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Inside Story - February 2003

This month, Graham and crew are back a very large, six section column. (Actually, it is a column, a tip section, and four mini-articles.) It starts with an in-depth discussion about motor constants, which has more hard-core equations than any EZone article, and then moves on to articles on actuators, servos, receivers, and a section devoted to building a cheap wattmeter, "Rapid Prototyping in the Microwave", and finally, and "LiPoly Voltage Monitor".

Splash

The First Bit

Hello all, we are back again for another exciting installment of (cue drum roll) The Inside Story. I want to start by thanking everyone for their encouragement and support for the column. Many of you who emailed me wanted to know more about actuators, in particular how to drive them, so this article contains a mini-article on just that. It is hard to write articles covering such a large field without feeling that you will frustrate the reader by not telling the whole story all at once. Please bear with us as we will try to cover plenty of the basics as quickly as possible but in the mean time always feel welcome to log on to the EZone's excellent discussion forums. Also in this issue is an excellent feature article by Joachim Bergmeyer. It covers the theory of how to work out what that mystery motor is good for. It contains a little mathematics but is set out in such a way that you can know as much as you want to know, from using the ready to fill in form to reading the derivations. After reading this article, I feel a personal cloud of ignorance had been lifted and I hope you will too. On a lighter note (pun intended) Gordon Johnson has written two mini-articles (gold star for that man!), one covering the lightening of normal micro servos and receivers and the other on the need to keep a close eye on your volts when using the new Lithium Polymer cells. I have also managed to squeeze out a second mini on making a simple circuit board to allow two multimeters to be used simultaneously and conveniently. Just to prove the microwave is good for something more than cooking ready meals we have a tip from Peter Frostick a free flight and micro RC guru on his unusual method of fast prototyping balsa props. You should probably be working while reading this so let's get started.

Feature Article - Motor Constants, How To Find Them And Use Them
by Joachim Bergmeyer

Introduction

Have you ever bought a tiny DC motor from a surplus store and asked yourself at which current and voltage it might work best? Do you believe that the well-known KP00 motor is an "amp hog" and eats up the current more quickly than it should? Then read on; you might find some help and a new point of view.

Abstract

This article describes how to measure and calculate the motor constants of small, brushed, DC motors at home with common and relatively cheap tools and instruments. Furthermore, it describes how to make use of them to choose a good point of operation for an unknown (or known) DC motor i.e. the working voltage and current. It also gives some insight to the physical basics. For this purpose, some high school level mathematics will be used.

What Is A Brushed DC Motor?

The motors we are referring to consist of very few parts.
The Stator: In most cases, it is the outer housing of the motor. It is made of a soft (does not remain magnetized) magnetic material, most commonly iron and contains magnets that are arranged to have one north and one south pole. There are also two bearings, either plain plastic or porous brass bushes or (in more expensive motors) ball bearings.
The Rotor: This either consists of an iron core carrying sets of windings (called poles) or is coreless. In the latter case, the windings are wound by a machine and then fixed by resin such that they stand alone without the iron core. The number of poles is most commonly three, and sometimes (in better motors) five. More poles tend to be used only with bigger motors that are too large for small indoor models.
The Commutator: This consists of two halves; one-half fixed to the rotor, the other to the stator. Its purpose is to route the current from the motor terminals to the rotor windings and switch the current in such a way that the windings of the poles generate magnetic forces that cause the rotor to turn indefinitely. This is the part of the brushed DC motor that makes it brushed. It is the brushes that connect the rotating rotor to the stationary stator.
How do voltage, current, torque and rpm relate?
A DC motor converts current into torque and voltage into rpm. Unfortunately, there are losses. Friction in the bearings must be overcome by an idle current I0. The resistance Ri of the rotor and alternator cause voltage losses, and there are also so-called "iron losses" caused by current in the core iron which is generated by the changing magnetic field in the rotor. In this article, the iron losses are ignored because they are also represented by the idle current (and because they are not calculated as easily as the other losses). Besides that, the often-used coreless motors do not have a core and therefore have much less iron losses, which make them more efficient than cored motors.
Here Are The Fundamental Motor Equations.
The generated voltage of a motor and the rpm have a fixed ratio. It is called the rpm constant :
(1)
The input power of a DC motor is the terminal voltage times the current:
(2)
The output power from a mechanical point of view is rotor torque times rotating speed (in radians per second):
(3)
Here arises a problem, as is difficult to measure the torque. You have to build some sort of test fixture and you need a good balance to measure the counteracting forces. Even if you do the former and already have or are willing to buy the latter, it is difficult to get exact readings. However, there is another possibility. We will not measure the torque, but calculate the output power as the input power minus the losses.
is the voltage that we can measure at the motor terminals. is the motor current. However, not all the voltage counts for the output power, only the voltage generated in the rotor windings according to formula (1) does. a part of the voltage is lost at the inner resistance when flows through it. therefore, we have to subtract the voltage loss across the motor resistance , (which is, according to Ohm's law, ) from the battery voltage:
(5)
The motor torque caused by the idle current is needed to compensate for the friction of the bearings and the alternator; we do not see any torque outside the motor from this part of the current so we subtract it from the battery current:
(6)
Now we multiply the effective voltage by the effective current and get the output power:
(4)
We can now find the output power without measuring torque!
The efficiency is the ratio between input and output power:
(7)
The rpm can be calculated using equation (1) and (5):
(8)
Now we have shown that with the help of the motor constants , and , we can calculate all these values including the . We just need the motor current and motor terminal voltage, both of which are easy to measure. We need a voltmeter and an ammeter. The cheapest and easiest way is to use two multimeters at the same time. These multimeters do not have to be very expensive, but those with digital display are easier to read, and of course, multimeters that are more exact are more expensive than simpler ones. There are also combined instruments available that show voltage, current and input power at the same time; these are a taste of luxury.
There are even more interesting values!
The most important questions when trying out a new motor are...
  • How much power can I expect?
  • At which current will I get maximum power?
  • Which efficiency can I expect?
  • At which current will I get maximum efficiency?
  • Which current should I use?
  • How fast will the motor turn?
All these questions can be answered by help of the motor constants. First, we should decide about the power source, because that will tell us how much voltage we can use. In general, more voltage leads to more efficiency, more power, more current and, worst, more weight. The latter is because we micro-flyers tend to use the smallest available batteries that provide us with the necessary power. Since energy to weight ratio is worse with smaller cells, we always should try to use less cells first, not smaller cells. The only reason to use smaller cells is when we are down to one LiPoly cell only and this one cell is too heavy!
The new star under the power sources is the Kokam Li-Poly cell with 145mAh, giving us about 3.6V under load and up to 1A. About the same can be expected from three Sanyo 50mAh NiCad cells (for a shorter time, of course) or from three GP 120mAh NiMH cells. One of these power sources is generally used in the "1oz-class" (models with an all-up weight of up to ~30g). More voltage needs a DC/DC booster which adds complexity, costs and weight, or more cells, which also adds weight and therefore is used more in the "2oz-class" (all-up weight up to ~60g). (Apart for some pager motor powered models that require a DC-DC and weigh <1oz, Graham) Therefore, depending on the class to which our model belongs, we assume that we have 3.6V or 7.2V and up to 1A.
Maximum Power
A formula for the current at the point of maximum power for a given motor can be derived from formula (4).
(10)
Here I just show the formula, if you are interested in the derivations then you can find it here.
Power, efficiency, and rpm can be calculated using formulae (4), (7), and (8). Just put in the motor constants, the chosen motor terminal voltage and use for .
We should never use more than in any case with any motor because if you pull more than then the efficiency goes down more quickly than the power consumption rises, so we will get less (not more) power. The motor will become very hot, since all the electrical power that is not converted into mechanical power will be converted into heat instead.
Of course, this tells nothing about how long the motor will stay alive at this power level. Formula (10) only tells us at which current this particular motor will have the most output power when used with the assumed voltage. Electrical motors generally have no fixed maximum working voltage, so this has to be answered by experience. As stated above, the motor could quit by overheating, by burning the brushes or by destroying the rotor.
The coreless motors are more sensitive because their rotor has little mass and thus will heat up very quickly, and since it has no iron core, if the resin that fixes the windings melts then the rotor will expand and rub to the outer housing of the motor, generating a lot of friction. In this way, I myself damaged some tiny coreless pager motors.
Maximum Efficiency
The motor current at the point of maximum efficiency is:
(11)
The derivation can again be seen here. We also have a nice formula to calculate the maximum efficiency directly (we could also put into formula (7) for the same result):
(12)
The motor current at full throttle should never be less than because the output power would be very low and the efficiency would be bad. So, we have some nice guidelines on what we can expect from a motor and which current we should try to reach by choosing prop and gear ratio.
The Rules
  • Use at least . If this is already too much power for your model, use a smaller motor.
  • Never use more than . Otherwise, your motor will burn and the output power will be low.
  • It is a good rule of thumb to use about 70-90% of for static current at full throttle for the small motors we normally use.
The output power, rpm and of course the efficiency can again be calculated using equations (4), (7) and (8) in any point of operation, so we will know anything that we need.
But how do we get the constants?
We get the constants by measuring values that are relatively easy to get, and then use some special formulas.
Firstly, we measure the idle current . This is dependent on the motor terminal voltage, so we measure it at about 2/3 of the operating voltage (means: battery voltage) that we want to use later. This is because it will more or less be the rpm level at which the motor will run later with the best compromise of efficiency and power. Measure the idle current without having a gearbox attached to the motor.
Then we choose two different loads that we believe to be near the upper and lower border of the operating range of the motor. In the simplest case, these are the two props that we think are ok but which we do not know which fits better. It does not matter whether the props are directly driven or geared, but in the geared case, of course we have to multiply the measured rpm with the chosen gear ratio.
We take two sets of data. , and with the first prop, , , and with the second prop. We should take the voltage directly at the motor terminals to avoid a measuring error. All values should be taken at the same moment, or it might be easier to use a battery with big capacity because the voltage will then not drop quickly during the measurements.
If we find later that the measured currents are about 30-60% of for the lighter load and about 80-100% of for the heavier load then we guessed right about the two different test loads. If the readings are much different from that then we should choose some other loads (means: props) and redo all our measurements and calculations. Otherwise, our results might be less precise than they could be.
The motor resistance can be calculated as:
(9-6)
...and the speed constant will be:
(8-4)
These formulas are also derived here.

Examples

1). The Kp00 Motor
I have measured the following data at this little workhorse and got the following results.
Measurement with load 1 (bigger prop):
U1 2.10 V
I1 0.66 A
n1 13900 rpm
Measurement with load 2 (smaller prop):
U2 2.08 V
I2 0.62 A
n2 14450 rpm
Measured idle current at 2.1V: I0 0.10 A
We put our values into the formulas and get the following motor constants:
(9-6)
Ri = 1.559 Ohm
(8-4)
kv = 12980 rpm/V
Assuming an average discharge voltage of 3.3V for one Kokam 145mAh Li-Poly cell, we get the following predicted data:
(10)
= 1,108 A
This fits well to the maximum current of the Li-Poly cell. We don't want to go all the way up to the maximum power anyway. (Very Good!)
(4)
= 1.59 W
This is quite a lot of power. A rule of thumb says that the INPUT power of a sports plane should be 50W/kg, So we could expect to be able to power models up to 3.3V*1.1A/50W*kg = 72g. This isn't a micro flyer anymore, right? Right! I would recommend this motor for models in the 1oz-class (around 30g maximum). (good again)
(7)
= 43%
This is a quite high efficiency at the maximum power point of a so small motor (50% is the theoretical maximum value here!), also very good.
(8)
= 20405 rpm
We can only use small props or have to use big gear ratios, since this motor turns fast! The micro scale builders will enjoy this, as most micro scale flyers will be able to use a prop of about scale diameter! If you are searching for efficiency of the prop then you will have to use a big gear ratio. We have all the possibilities, good again.
(11)
= 0.46 A
(12)
= 61%
(4)
= 0,93 W
(8)
= 33524 rpm
Now comes the excellent part. The point of maximum efficiency for this motor at this voltage is still within the area of useable power! Yes, this motor likes amps, but it gives us a lot of torque back for it also! Lighter models (around 20-25g) will be able to stay in the air with this power, and a motor this small performing with an efficiency of more than 60% is outstanding! Yes, we will have to use a high gear ratio to make use of this high rpm.
My conclusion is that the KP00 motor is a very efficient and powerful one if you use a light load (=small prop or high gearing) and let him turn as fast as he wants to. The useable current range reaches from 0.5A to 1A at 3.3V, which is exactly what we want for micro scale models around 1 ounce total weight. My Guided Mite did loops! Yes, I love this little powerhouse! :-)
2) The N-20 Lv Motor
It is available from many surplus sources; mine are from Allelectronics.
I've measured the following data with this motor and got the following results:
Measurement with load 1 (bigger prop):
U1 3.19 V
I1 0.84 A
n1 7900 rpm
Measurement with load 2 (smaller prop):
U2 3.36 V
I2 0.52 A
n2 15700 rpm
Measured idle current: I0 0.12 A
We get the following motor constants:
(9-6)
Ri = 2.592 Ohm
(8-4)
kv = 7803 rpm/V
I've used this motor with a DC/DC booster that happened to deliver 5.2V. We get the following predicted data:
(10)
= 1,063 A
This fits well to the maximum current that my DC/DC booster can deliver. (good)
(4)
= 2.3 W
This is more power than from the KP00. The rule of thumb (for bigger models) says 5.2V*1.06A/50W*kg = 110g. I would doubt that this motor would carry around a model that heavy, but it works at least for up to 60g. (good again)
(7)
= 42%
The efficiency of this motor is slightly lower, but still good.
(8)
= 19075 rpm
We still have all the possibilities. (good again)
(11)
= 0.491 A
(12)
= 57%
(4)
= 1,456 W
(8)
= 30653 rpm
As said above, at 5.2V this motor is not as efficient as the KP00 at 3.3V, but is still good. It is heavier, but it will carry around models that are heavier also.
The N-20 LV performs well at a minimum voltage of 5V. It works notably less efficiently from one Li-Poly cell without booster. It is therefore probably not ideal for the very small models. However, it works well in my models around 2oz. In addition, it is cheap! So, if one ever quits (no one did yet for me), take the next one! It will probably quit sooner if working from two Li-Poly cells, but this should be a quite efficient drive system if you keep the current low enough (say at about 1A maximum).
3) Your Own Motor:
You can download a blank form that can be printed out and filled in with your measured values here. You will need Adobe Acrobat Viewer to read it.

Conclusion

I wanted to show that these calculations are no black magic and that you don't have to grope in the dark when you ask yourself how to make the best use of the tiny motor that you have recently found somewhere. Some predictions are calculated easily, and it is even possible to develop the mathematics yourself.
Bibliography
The following books helped me to understand the theory of DC motors.
  • K. Gieck, Technische Formelsammlung, Gieck Verlag 1981, Heilbronn, Germany
  • Dipl.-Ing. L. Retzbach, Ratgeber Elektroflug, Neckar-Verlag 1999, Villingen-Schwenningen, Germany

Mini Article - Lightening Rotary Servos and the R4P Receiver
By Gordon Johnson

One of the keys to building a successful micro plane is an accurate estimate of the final AUW (All Up Weight). A major part of the AUW for any of our planes will be the receiver, speed control, servos, motor, and battery. A good estimate of what these will weigh allows us to determine in advance whether we will achieve a low enough wing loading for the type of flying we are interested in, indoor or outdoor. This in turn allows us to either (1) scale the plane up to achieve the target wing loading, or (2) select different, and usually more expensive, equipment. One problem we face is that the weights listed by the manufacturers for servos are not the effective weight including wires, plugs, and servo arms. Moreover, the manufacturers don't tell us how much weight we can save by lightening their products.
In the tables below, I give effective weights for common servos and receivers to aid in planning a model. I also give weights after relatively easy to perform lightening techniques. There are more aggressive lightening techniques that I do not cover here. For example, you can lighten a R4P receiver even more by removing the plugs and part of the circuit board and then soldering the wires from the servos directly to it. People have also lightened servos by taking apart the cases and drilling many small holes. If you do a search in the EZone micro forum, you will find examples of aggressive lightening of servos and receivers. My emphasis here is on less aggressive techniques, and I will give a breakdown of the amount of weight saved from each technique.
Now let's look at the table. First, even the very light Westechnik servos weigh almost half a gram more than their stated weights once the wires and plug are included. Similarly, a HS50 is listed by the manufacturer as weighing 5.8g, but is 6.24g with the wire and plug and a servo arm, and a GWS Pico listed at 5.4g weighs 6.20g for the version with a JST plug.
Where do we get the greatest reduction in weight for the least effort? The picture below shows the wires and plugs for (top to bottom) the HS50/Futaba, the GWS/JST, and very thin JST wires/plug from Dave Lewis. Their wire diameters including insulation are 0.7mm, 0.5mm, and 0.4mm, respectively. The "Potential Weight Reduction Breakdown" section of the table confirms that the biggest bang for the effort for the HS50's is replacing the wire/plug for a saving of 0.84g. The wire/plug substitution yields only a 0.35g savings for the GWS.
The next picture shows both servos (the HS50 on the right) after the external wires have been replaced. But, the wires inside the servo are also quite large. However, the HS50 uses very thin wires to the motor while the GWS uses the thick wires for all internal connections. I measured the wire inside the servos and estimated what the savings would be if thin wire were substituted. The savings would only be .07g for the HS50 and 0.12g for the GWS. Ralph Bradley has performed this substitution on the HS50's, and he says the resulting weight savings are generally not worth the effort.
The last two simple weight reduction techniques are to cut off the mounting tabs and leave off the bottom part of the case. Removing the mounting tabs save 0.08g and 0.13g for the HS50 and GWS respectively. This doesn't save much weight, but if the servos are going to be mounted with double-sided cellophane tape, it's easy to remove the tabs. Removing the bottom part of the case is also easy and saves roughly a third of a gram, but will leave the servo open to the elements and the circuit board floating free. This is a matter of choice, and it might depend on whether the servos are going to be mounted internally or externally.
What this shows is that with only a little work a conventional rotary micro servo like the HS50 can get to within 1.54g of the effective weight of the Westechnik "3.0" servo, and possibly closer if the servo arm is shortened on one side and cut off on the other. In addition, one does not have to resort to replacing the internal wires to achieve this. Given that micro planes are clearly moving to using LiPoly cells, many planes could now afford a three-gram increase in weight for using lightened, but less expensive, rotary servos.
Here are a final few notes on replacing wires and the GWS and Hitec servos. Sketch out the circuit board and color of the wires you are going to replace. For both the Hitec and GWS the wire colors correspond to the JST wire colors from Dave Lewis as follows. Red=Red, Yellow=White, and Black=Grey, where the second color is the JST color. When replacing the external wires I shortened the length to three inches since I've found that to be plenty for most applications. The HS50 seems to come with two lengths of wires. I recently purchased four of these from the same mail order house and one of them came with wires that were 6-inches long instead of 4-inches, and the weight was 0.3g higher for that one. The GWS Pico servos with the JST connector, but thicker wires, are available from Dave Lewis and are the standard version, not the ball bearing version. The motor used in the GWS is 7.8mm diameter while the one in the HS50 is 5.8mm diameter. I don't know the weights of the two motors, but this could be one reason the HS50 ultimately maintains a slim weight advantage over the somewhat less expensive GWS servo. Finally, the GWS Pico servos with regular Futaba plugs can be found for lower prices than the ones with the JST plugs. If you know you are going to replace the wires anyway, it may worth shopping around for the best price on the Futaba version.
Receiver Lightening
One of the most common receivers for micro planes is the GWS R4P, because of its low price and relatively low weight. Like servos, it can easily be lightened. The three major types of R4P receiver are shown in the picture below. They are (left to right) the JST plugs, Futaba side plugs, and Futaba end-plugs.
The table below shows the weights and some other receivers for comparison. Most people know they can save some weight by removing the case. However, weight can be saved by choosing the right version of this receiver. The JST plug version is the lightest, which also means the servos will use the lighter JST plugs. Even more weight can be saved by replacing the antenna. Again, different versions of this receiver can have different diameter antenna wires. It is worth checking yours. Better yet, replace it entirely with either a 1/2 length of fine wire or the Azarr antenna. The bottom line is that by choosing the GWS receiver with JST plugs, and then going with a litz wire antenna and no case, you can save about 3.3g over the heaviest version.

Mini Article - Actuator basics part II (Driving the actuator)
By Graham Stabler

I have brought this part of the planned (yeah right) series on actuators forward to the second issue as I have received several emails and have seen just as many messages appear on the boards asking if it possible to use these devices with standard receivers and if so, how? Therefore in this mini-article, I will attempt to explain how we can use actuators in practice and a little of the theory behind how they are driven for those who are interested. I will cover only proportional control, as it is likely that when bang-bang (on/off) control is used it will be with a system made just for that purpose.

A Recap

Let us look back at what we learned last month. If you place a magnet in a coil, and current flows through the coil, torque is applied to the magnet. This torque is proportional to the current. That means as the current increases so does the torque. This in turn means that in order to control the torque generated by an actuator we must be able to control the average current flowing through it.
The most common way to produce this controlled current is to use PWM (Pulse Width Modulation) this is what is used in most speed controllers. The way it works is quite simple; imagine standing on the edge of a dam with your hand on the outlet valve while listening to the ticking of a clock (as one often does). On each tick, you open the valve and wait until some percentage of the time between ticks before closing it again. If the percentage is 100% then the valve stays open and if 0%, it stays closed. Between 0 and 100%, the flow pulsates. Now if someone had built a flourmill downstream of the dam, they would be much happier as the percentage of the time the valve opened increased, as this would mean the average flow of water would be larger.
Obviously, there is no room for dams and clocks in our models but the system works in the same way. The valve is a mosfet, a transistor, or similar electronic switch and the poor guy with the timer is a microprocessor or similar piece of electronics. For those who are interested, the fraction/percentage of "on time" is known as the duty cycle in electronic circles.
The added complication of an actuator drive over a speed control is that it must be bi-directional, i.e. the current must be controllable in both directions so we can go left AND right. So just to recap, with the stick in the middle the PWM duty cycle is 0, if we then move the stick to the left and the duty cycle increases, as this happens, more torque is applied to the magnet/control surface. Moreover, the same thing happens if we move the stick to the right, except in this case the current flows in the opposite direction.

Variations

Some systems drive the coils straight from the microprocessor without the mosfet/transistors; other people flip the current back and forth. The PWM is a little different in this case, because the current is flowing one way or the other, rather than being off or on in a particular direction. This means that with left stick, it is mainly pulsing left and with right stick, it is mainly pulsing right. At neutral, it spends half the time between "ticks" left and half-right. You would think that this would cause the rudder to oscillate but the high frequency of the clock and mechanical damping of the rudder mean that it cannot be seen. The main advantage of this drive system is ease of building drive circuitry; the disadvantage is the constant current flow even with no rudder/elevator deflection.
Another more subtle variation is to smooth the PWM before it is amplified and fed to the actuators. This results in a constant current being applied to the coil and makes the operation silent. The only disadvantage it could be argued is the higher component count and the inherent lower efficiency of linear amplification. This however is not a problem in practice.
Very nice, but what do I buy, beg, borrow, or steal?
To actually use actuators, there are essentially two routes to go down.
  1. A dedicated receiver
  2. A piece of hardware between a standard micro receiver and actuator coil
In the case of a dedicated receiver, you have a couple of choices presently, the RFFS-100 by Dynamics Unlimited, the Micromag, to be made by FMA direct in near future, the Ztron system now out of production effectively but still around (www.aeronutz.flyer.co.uk), the new infrared system by Didel and Nick Leichty's ultralight system.
In terms of add on boards often know as DPCs (digital pulse converters), there are also a few options, such as Cloud9 for single coil drivers and Bob Selman for single and dual coil drivers. If there are others out there, drop me a line for a mention.
I'm Too Poor.
There are also options for the experimenter. You could look at Andy Birkett's website, there you can download info on building a programmer for PIC microprocessors as well as the software required to drive two actuators and a mosfet for speed control. This chip can then be connected to a standard but lightened micro receiver. In this case, the microprocessor is not used to drive mosfets / transistors but rather the coil is driven directly from its outputs. The current available is small (~25mA) so it is worth mentioning that as it will only drive coils of around 200Ohms (not less) but these are fine for sub ounce models. Actually, Andy has used 130-ohm coils without ill effect, but be cautious.
Another option is to cannibalize a micro servo. Take the servo and remove the small circuit board from inside. Then disconnect the motor and connect your coil where the motor had been connected. Then de-solder the potentiometer; this is a circular device with three pins and a shaft that rotates with the output arm of the servo. In its place, either solder two 2.5kOhm fixed resisters or a 5kOhm surface mount potentiometer. This will control an actuator proportionally because a servo drive without the feedback works like a bi-directional speed control. Replacing the original feedback with fixed or surface mounted resisters just makes it lighter and allows the current flowing through the coil to be set to zero when the stick is centered (will require tweaking if a pot is used). Unfortunately, most servos will draw about 100mA even when the coil is disconnected so they are hungry little things. That said they would work well with a rudder only model in the sub 2oz range especially with the new Lithium Polymer cells available. Here is a diagram showing the wiring, the important thing to know is that the potentiometer has a middle pin. This is the slider, it is important to make sure that you connect this wire to the slider of your replacement potentiometer or in-between the two fixed resisters.

Conclusion

There is more than one way to skin a cat but all the methods of driving a coil will help to produce a light model that keeps a smile on your face.

Mini Article - Super Cheap Wattmeter
By Graham Stabler

I first built one of these about a year ago when testing some DC-DC converters. In that instance, I wanted to know input power and output power to gauge efficiency. To do this I needed to measure current and voltage at the same time because power = current x voltage. I already had a couple of meters lying around and so I used them. Then after an hour of grappling with wires and clips, I yearned for one of those lovely devices that many use to measure power into their electric drive trains. There are several available, including the AstroFlight wattmeter. They are in no doubt highly useful but I really needed two of them and I had other things to spend my cash on at the time. So I bought some super cheap multimeters (5GBP/8USD for a pair on special offer) and set about making them a more convenient proposition for measuring current and voltage simultaneously. What I did was to make a simple circuit board with 4mm banana plugs attached that could be plugged into both meters at once. This made the simple circuit of one meter in parallel with the load and the other in series, the former measuring voltage, and the latter current, while also getting rid of several leads, and turning the two meters into a single "super" unit. Here are the steps.
1. Procure some copper clad PCB material (fiberglass board with copper on one side), the normal FR4 1.6mm stuff is ok, it is not that critical for micro applications. You only need a small bit so scrounge some like me.
Here is what you need.
2. Work out which hole is which on your meters and draw out your "circuit" on the copper clad using a pencil or pen. Also, mark out the holes for the banana sockets.
Here is the circuit that needs to be made. (Diagrammatic only!)
3. Drill the holes and remove the copper from the surface of the board where the lines were drawn.
Here is the copper clad after it has been engraved using my Dremel drill, you can of course etch it, scratch it or even use a different make of drill but the main thing is that the copper is removed between the islands that make the circuit.


Now the holes have been drilled, make them a little oversize unless you are very accurate.
4. Put the sockets in the board and insert into the meters, then solder them in place, be gentle, as you do not want to damage the meters.
Here you can see the board in place and soldered. Notice I have left a few holes blank. The top left hole is redundant but I thought it might make it more durable and hold the meters together, in the end I did not bother. Top right is the output from the meter for the low current range, if used as the positive output you can do sensitive measurements, as yet I haven't needed to.
5. Have a cup of tea.
To hook your battery/motor/dc-dc/electronic gubbins to the meter you have a few options. You can solder the wires direct for experimentation, Add flying leads with crock clips or even use special banana plugs that also include a socket in the top allowing a selection of leads.

Conclusion

Takes 5 min to make, saves clutter, costs pence/cents/groats. Add a web cam and you have a data capture system BUT the downside is you have to do the multiplication yourself to get the power.

Micro Tip - Rapid Prototyping in the Microwave
Courtesy of Peter Frostick

This little tip comes from Peter Frostick and describes his method of making small propeller blades FAST. I'll let the picture do the talking but it basically allows you to test lots of options quickly. Anything you can carve into a block you can mould. Balsa blades are ideal for floaters (slow flying models) and free flight models as well as the smaller electrics, guess that's most things then. You will need to make a hub of some sort from balsa or bamboo and glue the blades into that. More to come on prop making including hub manufacture and we have a few articles on carbon fiber lined up as well for the high-tech inclined.

Mini Article - LiPoly Voltage Monitor
By Gordon Johnson

When we first started flying models with single LiPoly cells, we all wondered how we would know if we were discharging the cell below its 3-volt limit. Discharging below 3 volts should be avoided and the cells should definitely not be discharged below 2.7 volts, which could cause permanent damage. In both cases, these are volts under load. At first people thought the plane would "land itself" because of insufficient power to keep it airborne and that it wouldn't be a problem. However, a number of people have been developing airborne systems to monitor voltage and signal when it drops below 3 volts via a LED. Bob Selman and Dynamic Web enterprises are both selling voltage monitors made by the same source. Bob also has a two-cell monitor that signals when the volts drop to 6v. Dave Lewis has recently introduced a single-cell monitor and will soon introduce one for two cells. And, Abbott Lahti has developed one that blinks when volts drop below 3 volts, and blinks faster when they drop below 2.7 volts. (This one will probably be available from Cloud 9 RC.) The monitor sold by DWE and Bob Selmanis shown in the picture below.
I installed the Dave Lewis voltage monitor (shown in the picture below) on a small Depron plane; I built to use the RFFS-100 and a geared M20-LV motor. I knew that this motor would pull about 0.75 amps out of the single Kokum 145 cell, so I thought this would be a good model to test the voltage monitor. I removed two unisex pins from their plastic housing. These plugged perfectly into the power lead pins on the RFFS-100. Then, I soldered the leads from the voltage monitor crosswise on the outside of the pins. This allows the pins to be plugged into the RFFS-100, and then the leads from the battery to be plugged into these pins. I mounted it on the LE of the wing pylon, where it would be easy to see, with a piece of foam sticky tape. An alternate position would have been to let it dangle a short distance below the fuselage.
Using the Dave Lewis voltage monitor I found that when my plane was flying noticeably slower, but still very flyable, the red light on the voltage monitor would go out indicating I had dropped below 3 volts. Most of the time if I throttled back to just barely cruise speed, the light would come back on as the load decreased, indicating in an increase in volts. Other times the light would not come back on until I landed the plane and taxied it back across the floor at low throttle.
What this tells me is that for my planes that have higher current draw propulsion set-ups I may want to consider a voltage monitor if I intend to fly them as long as possible. An exception is my planes with a Sky Hooks and Rigging hybrid receiver that can be set for a soft 1/4 throttle cut-off at six volts when using a two-cell pack. In any case, micro fliers may want to consider the amp draw of their propulsion system and their flying style and decide if a voltage monitor makes sense for them.

The Last Bit

This is the end of this month's installment. Keep the suggestions coming in and lets have some photos of what you lot are up to, the wackier the better. It struck me while sticking this issue together that one thing we are lacking is models. In fact, I don't think we have had a single picture of a complete model. I would like to do some plan features so any budding designers out there get drawing. You know who you are! Also, you will have noticed how simple my second mini-article and Peter's microtip was, so why not have a go if you have something useful to share.
Happy flying,
Graham

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