Archive for August, 2013
Posted by rrritchey |
08-14-2013 @ 10:47 AM | 15,997 Views
Hi, its been a while since I have made a blog entry but a tech support issue today has reignited one of my pet peeves about this hobby. A vast majority of people flying Giant Scale aircraft and very expensive Jets seem to feel that if you plug all the electronics together and it works then everything is good. Well, it really might not be. I just had a support issue where someone said their receiver was getting 3.7V. I am positive I asked if anything besides the Smart-Fly PowerSystem receiver pigtails was connected to the receiver and he said no. It turns out that he had a choke servo connected to the receiver and it had partially failed, pulling the receiver regulator down to 3.7V. I have a feeling he misunderstood my question but I have gone to great lengths to try and document that no servos should be connected to the receiver. These days, with the electronics needed to fly giant scale and jets you really, really need to read instructions and make sure you follow them. Yes, having the choke servo on the receiver did work but it was not safe and it was documented. I get calls all the time from people that really have not take 15 minutes of their time to read the documentation and its caused them hours of problems. I understand this, sometimes I just plug things up but in the case of my planes, I never take them out without fully reading all the documents and making sure I have not made an assumption that will cause problems later.
On this same line, its been interesting to see some of the responses when people use my Digital Voltmeter in their planes and check the lowest voltage at the end of the flight. A lot of these people are seeing readings under 4V on their receivers after a flight. This is another case of "If it doesn't crash it must be OK" syndrome. Yes, they are getting away with running their servos off their receivers but they are on the hairy edge of loosing their planes. You can get spikes under the "brown-out" voltage of a receiver and it will not brown-out immediately because of the capacitors in the receiver supporting the voltage to the receiver chips through the spike but eventually you are going to create a low voltage long enough, which is probably 50-100 milliseconds or so (yes, 1/10th-1/20th of a second) that the receiver processor's brown-out circuit is going to kick in and reset the processor. Then you have lost control of your plane for some amount of time.
Another guy called in and said he thought one of my SuperSwitches had deteriorated, it was 4-5 years old, because he thought he was getting brownouts. I told him about my DVM and he bought one and confirmed he was getting a large drop between his batteries and the receiver. When he plugged the LiFes directly into the receiver most of the drop went away. It turns out that he did not supply all the information to me the first time. One, he was only using one output of the SuperSwitch and, two, he had an extension on the one SuperSwitch lead he was using. No wonder he was getting around a 3V drop between the battery and the receiver. If there is anything everyone should learn its Ohms law and how it applies to our hobby. The simple relationship, V=IR, can save your plane. Its telling you the more current you put across wires and connectors the more voltage drop you will have between points. The typical R/C connector has about 0.075-0.125 ohms of resistance depending on the quality of the gold. This means you can get about 0.333V drop across each connector for ever 3A of current. At the currents our servos operate these days, this drop adds up on the battery leads across multiple connectors and switch contacts (which may be as bad or worse than the connectors). Also, this voltage drop is instantaneous, if you pull 10A down the battery wire for 15 milliseconds your voltage is going to drip that whole 15 milliseconds. There is no grace period here. Just realizing that you can pull 16A-18 on a 100cc plane doing hard 3D should make everyone pause and think about their battery supply wiring. As an engineer I like lots of margin because I know at times that margin is going to be tested because unexpected things happen. If you want to keep your investment in your plane for several years you really need to think about your margins.
Also, I should mention just one thing about servos. When you use a current meter on them you are reading and AVERAGE current. A servo does not constantly draw current. Digital servos have a microprocessor in them, the microprocessor does not turn the motor on constantly, even at the highest current loads. Pretty much, a motor gets driven at most about 50% of the time. What this means to you is that, remembering that voltage drop is instantaneous, if you read that your servo is drawing 4A on a meter then if you put this on an oscilloscope and looked at the current spikes you would see 8A current spikes for around 1 millisecond followed by zero current for 1 millisecond. So, if you put a current monitor on your battery and you read a 8A you are probably really spiking the battery with around 15-16A current spikes, double what you might think.
In addition, everyone is buying, and paying big bucks, for servos that put out 500 in/oz at 7.4 volts but when you need this torque you are really getting about 5V to the servos and you are getting around 250 in/oz out of it. You paid big bucks for a servo that you are failing to utilize the capabilities of. This just amazes me. One, you could buy smaller servos, use a power system from anyone, and get the same performance probably for less money or you can use the same servos with the power system and get great performance from the servos. Everyone complains my stuff is too expensive but in general it constitutes around 2.5%-5% of the total expense of putting together a plane. A lot of that expense could be recouped if you bought 300 in/oz servos instead of 500 in/oz servos and get the same performance.
I know a lot of people are going to say I am just hyping my products but I really hate to hear that someone lost a plane and I really hate to hear its because they just did not spend a little time analyzing the system and making sure they had enough margin in the system so if unexpected things came up the system would not fail. I have been singing the same tune for about 10 years now, when I started analyzing the wiring in the giant scale planes I was introduced to in 2002, that the old wiring system just cannot stand up to planes bigger than about 1.20 glow, around 12 lbs. And this has just gotten worse because digital servos are current hogs. I don't know how many of you made it to the end of this blog but I hope it made you think a bit about your investment and how you can check to be sure you are not going to loose it.
On this same line, its been interesting to see some of the responses when people use my Digital Voltmeter in their planes and check the lowest voltage at the end of the flight. A lot of these people are seeing readings under 4V on their receivers after a flight. This is another case of "If it doesn't crash it must be OK" syndrome. Yes, they are getting away with running their servos off their receivers but they are on the hairy edge of loosing their planes. You can get spikes under the "brown-out" voltage of a receiver and it will not brown-out immediately because of the capacitors in the receiver supporting the voltage to the receiver chips through the spike but eventually you are going to create a low voltage long enough, which is probably 50-100 milliseconds or so (yes, 1/10th-1/20th of a second) that the receiver processor's brown-out circuit is going to kick in and reset the processor. Then you have lost control of your plane for some amount of time.
Another guy called in and said he thought one of my SuperSwitches had deteriorated, it was 4-5 years old, because he thought he was getting brownouts. I told him about my DVM and he bought one and confirmed he was getting a large drop between his batteries and the receiver. When he plugged the LiFes directly into the receiver most of the drop went away. It turns out that he did not supply all the information to me the first time. One, he was only using one output of the SuperSwitch and, two, he had an extension on the one SuperSwitch lead he was using. No wonder he was getting around a 3V drop between the battery and the receiver. If there is anything everyone should learn its Ohms law and how it applies to our hobby. The simple relationship, V=IR, can save your plane. Its telling you the more current you put across wires and connectors the more voltage drop you will have between points. The typical R/C connector has about 0.075-0.125 ohms of resistance depending on the quality of the gold. This means you can get about 0.333V drop across each connector for ever 3A of current. At the currents our servos operate these days, this drop adds up on the battery leads across multiple connectors and switch contacts (which may be as bad or worse than the connectors). Also, this voltage drop is instantaneous, if you pull 10A down the battery wire for 15 milliseconds your voltage is going to drip that whole 15 milliseconds. There is no grace period here. Just realizing that you can pull 16A-18 on a 100cc plane doing hard 3D should make everyone pause and think about their battery supply wiring. As an engineer I like lots of margin because I know at times that margin is going to be tested because unexpected things happen. If you want to keep your investment in your plane for several years you really need to think about your margins.
Also, I should mention just one thing about servos. When you use a current meter on them you are reading and AVERAGE current. A servo does not constantly draw current. Digital servos have a microprocessor in them, the microprocessor does not turn the motor on constantly, even at the highest current loads. Pretty much, a motor gets driven at most about 50% of the time. What this means to you is that, remembering that voltage drop is instantaneous, if you read that your servo is drawing 4A on a meter then if you put this on an oscilloscope and looked at the current spikes you would see 8A current spikes for around 1 millisecond followed by zero current for 1 millisecond. So, if you put a current monitor on your battery and you read a 8A you are probably really spiking the battery with around 15-16A current spikes, double what you might think.
In addition, everyone is buying, and paying big bucks, for servos that put out 500 in/oz at 7.4 volts but when you need this torque you are really getting about 5V to the servos and you are getting around 250 in/oz out of it. You paid big bucks for a servo that you are failing to utilize the capabilities of. This just amazes me. One, you could buy smaller servos, use a power system from anyone, and get the same performance probably for less money or you can use the same servos with the power system and get great performance from the servos. Everyone complains my stuff is too expensive but in general it constitutes around 2.5%-5% of the total expense of putting together a plane. A lot of that expense could be recouped if you bought 300 in/oz servos instead of 500 in/oz servos and get the same performance.
I know a lot of people are going to say I am just hyping my products but I really hate to hear that someone lost a plane and I really hate to hear its because they just did not spend a little time analyzing the system and making sure they had enough margin in the system so if unexpected things came up the system would not fail. I have been singing the same tune for about 10 years now, when I started analyzing the wiring in the giant scale planes I was introduced to in 2002, that the old wiring system just cannot stand up to planes bigger than about 1.20 glow, around 12 lbs. And this has just gotten worse because digital servos are current hogs. I don't know how many of you made it to the end of this blog but I hope it made you think a bit about your investment and how you can check to be sure you are not going to loose it.
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