Battery Charging
I will discuss charging Nicasds and Metal Hydrides
Both batteries have nominal voltages of 1.2V but when fully charged(peaked), the V goes to 1.4V. Thus a flight pack of 4 cells should have a voltage of 5.6V when freshly charged. the voltage drops suddenly when the V reaches 1.1V. Thus 4.4V is a critical level.
How to charge them
In technospeak, C is charging rate. Thus a charger is known to have percentC charging rate. A .10C means the charging rate is 10% of capacity(less the hr). The safest charging rate is .10 C. for example, if the pack is a 1000mah pack, the charging rate at 10% is 100ma. For a fully discharge pack, the time to restore the battery to full charge is 15 hours. It means 50% more is inserted into the battery to charge it.(15 hoursx100ma is 1500 mah). .10 is also called C10.
The enemy of the battery is high temperature. As the C rate goes up, heat build up goes up. And it is really bad to leave a battery under charge beyond the time needed at high C rates. The safest is the .10C charger.
Most of the radio sold in the market with batteries come with a 500 mah packl. And the call charger is usually a .10C charger. (You might say charger for dummies who will surely forget to unplus the charger after 15 hours.) . One other good reason why a .10C charger is used is because the pack is not really depleted before it is recharged. So, the 10C charger is safe.
Modern chargers with peak detection allows us to plug in the charger and batt anytime without worry. It dos not matter how much charge was there at the start.
We can make a very simple rugged charger like the ones used on RC cars charging a fully discharged pack. Usually, there was a 15 mins timer in those charger. Tose chargers were as high a 3C chargers.
15 mins RC chargers are no longer in voque but once in while, there is a need to make a simple rugged charger to charge metal hydrides used in model planes. Normally 7-8 cells are used. The peaked voltage os 8 cells is 8 x 1.4 or 11.2V. We can charge them in series from the car battery.
A simple rule of thumb is to charge it at 50% more opf capacity. If we use a charger that is 1C, we need a timer for 1.5 hours. or if we charge it at 1.5C, we charge it for 1 hour. The formula is rate x time = 1.5 x capacity.
For example, for a 1000 mah pack, we can charge at 1.5 amps (1500 ma) for one hour. If we want it for 30 mins, then we double the rate to 3 amps.
A good good guideline is to decide on how many packs one can afford in the field and use the longest time to charge acceptable. for example, using 4 packs, each pack lasting 10 minutes, with a rest in between. When the first pack is used, the nest 3 packs is good for 30 minutes plus rest time of 30 minutes. Then a 1 hour charger is good.
Can we charge more than 8 cells from the 12V car battery. Well, yes. Say for a 10 cell pack, we break them up into 5 cell packs. We charge them as parallel packs. After they are charged, they can be plugged together in series. Only one timer has to be used and there is no need for two chargers really.
Current is the critical parameter. The charger voltage has to be higher than the peak V of the pack. Current is calibrated using resistors in series with the right wattage.
BK
Wednesday, December 2, 2009
Wednesday, August 5, 2009
Tuesday, July 7, 2009
RC RADIO
July 7, 2009
Don Mathers and Dough Spreng developed the truly proportional system still in use today. Different modes of modulation were later introduced (FM) other than AM, PCM was introduced, but still, it is the Mathers/Spreng servo at work at the end. Even digital servos still use the system.
AM to FM. This is often confused, many thinking that we have to go FM once we are on the upper band. No, in fact, there were FMs on 27Mhz. FM is a modulation form to transmit information. It does not change the the Mathers/Spreng system in positioning the servo. In AM, the strength of the signal is varied to send info. In fact, in RC, the carrier wave(CW) is momentarily shut off to represent a signal. This is also practical since no extra power is use to send a short pulse. The receiver takes care of recognizing the blank and convert it to an ON signal at its output. In both AM and FM, a very short trigger pulse (.25MS) is sent at intervals. The intervals represent the time a port is ON at the decoder, which sets the servo position. This is called Pulse Width Modulation(PWM). PCM which stands for Pulse Code Modulation can be viewed by the layman as also a modulation form. Instead of the short pulse sent, the TX actually sends a binary code to the RX. The RX has a CPU which produces the command pulse for a servo (from 1 MS to 2 MS). In PWM, the TX sends the ON/OFF commands to several servos in sequence. Thus, usually,it can only send about 60 commands to each servo per second. In PCM, in as long as there is no new command from the TX, the RX keeps generating the same pulse width. So, an interference on the signal does not mean anything to the RX since it is unintelligible to it. PCM today is usually on FM. It remains a very good system against interference. But it should not be confused as to what the spread spectrum (SS) is doing. In PCM, The system is very good against unintellgible interference. But another TX on the same channel can of course interfere with one's RX. The spread spectrum takes care of this aspect. In frequency hopping (FHSS) for example, the signal is rotated among different spot channels and the sequence of the combination code is locked between the TX and the RX (called binding), and there are thousands of combinations possible, thus probability for two radios using the same code is very low. So, the SS is not an improvement on the modulation. It is an improvement on the use of frequency space. It could well be that PCM is used and transmitted the SS way on 2.4Gig. The 2.4G is just coincidental that the FCC opened up this band for wireless communication. Again, it could well be that SS were used at 72-75 Megs, or even 27 megs.
What is the Mathers/Spreng system?
Well, the best way to appreciate it is to look at the servo. In a way, this was the heart of their invention. Before them, the servo was analog. A motor is geared down to developed torque and there was a feedback pot on the final shaft. In the analog system, the pot adjusted the bias on a transistor. When an increased/decreased drive voltage turns ON/Off the transistor, the pot counters the move(opposite wise). So, the changed is canceled by the changed in the transistor bias controlled by the pot. The problem of the analog servo was delayed reaction, non-proportionality and overshoot, among others perhaps. You see, the voltage level moving was purely analog, meaning, it was on a curved path. There must be something discrete like On then OFF but sent continously and when a change is introduced, the interval between the ON/Off is changed.
The concept was an incoming pulse with a width from 1 MS to 2MS. A local pulse is also started by the incoming pulse but is inverted (positive incoming and negative local pulse). When one is longer than the other, a residual pulse remains. TX sends the trigger pulses which the decoder vonvrts to a positive pulse of a duration. The TX sends the trigger pulses on a stream (repeated). When the residual pulse is positive, the motor is turned one way. When the residual pulse is negative, the motor is turned the other way. Now, a pot (normally 5k today) is again, attached to the final output shaft of the servo and controls the width of the local pulse. It is wired so that the width of the local pulse matches that of the incoming pulse as the pot is turned which then stops the motor. By that time, the servo horn is at a different position.
How is the command pulse created?
It is created at the decoder. The TX sends two short pulses that we can call trigger pulses(.25MS). This is not the servo command pulse. It is the signal for a decoder to turn a port ON on the firsts pulse and OFF at the second pulse. This port is the servo channel port. In a hypothetical single channel system, the TX will just keep sending two trigger pulses with intervals controlled by a joystick. And since the max interval is 2 MS and there are 1000 MS in one second, the TX can send 500 command pulses per second to 1000 command pulses when the delay is at 1MS. But all of this changes when more than one channel is sent. First, the OFF trigger pulse of the fist channel can be made to be the ON trigger pulse of the second channel. Thus, only three trigger pulses are needed to create two command pulses at the decoder. However, to make sure that the first trigger pulse turns on the first port, a long inverval called a pause is introduced in the stram of trigger pulses. This must be at least twice the longest interval which is 2MS, thus 4MS. So, in a 2 channel system, the max intervals is 2x 2MS plus the 4 MS for a total of 8MS. Thus, a system can be designed to send 1000MS/8MS commands per second. A set of command pulses for the two servos is called a frame. That will be 150 frames per second. Mind you fellow modellers, if you are using an 8 channel TX and RX to control only two servos, the resolution is wasted.
PCM
Perhaps the above will make us appreciate PCM more. In PCM, the above computation becomes irrelevant.The TX in a PCM does not send pulses on merry go around such that we add all the intervals which becomes longer the more the servo channels. In PCM, it is like sending the juke box a number for a record to play and you do it only once. In PCM, the TX tells the RX-CPU to generate a pulse for a given channel only once until you had moved that joystick for a new command. There is no stream of commands to a set of servos. In PWM (or PPM), a cut in the stream sends the servos chattering. In PCM, a cut in the transmission is merel;y recognized as silence and does not mean much to the RX-CPU which is waiting only for intelligible commands to shift the pulse widths to a set of servos. PCM is truly intereference proof but not from same channel TX. That is where the SS comes in. By the way, PCM was not developed for the RC industry. It was developed for general communications works but was particularly used to send commands to the Mars probe and to send images from the Mars probe to Earth.
A short explanation of
Spread Spectrum
Let say we have 3 channels only on 72 MHZ. The way to allow more that 3 TX to be used at the same time is to make a system wherein the TX hops channels. Let us call the channels A,B, and C. Tx1 will hop ABC,TX2 ACB, TX3 BAC, TX4BCA, TX5CAB,TX6CBA. This of course is a very simplified semblance on the principle used. The pattern that the TX will adopt is copied into the RX when they are bounded(binding). Thus it is nearly impossible for two systems to cross.
The 2.4Ghz band is very high frequency of course and is not limited to RC. Wireless phones, cell phones and other services are in this band. The antenna is very short indeed as the wavelength of the signal is 300/2400 or just .125 meter or 1.25 CM. The problem at this frequency, are wires around the antenna such as servo extension cables which robs the RX of range. And that is one reason 2.4's suffer momentary cut offs. SS is good but the band gives this problem. One remedy is to wrap the extension cables around a ferrite ring.
To follow is a detailed explanation of the "hardware" of a typical TX and RX with decoder. Complete circuitry will be explained.
BK
Don Mathers and Dough Spreng developed the truly proportional system still in use today. Different modes of modulation were later introduced (FM) other than AM, PCM was introduced, but still, it is the Mathers/Spreng servo at work at the end. Even digital servos still use the system.
AM to FM. This is often confused, many thinking that we have to go FM once we are on the upper band. No, in fact, there were FMs on 27Mhz. FM is a modulation form to transmit information. It does not change the the Mathers/Spreng system in positioning the servo. In AM, the strength of the signal is varied to send info. In fact, in RC, the carrier wave(CW) is momentarily shut off to represent a signal. This is also practical since no extra power is use to send a short pulse. The receiver takes care of recognizing the blank and convert it to an ON signal at its output. In both AM and FM, a very short trigger pulse (.25MS) is sent at intervals. The intervals represent the time a port is ON at the decoder, which sets the servo position. This is called Pulse Width Modulation(PWM). PCM which stands for Pulse Code Modulation can be viewed by the layman as also a modulation form. Instead of the short pulse sent, the TX actually sends a binary code to the RX. The RX has a CPU which produces the command pulse for a servo (from 1 MS to 2 MS). In PWM, the TX sends the ON/OFF commands to several servos in sequence. Thus, usually,it can only send about 60 commands to each servo per second. In PCM, in as long as there is no new command from the TX, the RX keeps generating the same pulse width. So, an interference on the signal does not mean anything to the RX since it is unintelligible to it. PCM today is usually on FM. It remains a very good system against interference. But it should not be confused as to what the spread spectrum (SS) is doing. In PCM, The system is very good against unintellgible interference. But another TX on the same channel can of course interfere with one's RX. The spread spectrum takes care of this aspect. In frequency hopping (FHSS) for example, the signal is rotated among different spot channels and the sequence of the combination code is locked between the TX and the RX (called binding), and there are thousands of combinations possible, thus probability for two radios using the same code is very low. So, the SS is not an improvement on the modulation. It is an improvement on the use of frequency space. It could well be that PCM is used and transmitted the SS way on 2.4Gig. The 2.4G is just coincidental that the FCC opened up this band for wireless communication. Again, it could well be that SS were used at 72-75 Megs, or even 27 megs.
What is the Mathers/Spreng system?
Well, the best way to appreciate it is to look at the servo. In a way, this was the heart of their invention. Before them, the servo was analog. A motor is geared down to developed torque and there was a feedback pot on the final shaft. In the analog system, the pot adjusted the bias on a transistor. When an increased/decreased drive voltage turns ON/Off the transistor, the pot counters the move(opposite wise). So, the changed is canceled by the changed in the transistor bias controlled by the pot. The problem of the analog servo was delayed reaction, non-proportionality and overshoot, among others perhaps. You see, the voltage level moving was purely analog, meaning, it was on a curved path. There must be something discrete like On then OFF but sent continously and when a change is introduced, the interval between the ON/Off is changed.
The concept was an incoming pulse with a width from 1 MS to 2MS. A local pulse is also started by the incoming pulse but is inverted (positive incoming and negative local pulse). When one is longer than the other, a residual pulse remains. TX sends the trigger pulses which the decoder vonvrts to a positive pulse of a duration. The TX sends the trigger pulses on a stream (repeated). When the residual pulse is positive, the motor is turned one way. When the residual pulse is negative, the motor is turned the other way. Now, a pot (normally 5k today) is again, attached to the final output shaft of the servo and controls the width of the local pulse. It is wired so that the width of the local pulse matches that of the incoming pulse as the pot is turned which then stops the motor. By that time, the servo horn is at a different position.
How is the command pulse created?
It is created at the decoder. The TX sends two short pulses that we can call trigger pulses(.25MS). This is not the servo command pulse. It is the signal for a decoder to turn a port ON on the firsts pulse and OFF at the second pulse. This port is the servo channel port. In a hypothetical single channel system, the TX will just keep sending two trigger pulses with intervals controlled by a joystick. And since the max interval is 2 MS and there are 1000 MS in one second, the TX can send 500 command pulses per second to 1000 command pulses when the delay is at 1MS. But all of this changes when more than one channel is sent. First, the OFF trigger pulse of the fist channel can be made to be the ON trigger pulse of the second channel. Thus, only three trigger pulses are needed to create two command pulses at the decoder. However, to make sure that the first trigger pulse turns on the first port, a long inverval called a pause is introduced in the stram of trigger pulses. This must be at least twice the longest interval which is 2MS, thus 4MS. So, in a 2 channel system, the max intervals is 2x 2MS plus the 4 MS for a total of 8MS. Thus, a system can be designed to send 1000MS/8MS commands per second. A set of command pulses for the two servos is called a frame. That will be 150 frames per second. Mind you fellow modellers, if you are using an 8 channel TX and RX to control only two servos, the resolution is wasted.
PCM
Perhaps the above will make us appreciate PCM more. In PCM, the above computation becomes irrelevant.The TX in a PCM does not send pulses on merry go around such that we add all the intervals which becomes longer the more the servo channels. In PCM, it is like sending the juke box a number for a record to play and you do it only once. In PCM, the TX tells the RX-CPU to generate a pulse for a given channel only once until you had moved that joystick for a new command. There is no stream of commands to a set of servos. In PWM (or PPM), a cut in the stream sends the servos chattering. In PCM, a cut in the transmission is merel;y recognized as silence and does not mean much to the RX-CPU which is waiting only for intelligible commands to shift the pulse widths to a set of servos. PCM is truly intereference proof but not from same channel TX. That is where the SS comes in. By the way, PCM was not developed for the RC industry. It was developed for general communications works but was particularly used to send commands to the Mars probe and to send images from the Mars probe to Earth.
A short explanation of
Spread Spectrum
Let say we have 3 channels only on 72 MHZ. The way to allow more that 3 TX to be used at the same time is to make a system wherein the TX hops channels. Let us call the channels A,B, and C. Tx1 will hop ABC,TX2 ACB, TX3 BAC, TX4BCA, TX5CAB,TX6CBA. This of course is a very simplified semblance on the principle used. The pattern that the TX will adopt is copied into the RX when they are bounded(binding). Thus it is nearly impossible for two systems to cross.
The 2.4Ghz band is very high frequency of course and is not limited to RC. Wireless phones, cell phones and other services are in this band. The antenna is very short indeed as the wavelength of the signal is 300/2400 or just .125 meter or 1.25 CM. The problem at this frequency, are wires around the antenna such as servo extension cables which robs the RX of range. And that is one reason 2.4's suffer momentary cut offs. SS is good but the band gives this problem. One remedy is to wrap the extension cables around a ferrite ring.
To follow is a detailed explanation of the "hardware" of a typical TX and RX with decoder. Complete circuitry will be explained.
BK
Monday, June 29, 2009
WIRELESS REMOTE CONTROL
INTRODUCTION
Probably Man's age old dream: wireless remote control of objects. This was realized only in the 20th century after the vacuum tube was developed ( which was discovered after Edison invented the lightbulb). Well, Samuel Morse conceptualized and developed the Morse Code, perhaps unknowingly, stumbling into digital concept, a two state mode communication, dot and dash. Today, dot and dash is reresented by 1 and 0 which can be manipulated mathematically. It was Marconi who made it wireless. Instead of voice over the Atlantic, Marconi sent dots and dashes, using Morse's code. There were only two sounds to recognize. Of course voice followed but it is interesting to note that today, we are back to using only two distinct symbols to send voice great distances, the farthest is to the moon when austronauts were there. In reality, 0 and 1 are sent but the way it is electrically sent is on or off. On for 1 and off for 0. Voice is converted to 0 and 1's (binary) sent, then reconstructed in the RX.
The modellers were at the forefront of RC. Actually, before the vacuum tube era, a spark gap was used to transmit a wideband electrical noise. The receiver was a coherer tube, a glass tube with iron fillings. When electromagnetic energy was detected, the iron fillings lined up inside the tube in an orderly North-South arrangement, thus making the iron fillings conduct. The first public display was somehow controlling a balloon inside a building, probably looking like a dirigible. Rapid progress was made in the 30's when vacuum tubes was used to oscillate at a frequency kept steady by a crystal. A fixed frequency on a narrow bandwith was transmitted.(this was broadband by today's standard). A broadband receiver with very little selectivity picked up the propagation which at the beginning was nothing but the carrier wave of the TX. Modellers put a button on a TX and a relay in the RX. The world was "quiet" in those days (transmission wise). Later, it was found out an audio tone modulated on the CW (AM) was better. To move the control surface of a model aircraft, an escapement was developed which multiplied the force of the relay. The escapement was mechanical where rubber band was wound to generate torque on a spindle. The spindle had an dual arms caught on a latch to hold it from turning. The relay released the latch and engage it in the next arm, thus changing the positon of a horn attached to the spindle. Thus rudder was controllable to be at center, left or right by pressing the TX button. This was called "single channel" in those days and was probably like owning a I-pod today! If only we could control the elevator as well was the main wish. There were various attempts and the one that became widely acceptable was to use tones to send different commands on the same frequency. The sound discriminator in the RX was mechanical: reeds. Reeds resonant to the tone were placed on top of an electromagnet. The corresponding reed vibrating given a tone. Soon, there were 8 channels, eight reeds on top of an electromagnet. These reeds touched a contact when vibrating which turns on a relay which activates escapements. This went on for a time. In 1949, the Americans discovered semi conductors called transistors, much much smaller than a vacuum tube and did not need heaters. By the fifties, transistors were appearing everywhere. Still, tones and escapements were used. Eventually, small dc motors replaced the rubber band and the escapement itself. High speed motors were geared down to have torque to turn the control surfaces(Rudder and elevator, ailerons and throttle). These were called "servos". In the forefront of servo development was Howard Bonner(American). Bonner servos reached moddellers across the globe. But this servos were not proportional, meaning they did not move linearly with stick movement in the TX. In the beginning, they only had end switches to stop at extreme ends. In fact, a transition was a servo that was swinging back and forth to a position based on an ON/Off switcher in the TX. The rudder appeared like a dorsal fin of a fish on flight. It became known as the galloping ghost.
The first attempt at proportional control was called analog in nature. The TX transmitted a signal which can increase/decrease a voltage level. A transistor is turn on but a variable resistor connected to the output shaft of the servo changes the sensitivity of the transistor thus bringing it back to an OFF state and shuts the motor. By then the horn connected to the control surface has changed position. The problem was delayed reaction, proportionality,and overshoot. By then, two young Americans electronics buffs were working on a concept. They were Mathers and Spreng(Dough).
Probably Man's age old dream: wireless remote control of objects. This was realized only in the 20th century after the vacuum tube was developed ( which was discovered after Edison invented the lightbulb). Well, Samuel Morse conceptualized and developed the Morse Code, perhaps unknowingly, stumbling into digital concept, a two state mode communication, dot and dash. Today, dot and dash is reresented by 1 and 0 which can be manipulated mathematically. It was Marconi who made it wireless. Instead of voice over the Atlantic, Marconi sent dots and dashes, using Morse's code. There were only two sounds to recognize. Of course voice followed but it is interesting to note that today, we are back to using only two distinct symbols to send voice great distances, the farthest is to the moon when austronauts were there. In reality, 0 and 1 are sent but the way it is electrically sent is on or off. On for 1 and off for 0. Voice is converted to 0 and 1's (binary) sent, then reconstructed in the RX.
The modellers were at the forefront of RC. Actually, before the vacuum tube era, a spark gap was used to transmit a wideband electrical noise. The receiver was a coherer tube, a glass tube with iron fillings. When electromagnetic energy was detected, the iron fillings lined up inside the tube in an orderly North-South arrangement, thus making the iron fillings conduct. The first public display was somehow controlling a balloon inside a building, probably looking like a dirigible. Rapid progress was made in the 30's when vacuum tubes was used to oscillate at a frequency kept steady by a crystal. A fixed frequency on a narrow bandwith was transmitted.(this was broadband by today's standard). A broadband receiver with very little selectivity picked up the propagation which at the beginning was nothing but the carrier wave of the TX. Modellers put a button on a TX and a relay in the RX. The world was "quiet" in those days (transmission wise). Later, it was found out an audio tone modulated on the CW (AM) was better. To move the control surface of a model aircraft, an escapement was developed which multiplied the force of the relay. The escapement was mechanical where rubber band was wound to generate torque on a spindle. The spindle had an dual arms caught on a latch to hold it from turning. The relay released the latch and engage it in the next arm, thus changing the positon of a horn attached to the spindle. Thus rudder was controllable to be at center, left or right by pressing the TX button. This was called "single channel" in those days and was probably like owning a I-pod today! If only we could control the elevator as well was the main wish. There were various attempts and the one that became widely acceptable was to use tones to send different commands on the same frequency. The sound discriminator in the RX was mechanical: reeds. Reeds resonant to the tone were placed on top of an electromagnet. The corresponding reed vibrating given a tone. Soon, there were 8 channels, eight reeds on top of an electromagnet. These reeds touched a contact when vibrating which turns on a relay which activates escapements. This went on for a time. In 1949, the Americans discovered semi conductors called transistors, much much smaller than a vacuum tube and did not need heaters. By the fifties, transistors were appearing everywhere. Still, tones and escapements were used. Eventually, small dc motors replaced the rubber band and the escapement itself. High speed motors were geared down to have torque to turn the control surfaces(Rudder and elevator, ailerons and throttle). These were called "servos". In the forefront of servo development was Howard Bonner(American). Bonner servos reached moddellers across the globe. But this servos were not proportional, meaning they did not move linearly with stick movement in the TX. In the beginning, they only had end switches to stop at extreme ends. In fact, a transition was a servo that was swinging back and forth to a position based on an ON/Off switcher in the TX. The rudder appeared like a dorsal fin of a fish on flight. It became known as the galloping ghost.
The first attempt at proportional control was called analog in nature. The TX transmitted a signal which can increase/decrease a voltage level. A transistor is turn on but a variable resistor connected to the output shaft of the servo changes the sensitivity of the transistor thus bringing it back to an OFF state and shuts the motor. By then the horn connected to the control surface has changed position. The problem was delayed reaction, proportionality,and overshoot. By then, two young Americans electronics buffs were working on a concept. They were Mathers and Spreng(Dough).
Thursday, April 16, 2009
KNOWING YOUR BATTERY PACK(for Nitro planes)
Most of us put our confidence in what the label says in a battery pack. When we used to have the standard 500mah pack and later on got a 650mah pack,we falsely conclude that we have a better pack. To compound the problem, modern chargers will indicate how much capacity it has injected into the pack while charging. Another confidence booster. But there is no way actually that a charger can record how much your battery pack absorbed. What is then the best way to evaluate a battery pack? Well, many modellers do not really appreciate the discharge feature of newer chargers. This feature can give us a better evaluation of a battery pack. This is the procedure:
1. Fully charge the pack with the charger which has delta peak feature.
2. Discharge the pack at a known rate till the voltage has reached 4x1.1=4.4V.
3.Record the time elapsed.
4. The discharge rate x time elapse=true usable capacity of the pack.
Example:
You have a650 mah pack and after fully charging it, you discharged it at 650 ma. It lasted an hour before falling to the 4.4V threshold. It has a 650MAH useful capacity. Now,supposed it lasted only 30minutes, then the useful capacity is half or 325 mah.
Most of us put our confidence in what the label says in a battery pack. When we used to have the standard 500mah pack and later on got a 650mah pack,we falsely conclude that we have a better pack. To compound the problem, modern chargers will indicate how much capacity it has injected into the pack while charging. Another confidence booster. But there is no way actually that a charger can record how much your battery pack absorbed. What is then the best way to evaluate a battery pack? Well, many modellers do not really appreciate the discharge feature of newer chargers. This feature can give us a better evaluation of a battery pack. This is the procedure:
1. Fully charge the pack with the charger which has delta peak feature.
2. Discharge the pack at a known rate till the voltage has reached 4x1.1=4.4V.
3.Record the time elapsed.
4. The discharge rate x time elapse=true usable capacity of the pack.
Example:
You have a650 mah pack and after fully charging it, you discharged it at 650 ma. It lasted an hour before falling to the 4.4V threshold. It has a 650MAH useful capacity. Now,supposed it lasted only 30minutes, then the useful capacity is half or 325 mah.
Saturday, March 28, 2009
Hello guys. Sorry for the long vacation. Have been busy with RC planes. Our last topic was encoders and decoders in RC gear. Let's hightlight some points about the non- digital way of driving the servo. In the ordinary PWM system, the Tx sends about 60 frames of commands to many servos. The more the channels, the less command is given to a servo per unit time. Let's take a 4 channel system. The allocation is 2 MS for each channel totalling 8 ms and 10ms for the pause. That's 18ms divided into 1000MS in a second. At 20 MS, that would be 50 times a second. If you have 6 channels, that would have been 6 x 2m=12, plusa 10ms pause to make a total of 22 ms. There will be less than 50 c0mmands to a servo per second. Now, how does this affect the servo strength? Well, everytime a servo arm is pull out of its position, the pulses drives the motor to pull it back in. If we had more pulses, the pulling in would be finer. That is where the digital servos come in. In a digital servo, the number of pulses "correcting" the servo position is about 300 times vs 50-60 for the analog system. That is about five times. It's like a gear of 300 teeth vs one with 60. The drive is finer as it can correct a smaller error and in smaller units. The drawback is that digital servos also draw five times the current! How is this done. Well, it cannot be possibly converting the 60 pulses per second from the transmitter and convert it to 300 pulses. That is not possible. It became possible with PCM. Pulse code instead of pulse width. In PCM, the Tx sends a binary code to tell a CPU in the RX to position a servo. For example the Futaba 1024 PCM, the servo can take up 1,024 positions. And the beaty of it all, the TX only have to say it once, like sending in the code to take up the position 512 which is center. It's like the command is "Don't change position until I tell you". The local CPU will generate the pulse of 1.5MS but at 300 pulses per second. And that is why a PCM RX is immune to electrical noise since the code has to be exactly legible for it to recognize as a command. In PWM, a glitch can be read as a pulse. Theoretically, if you had switch off a PCM TX momentarily, the servos will remain in their position since no new command was received. This is not the fail safe feature in PCM. The failsafe feature is when a given setting for each of the servos at a certain moment was stored in the CPU in the RX. If the RX does not hear from the ground after X seconds, the CPU switches to this mode even when nothing was heard from the TX again. The plane took on a circling mode for example. Another thing that can be memorized by the CPU is say, a loop. Next time you press the repeat button, the plane does it again. It's like putting the music organ to recording. Pressing a button will play it again and again.
bk
bk
Tuesday, January 6, 2009
voltage, ohms and amps.
Hi guys, sorry there has been a long silence.
Ok, I promised to discuss VOA. These are the unit measures for emf,resistance, and current. EMF is electromagnetic force. They are related and my favorite analogy is your plumbing. Voltage is the water pressure you have. Resistance is the pipe you are using, and the current in amperes is akin to gallons per minute, your volume. If you increase the resistance of the water flow by making the pipe smaller, you will increase pressure and lower current. If you make the pipe bigger, you will bring in more water per time but at a lower presure. OHMS LAW is :
E=IR
Let's apply to brushless motors(same as brushed motors anyway).
People bring me their motors and tell me to make it stronger. But you see stronger refers to power and power is a combination of speed and torque. Well, I can it make run faster or at a higher torque or both to make it "stronger". You see, by the use of gears, speed and torque are inversely proportional and can conveted from one to the other. The quickest way to make it run faster is to lower the number of wire turns. In RC cars we hear of 27T as stock and 24T as souped up. It will heat up tho, assuming all else the same (prop size, weight of model). It is because the lower number of turns means less resistance and the current draw will go up. (V/R) Now, since power in watts is VA, power goes up. To reduce heat, use a thicker wire. Something is interesting here. The turns you had removed should be placed back into the motor in terms of thickness. That is why rewinders use weight. It should be the same weight of copper wires for a given wattage motor. If you bring me a 12V motor and wants it converted to 6V, I will half the turns and double the thickness. (and vice versa to convert a 6V to 12V motors). You see something there, the same watts but different combination of V and Amps. Yes, but I thought we were changing the resistance. Sure and it changes the amps in turn.
Why do motors burn? Well, you can burn it even if you had used the correct battery voltage if you had used a wrong prop(overprop). A loaded motor will draw excessive current. The more common cause I suspect is using hihger voltage. Many many modellers burn their brushed motors by using li-poly, especially 3S. A seven cell mH pack is 1.4x7=9.8V max and it drops more rapidly than a li-poly pack. A 2S is 8.4V and a 3s is 12.6V peaked. Now, even a 2S can burn up a brushed motors even if the V is lower. That is because the 2s can maintain delivery of strong current longer than an MH. It is wise to throttle back once you shot that plane up. So, if you want more speed , you can lower the turns or raise the V.
I think I have said this before. If you have a 12V motor and uses 2 packs of 6V each, if we rewind it to 6V, to get the same duration , we have to parallel the batteries anyway. We do not really save on battery weight. We just reconfigured the VA combination. The only real advantage is that we can carry one pack first, land then switch to the second pack. We didn't have to carry 2 packs. What is the conclusion anyway? Well, say you have a 12V motor. Do not rewind. Find a battery pack of 6V each at half the capacity, presumably at half the weight. Series 2 of them to get 12V. Yyou will have the same weight as using a 6V motor with a 6V pack but at twice the capacity.
THE RADIO SYSTEM
----------------------
Two Americans invented the proportional system when transistors were already available(post 1950). The heart of it all was the servo. The servo developed is the one still widely used today. The circuit makes the motor turns on pulses smoothed out by capacitors. An incoming pulse from the receiver is added to a locally generated pulse with an opposite polarity. If they are the same, they cancell each other and no differential pulse appears. But if one is longer than the order(duration they are on), a residual pulse appears of either polarity. This residual pulse keeps coming on at about 60 times per second. The circuit senses the polarity and turns on the appropriate drivers to turn the motor CW or CCW. But as the motor turns gears to multiply its torque, a pot is also turned adjusting the local pulse to match the incoming pulse. Once matched, the motors stops but the motor output shaft is now at a different position. The inventors decided on 1.5MS as the centering pulse and 1MS for full left and 2MS for full right. One MS is one millisecond. The size of the pulse limits the repetition rate per second and it cannot be made too small either due to bandwith problems in transmission. Imagine a single servo system. The TX can be turned on between 1MS to 2MS repetitively for the RX to interpret as the pulse. Another appraoch to save power is to momentarily turn the TX ON. At the RX end, the On switches a Flip-Flop ON, then OFF at the second ON of the TX. The OFF time will be the TX at rest and will save on power. But it was recognized that the OFF time will make the RX more prone to pick up an interference. The reverse was adopted. Keep the TX carrier ON. Switch it OFF momentarily twice, the gap representing the ON time of the pulse created by the Flip-Flop at the RX.(the Off is momentary). To have multi channels, we add more FFs at the receiver for the next servo. the TX then simply have more OFFS in the chain of commands.(number of servos + 1 for the starting OFF). Today a number of FFs are in one IC called the shift register. The RX converts the OFF time of the TX into a positive short pulses called clocks. First clock turns ch 1 ON, clock two OFF but CH 2 ON, etc.
to be continued...
Ok, I promised to discuss VOA. These are the unit measures for emf,resistance, and current. EMF is electromagnetic force. They are related and my favorite analogy is your plumbing. Voltage is the water pressure you have. Resistance is the pipe you are using, and the current in amperes is akin to gallons per minute, your volume. If you increase the resistance of the water flow by making the pipe smaller, you will increase pressure and lower current. If you make the pipe bigger, you will bring in more water per time but at a lower presure. OHMS LAW is :
E=IR
Let's apply to brushless motors(same as brushed motors anyway).
People bring me their motors and tell me to make it stronger. But you see stronger refers to power and power is a combination of speed and torque. Well, I can it make run faster or at a higher torque or both to make it "stronger". You see, by the use of gears, speed and torque are inversely proportional and can conveted from one to the other. The quickest way to make it run faster is to lower the number of wire turns. In RC cars we hear of 27T as stock and 24T as souped up. It will heat up tho, assuming all else the same (prop size, weight of model). It is because the lower number of turns means less resistance and the current draw will go up. (V/R) Now, since power in watts is VA, power goes up. To reduce heat, use a thicker wire. Something is interesting here. The turns you had removed should be placed back into the motor in terms of thickness. That is why rewinders use weight. It should be the same weight of copper wires for a given wattage motor. If you bring me a 12V motor and wants it converted to 6V, I will half the turns and double the thickness. (and vice versa to convert a 6V to 12V motors). You see something there, the same watts but different combination of V and Amps. Yes, but I thought we were changing the resistance. Sure and it changes the amps in turn.
Why do motors burn? Well, you can burn it even if you had used the correct battery voltage if you had used a wrong prop(overprop). A loaded motor will draw excessive current. The more common cause I suspect is using hihger voltage. Many many modellers burn their brushed motors by using li-poly, especially 3S. A seven cell mH pack is 1.4x7=9.8V max and it drops more rapidly than a li-poly pack. A 2S is 8.4V and a 3s is 12.6V peaked. Now, even a 2S can burn up a brushed motors even if the V is lower. That is because the 2s can maintain delivery of strong current longer than an MH. It is wise to throttle back once you shot that plane up. So, if you want more speed , you can lower the turns or raise the V.
I think I have said this before. If you have a 12V motor and uses 2 packs of 6V each, if we rewind it to 6V, to get the same duration , we have to parallel the batteries anyway. We do not really save on battery weight. We just reconfigured the VA combination. The only real advantage is that we can carry one pack first, land then switch to the second pack. We didn't have to carry 2 packs. What is the conclusion anyway? Well, say you have a 12V motor. Do not rewind. Find a battery pack of 6V each at half the capacity, presumably at half the weight. Series 2 of them to get 12V. Yyou will have the same weight as using a 6V motor with a 6V pack but at twice the capacity.
THE RADIO SYSTEM
----------------------
Two Americans invented the proportional system when transistors were already available(post 1950). The heart of it all was the servo. The servo developed is the one still widely used today. The circuit makes the motor turns on pulses smoothed out by capacitors. An incoming pulse from the receiver is added to a locally generated pulse with an opposite polarity. If they are the same, they cancell each other and no differential pulse appears. But if one is longer than the order(duration they are on), a residual pulse appears of either polarity. This residual pulse keeps coming on at about 60 times per second. The circuit senses the polarity and turns on the appropriate drivers to turn the motor CW or CCW. But as the motor turns gears to multiply its torque, a pot is also turned adjusting the local pulse to match the incoming pulse. Once matched, the motors stops but the motor output shaft is now at a different position. The inventors decided on 1.5MS as the centering pulse and 1MS for full left and 2MS for full right. One MS is one millisecond. The size of the pulse limits the repetition rate per second and it cannot be made too small either due to bandwith problems in transmission. Imagine a single servo system. The TX can be turned on between 1MS to 2MS repetitively for the RX to interpret as the pulse. Another appraoch to save power is to momentarily turn the TX ON. At the RX end, the On switches a Flip-Flop ON, then OFF at the second ON of the TX. The OFF time will be the TX at rest and will save on power. But it was recognized that the OFF time will make the RX more prone to pick up an interference. The reverse was adopted. Keep the TX carrier ON. Switch it OFF momentarily twice, the gap representing the ON time of the pulse created by the Flip-Flop at the RX.(the Off is momentary). To have multi channels, we add more FFs at the receiver for the next servo. the TX then simply have more OFFS in the chain of commands.(number of servos + 1 for the starting OFF). Today a number of FFs are in one IC called the shift register. The RX converts the OFF time of the TX into a positive short pulses called clocks. First clock turns ch 1 ON, clock two OFF but CH 2 ON, etc.
to be continued...
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