the Rm value of the motor. Normally, the windings of the motor take several seconds to heat up and start to burn in this condition, but the FET transistors in the speed controller cannot handle that much current, so within about 2 seconds they start blowing out.
If you are lucky, the ESC burns open quickly and removes the load from the battery and motor and they survive the incident. In some cases though, the ESC welds shut from the current and takes out the motor and sometimes the battery as well.
The really sad thing is that the ESC itself is not at fault in this kind of failure. The complete fault for the incident lies in the current draw of the servos that exceeds the design specifications of the BEC. The worst part about it is that virtually none of the servo manufacturers out there give the full current specs for their servos, and some of them give absolutely no current specs at all. This places the blame for a huge number of speed controller failures squarely in the laps of the servo manufacturers.
I learned a lot from reading this..
Its quite long and based on rc hely's but still very useful I hope!
--------------------------------------------------------------------------
Digital Servos and BEC's
This is an extremely important post, and I urge everyone to read it. It is quite long, but it contains VITAL information concerning motors, speed controller and servos
We have been having an above average rate of failures of ESC's lately, and as a result, I have taken a great deal of time investigating this issue to discover the root cause. From what I have been reading in other posts in other threads and on other sites, this is not an isolated incident, and is affecting all brands of speed controllers. These failures are occuring about 98% of the time in helicopters as compared to airplanes, so obviously, there is something inherent to helicopters that is the cause.
The conclusion that I have come up with is that the true cause of the ESC failures is the servos that are being used in models these days. With the advent of the newer Digital Servos, and the current availablity of these servos are reasonable prices, just about everyone has switched to them.
a bit of education is necessary so here it is. I will attempt to keep it as simple as I can, but there are some electronics terms that have to be used, so try to follow along as best you can.
For those of you that do not know how a servo actually works, here is a crash course. A servo consists of a motor, a set of gears that reduce the speed and increase the torque of the motor output, a feedback potentiometer, a feedback amplifier circuit and a drive circuit.
A servo receives a pulse from the radio receiver that tells the servo what position it should move to. In a typical radio system, the pulse has a width that varies from 1.0 milliseconds at one extreme to 2.0 milliseconds at the other extreme, with 1.5 milliseconds considered to be the center point.
The feedback potentiometer in the servo provide a variable resistance that is converted to a varying pulse signal inside the feedback amplifier. The feedback amplifier then compares the width of this signal to the one that is coming in from the radio receiver. If the width of the two pulses are the same, then the servo sits still at that position.
If you move the control stick a bit, the width of the pulse coming from the radio receiver will change and the feedback amplifier will now sense a difference between the two signals. The feedback amplifier will then send out a signal to the servo's drive circuit, and this causes the motor to spin in the proper direction to match the new signal input. As the motor turns, it spins the gears in the servo. These gears eventually attach to the output arm of the servo and to the feedback potentiometer. As the output arm turns the potentiometer, the resistance value changes until a point is reached where it matches the position of the control stick and the servo stops at the new position.
This process repeats itself over and over again, hundreds of times per minute as we fly our models around, constantly matching the servo outposition to match the control inpuuts that we give at the transmitter. Now that we know how the system works, we can take a look at the difference between the older analog servos versus the newer digital servos.
In analog servos, the transistors used in the driver circuit were normaly traditional NPN and PNP bi-polar transistors. When these servos are set up in an amplifier circuit, there is a small range of operation on either side of neutral where the servos operate in a linear mode. What this means is that if you move your stick a tiny bit, the servo would react slowly at a lower power level. This would pull less current that normal, and the servo would move a little slower than normal. However, if you made a large stick movement, the servo would quickly ramp up to full power and full speed and move to the new position.
Since we are talking about current, I want to clarify a few things here about the different types of servo current. There are basically 3 different current levels you need to wory about. First is the Idle current. This is how much current the servo pulls when it is sitting still doing no work. In most cases, this value is very small, somewhere in the 5mA to 20mA range, which is very negligible.
The second current is the Working Current. This is how much current the servo pulls when it is in the process of moving from one position to another, with normal flight loads applied to the output arm. Depending on the size of the servo, and the applied load, this value can range from around 200mA up to 1 amp or more.
The last current is the Stall Current. This is how much current the servo draws if you hold the output arm from moving and apply a command to make the servo move. It is called Stall Current because the motor is stalled and cannot move. In this condition, the motor acts almost like a dead short, and pulls a lot of current. Again, depending on the size of the servo, and primarily the size and quality of the motor in the servo, this value can be anywhere from 500mA to 2 amps or more.
Another current value that has become very important is the Start Current of the servos. When a servo is sitting still at a fixed position, it only pulls the Idle Current. However, whenever a control signal is given, the motor has to go from a dead stop and accelerate to full speed. At the instant that the control signal is given, the motor is not spinning, so for a very brief period of time, the motor draws the stall current, and then as the motor starts turning, this current level drops down to the Working Current value of the motor.
With the earlier analog type servos, this start-up was softened somewhat because of the slight linear region of the transistors, so it never really got up to the short circuit current. However, with the newer Digital Servos, this is not the case.
The new digital servos use FET type transistors in the drive circuit, and these have almost no linear range around neutral. They also sent command signals to the motor much more quickly that the analog servos do, so the respond much more quickly. This change is what makes Digital Servos so popular with helicopter pilots. If you move the stick the smallest amount, the servo instantly reacts with full power to provide the desired control input. Helicopter pilots see this as a God-send, and use this power to perform amazing stunts with their helicopters.
The bad news is that this speed and responsiveness does not come without a very high cost. Unfortunately, very few pilots are aware of this, and it is this fact that has been the root cause of speed controller failures all over the world. ( I am sure you were al wondering when I was going to get back to the speed controllers. )
Because of the insanely fast response of the new Digital Servos, and the fact that they instantly go to full power every time you move the stick, they pull HUGE amounts of current every time they move. The new digital servos basically pull the full stall current of the servo every single time you make any control movement on the sticks. Due to the fact that almost all of the helicopters made today use CCPM mixing, there are 3 servos attached directly to the swashplate.
Any time you make a collective pitch change, all 3 servos move together in unison, starting and stopping at exactly the same time. This means that every single time you move the collective stick, you are hitting full stall current on all three cyclic servos for a brief period of time. As I have said earlier, these new digital often pull 2 amps of current or more in a stall, so when you multiply that by 3 servos, you are pulling current spikes that are 6 amps or more every time the colective stick is moved.
As you know, any time you make a collective change, the torque from the head changes, and the gyro compensates with a rudder input to the tail rotor. This servo will also react, adding to the current. When you start adding all of this up, you can quickly see how the BEC circuit is getting constantly hammered with HUGE current surges.
Most of the on-board BEC circuits are rated for around 3 amps with a 4 amp surge. For a 400 or 450 size machine with 325mm blades, this is usually sufficient, even with the smaller digital servos. However, when you start getting into larger machines such as the Logo 400, Trex 500, and others with 400mm or larger blades, the current levels from the servos can quickly out-strip the ability of the BEC circuit to provide the required current without over-heating.
When the BEC circuit gets overloaded, they either go into an over-current or over temperature protection mode and shut down for a while, or just burn out all together. If you lose the BEC voltage, the microprocessor in the ESC can no longer function, and whatever phase was turned on in the ESC when the power goes out usually stays stuck on. This pulls full short circuit from the battery, through the ESC ind into the motor. This current can be several hundred amps for a brief period of time, depending on the Rm value of the motor. Normally, the windings of the motor take several seconds to heat up and start to burn in this condition, but the FET transistors in the speed controller cannot handle that much current, so within about 2 seconds they start blowing out.
If you are lucky, the ESC burns open quickly and removes the load from the battery and motor and they survive the incident. In some cases though, the ESC welds shut from the current and takes out the motor and sometimes the battery as well.
The really sad thing is that the ESC itself is not at fault in this kind of failure. The complete fault for the incident lies in the current draw of the servos that exceeds the design specifications of the BEC. The worst part about it is that virtually none of the servo manufacturers out there give the full current specs for their servos, and some of them give absolutely no current specs at all. This places the blame for a huge number of speed controller failures squarely in the laps of the servo manufacturers.
(crazy binds etc)
