Servo Brownout and Solution
03-12-2012, 06:07 PM
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Servo Brownout and Solution
Recently I have been working on my FPV Raptor from Hobby King. I am planning to install a flight stabilization system and one of my friends advised me that I should upgrade all servos with metal gear servos, due to the control surfaces will be very busily adjusted by the automated system. So I installed 4 metal gear servos from Hobby King (HK939MG). After I hooked up my receiver and all servos, everything seems working fine. However, when I moved the aileron stick a bit faster, suddenly all servos moved simultaneously like crazy for a few seconds then quiet down. I thought there was something wrong with my receiver which might be not compatible with those metal gear servos.
I consulted a friend of mine and he suggested it might be the ESC issue. My BEC might be too wimpy for heavy current draw by those metal gear servos. I do not have the spec of the ESC and it only has a small label of "30A". The spec of those servos states that each one will draw 240mA and it will be under 1A in total. Most of the 25A+ ESC with BEC should provide at least 2A capacity. To further identify the problem, I disconnect one of the aileron servos and everything went normally. I replaced with two 9 gram nylon gear servos and it was OK as well. Furthermore, I tried to disconnect the BEC and use a pack of battery from my nitro plane. It behaved well too. Now I have my conclusion. The BEC is not strong enough to support those metal gear servos – a typical Servo Brownout problem.
How to address the problem? I can upgrade my ESC to have a more powerful BEC, or use a dedicated BEC to drive the servos. After quite a long thinking, I thought that the ESC does provide enough energy but the instant current surge pulls down the power. A large capacitor should be able to ease or eliminate the situation.
First, I connected a 470µF 16V electrolytic capacitor between the ground wire and the middle (power) wire. I had found the brownout was eased but not eliminated. Then I used a 2200µF 16V. This time, no matter how I move both sticks, no brownout occurred. Problem solved.
Over the week end, a friend of mine loaned me an oscilloscope so I can validate the situation and find out how bad was the brownout.
Fig 1 illustrates the original brownout situation. When I moved the sticks quickly, the voltage dropped down to 1.9V. Certainly it was a big problem. Not only that, the recovery time was very long. It took more than 10ms to get back to around 3V.
Fig 2 illustrates the effect of the surge suppressor. I did the same stick movement and the voltage only dropped by 2V and the recovery time was a lot shorter.
Furthermore, I did another research to measure the exact current draw. Fig 3 illustrates the schematic and the result. I inserted a 0.47Ω resistor between the BEC and the receiver and used the oscilloscope to monitor the voltage drop over this transistor to reveal the current. The result indicates that the maximum current draw was about 1.3A.
Fig 4 illustrates my test connection. Fig 5 shows the actual assembly. There are two shapes of electrolytic capacitors: Axial (leads from both ends) or Radial (both leads on one end). It happened that I had an axial type, bought from Radio Shack years ago (20 assorted electrolytic capacitors, 10 µF to 3300 µF, for $2.49). So I soldered it in a servo extension, between the red and black wires and installed it between the ESC and the receiver. Radial capacitor can be soldered to wires with only the receiver connector (Fig 6), then plug it into any open port of the receiver. This configuration requires an open port on the receiver. One thing need to be aware: Electrolytic capacitors are polar devices. Make sure the positive lead goes to the red wire.
I am not sure you have noticed that March 2012 issue of the Model Aviation magazine has an article in the Product Spotlight section (page 20) talking about a product called Glitch Buster. It is just an electrolytic capacitor. The article did not mention the exact capacity but I guess it is about 2200µF 16V. It is a very simple solution to address the servo brownout problem. It sells at $5.99 + S&H.
By the way, what about other control system? I cut off the ESC power line and used a NiMh battery pack (4.8V 1650mAh) to drive the servos. With the capacitor, the power drop was about 0.5V. Without the capacitor, the power drop was about 0.75V and last a bit longer for recovery (1.5ms vs 2ms). Then I tested my Cherokee. It has an 85A ESC with 5A built-in BEC and 6 standard servos. Without extra capacitor, when I moved the sticks rapidly, the peak current draw was almost 3A, but the voltage drop was barely noticeable from the oscilloscope. No brownout what so ever! I also tested Turnigy 3A UBEC on FPV Raptor and it worked very well. Less than 0.2 V voltage drop was observed even without external capacitor.
I was using Futaba 7c 2.4GHz SS transmitter with a Hobby King generic receiver. <xml><w:worddocument><w:browserlevel></w:browserlevel><m:mathpr><m:mathfont m:val="Cambria Math"></m:mathfont><m:brkbin m:val="before"></m:brkbin><m:brkbinsub m:val=""></m:brkbinsub><m:smallfrac m:val="off"></m:smallfrac><m:dispdef></m:dispdef><m:lmargin m:val="0"></m:lmargin><m:rmargin m:val="0"></m:rmargin><m:defjc m:val="centerGroup"></m:defjc><m:wrapindent m:val="1440"></m:wrapindent><m:intlim m:val="subSup"></m:intlim><m:narylim m:val="undOvr"></m:narylim></m:mathpr></w:worddocument></xml><xml><w:latentstyles latentstylecount="267" defpriority="99" defqformat="false" defsemihidden="true" defunhidewhenused="true" deflockedstate="false"><w:lsdexception name="Normal" qformat="true" unhidewhenused="false" semihidden="false" priority="0" locked="false"></w:lsdexception><w:lsdexception name="heading 1" qformat="true" unhidewhenused="false" semihidden="false" priority="9" 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For more information, please watch my video:
www.youtube.com/watch
I consulted a friend of mine and he suggested it might be the ESC issue. My BEC might be too wimpy for heavy current draw by those metal gear servos. I do not have the spec of the ESC and it only has a small label of "30A". The spec of those servos states that each one will draw 240mA and it will be under 1A in total. Most of the 25A+ ESC with BEC should provide at least 2A capacity. To further identify the problem, I disconnect one of the aileron servos and everything went normally. I replaced with two 9 gram nylon gear servos and it was OK as well. Furthermore, I tried to disconnect the BEC and use a pack of battery from my nitro plane. It behaved well too. Now I have my conclusion. The BEC is not strong enough to support those metal gear servos – a typical Servo Brownout problem.
How to address the problem? I can upgrade my ESC to have a more powerful BEC, or use a dedicated BEC to drive the servos. After quite a long thinking, I thought that the ESC does provide enough energy but the instant current surge pulls down the power. A large capacitor should be able to ease or eliminate the situation.
First, I connected a 470µF 16V electrolytic capacitor between the ground wire and the middle (power) wire. I had found the brownout was eased but not eliminated. Then I used a 2200µF 16V. This time, no matter how I move both sticks, no brownout occurred. Problem solved.
Over the week end, a friend of mine loaned me an oscilloscope so I can validate the situation and find out how bad was the brownout.
Fig 1 illustrates the original brownout situation. When I moved the sticks quickly, the voltage dropped down to 1.9V. Certainly it was a big problem. Not only that, the recovery time was very long. It took more than 10ms to get back to around 3V.
Fig 2 illustrates the effect of the surge suppressor. I did the same stick movement and the voltage only dropped by 2V and the recovery time was a lot shorter.
Furthermore, I did another research to measure the exact current draw. Fig 3 illustrates the schematic and the result. I inserted a 0.47Ω resistor between the BEC and the receiver and used the oscilloscope to monitor the voltage drop over this transistor to reveal the current. The result indicates that the maximum current draw was about 1.3A.
Fig 4 illustrates my test connection. Fig 5 shows the actual assembly. There are two shapes of electrolytic capacitors: Axial (leads from both ends) or Radial (both leads on one end). It happened that I had an axial type, bought from Radio Shack years ago (20 assorted electrolytic capacitors, 10 µF to 3300 µF, for $2.49). So I soldered it in a servo extension, between the red and black wires and installed it between the ESC and the receiver. Radial capacitor can be soldered to wires with only the receiver connector (Fig 6), then plug it into any open port of the receiver. This configuration requires an open port on the receiver. One thing need to be aware: Electrolytic capacitors are polar devices. Make sure the positive lead goes to the red wire.
I am not sure you have noticed that March 2012 issue of the Model Aviation magazine has an article in the Product Spotlight section (page 20) talking about a product called Glitch Buster. It is just an electrolytic capacitor. The article did not mention the exact capacity but I guess it is about 2200µF 16V. It is a very simple solution to address the servo brownout problem. It sells at $5.99 + S&H.
By the way, what about other control system? I cut off the ESC power line and used a NiMh battery pack (4.8V 1650mAh) to drive the servos. With the capacitor, the power drop was about 0.5V. Without the capacitor, the power drop was about 0.75V and last a bit longer for recovery (1.5ms vs 2ms). Then I tested my Cherokee. It has an 85A ESC with 5A built-in BEC and 6 standard servos. Without extra capacitor, when I moved the sticks rapidly, the peak current draw was almost 3A, but the voltage drop was barely noticeable from the oscilloscope. No brownout what so ever! I also tested Turnigy 3A UBEC on FPV Raptor and it worked very well. Less than 0.2 V voltage drop was observed even without external capacitor.
I was using Futaba 7c 2.4GHz SS transmitter with a Hobby King generic receiver. <xml><w:worddocument><w:browserlevel></w:browserlevel><m:mathpr><m:mathfont m:val="Cambria Math"></m:mathfont><m:brkbin m:val="before"></m:brkbin><m:brkbinsub m:val=""></m:brkbinsub><m:smallfrac m:val="off"></m:smallfrac><m:dispdef></m:dispdef><m:lmargin m:val="0"></m:lmargin><m:rmargin m:val="0"></m:rmargin><m:defjc m:val="centerGroup"></m:defjc><m:wrapindent m:val="1440"></m:wrapindent><m:intlim m:val="subSup"></m:intlim><m:narylim m:val="undOvr"></m:narylim></m:mathpr></w:worddocument></xml><xml><w:latentstyles latentstylecount="267" defpriority="99" defqformat="false" defsemihidden="true" defunhidewhenused="true" deflockedstate="false"><w:lsdexception name="Normal" qformat="true" unhidewhenused="false" semihidden="false" priority="0" locked="false"></w:lsdexception><w:lsdexception name="heading 1" qformat="true" unhidewhenused="false" semihidden="false" priority="9" 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02-24-2015, 03:59 AM
#2
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Very useful write up and information. A small recommendation. The ideal is to use capacitor leads as short as possible and have the capacitor mounted as near to the receiver o and 5V bus as possible.I used an axial capacitor mounted directl onto a 3pin servo connector which then plugs into a spare receiver channel directly on the receiver. All setup of course has their own installation constraints and this may not Always be possible. Thanks again for also showing good measured results..!
02-24-2015, 08:44 AM
#3
My Feedback: (19)
peter-. I believe you are way over thinking this problem and not looking at some of the real issues. First, the ratings on the servos. My bet is the ratings you state are way low and would tend to be the current draw with the servo running but not under load. Your later tests confirm this. The capacitor is just a band-aid fix at best and will only mask the real problem and again, your tests confirm this. You simply need more available current capacity to drive those servos and a separate BEC of adequate capacity will probably resolve all your problems.
02-24-2015, 08:44 AM
#4
My Feedback: (19)
peter-. I believe you are way over thinking this problem and not looking at some of the real issues. First, the ratings on the servos. My bet is the ratings you state are way low and would tend to be the current draw with the servo running but not under load. Your later tests confirm this. The capacitor is just a band-aid fix at best and will only mask the real problem and again, your tests confirm this. You simply need more available current capacity to drive those servos and a separate BEC of adequate capacity will probably resolve all your problems.
