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REMOTE CONTROL BY BOB YOUNG Installing & adjusting the low-cost speed controller; Pt.3 Installing the speed controller is a matter of connecting it to the motor & receiver, & wiring the all-important suppression components across the motor. The speed controller must then be fine-tuned to match the transmitter & receiver. Last month, we ended by saying that the spark suppression components must be mounted directly on the motor terminals despite the fact that this is usually not very convenient, due to space restrictions in the model. Also, soldering three or four leads to each motor terminal can be difficult and frustrating at times, so why do it when some commercial speed controllers have these components built right into the speed control module? The laws of physics were written by a much higher authority than Bob Young and these laws state that the radiation from a spark gap transmitter (commutator) must be killed at the source. If this radiation is allowed to reach the motor interwiring, then it is all over. Marconi transmitted across the Atlantic with a spark gap transmitter and remember, you have a receiver antenna within just a few centimetres of this spark source. Much of the commotion over opto- coupling in speed controllers is largely a result of poor installation practices regarding this point. There is no need for optocoupling if the installation is carried out correctly. Antenna range tests carried out with diodes fitted in various places in the installation, and with the varistor fitted and deleted, all indicated quite clearly the importance of fitting these items directly at the motor terminals. The range decreased as the varistor was moved further away from the motor terminals. Leaving the diode off results in the FETs running extremely hot very quickly and some or all of the FETs being damaged. Make sure that the diode is installed with its banded end towards the positive battery lead. The varistor and capacitor are non-polarised. Ifby chance the motor spins in the wrong direction for your prop or application, reverse the wiring at the motor terminals and don't forget to reverse the diode! Finally, keep all of these component leads as short as possible. One point not mentioned so far is that the suppression diode must be a fast recovery type capable of operation at the full motor current. A Schottky MU810 or similar will do the job nicely. Fine tuning One of the test beds used for the SpeedlB has been this twin engined model. The controllers were mounted as close as possible to the motors, while still being housed in the main fuselage. 66 SILICON CHIP We now come to the interesting bit: fine tuning the motor/speed control/ radio combination using VRl, VR2 and VR3. You will need a tachometer (eg, the optical tachometer described in the May 1988 issue), a moving coil ammeter and a voltmeter (multimeter). It is also desirable although not essential to have access to an oscilloscope and a frequency counter. (Editor's note: you cannot use a digital multimeter to monitor the current unless you know that it has a response which is flat to 3kHz or more on the DC current range. Nor will a moving iron ammeter do the job since its response tapers off quite rapidly above 100Hz. That leaves a conventional moving coil meter as the only choice. Such a meter will respond to the average value of the current and not the RMS value but, for the purpose of this test, this should not be a problem. You can distinguish a moving iron meter from a moving coil meter by the fact that it does not have a linear scale - ie, it is cramped towards zero - and it has no polarity markings). Due to the fact that not all radios use a standard pulse neutral, pulse width variation or pulse frame rate, some adjustment must be made in the speed control to match these factors. For example, the old Futaba 2-channel sets used a 1.3ms neutral, 0.71.9ms pulse width variation, and a 14ms frame rate. The new Futaba and J. R. PPM sets use a 1.5ms neutral, 12ms pulse width variation and a 20ms frame rate. The standard unit as delivered is set to 1.5ms neutral, with variation between 1-2ms. The frame (repetition) rate will often va:r: with the number of channels and some sets use swinging frame rates which keep the sync pause constant and thus deliver the fastest possible system response time. All of these factors influence the speed controller performance to some extent. For example, the frame rate will have some affect on the voltage tripler. 1 Throttle settings That said, let us proceed. First, the travel direction and the 0V DC condition must be set with the throttle stick at the low throttle position. This is usually done with the trim lever in the mid position to allow for small variations over time. Switch on the transmitter and set the throttle to low, with the trim lever in the mid point. The usual convention in Australia is for full throttle to be applied by pushing the throttle stick forward (to the top of the Tx case). If you have an old transmitter with no servo reversing switch, the flexible wire jumpers between JlA, JlB, J2A and J2B need to be correctly connected. If your radio gives an increasing pulse length with increasing throttle, connectJlA to JlB This photo shows the test setup used to measure the change in efficiency at part throttle for various switchmode frequencies. No definitive answer emerged to give a best frequency for all conditions. Instead, there tends to a broad range of frequencies that will best suit a particular motor. and J2A to J2B as shown in Fig.2 (p.41, Dec. 1992). If your radio gives a decreasing pulse length with increasing throttle, connect JlA to J2B and JlB to J2A. If you haven't the slightest idea what your radio does, jumper the pins as shown in Fig.2 or use it as deliveredyou'll have a 50-50 chance that this is correct. If the throttle works backwards, you've got it wrong and the leads should be swapped over. With the speed control connected, restrain the model and switch on the Rx, making sure that you are well clear of the propeller. If the motor leaps into life, check to see if moving the throttle to full power slows the motor. If this is the case, reverse the direction of the throttle with the Tx switch or the PC board jumpers Jl and J2. All being well, the motor will sit quietly, eagerly awaiting your first command. Adjust VRl until the motor begins to emit a tone and then back offVRl slightly, until no noise is heard. If your scope is hooked up to the common gate line (ie, emitter of Q2), very narrow voltage spikes will appear just before the motor noise is heard. Set VRl to deliver a dead flat trace, with no sign of switching. As the throttle lever is gradually advanced, the gate pulses will become wider and current will begin to flow. At some point, depending on the number of poles in the motor, the resistance of the windings, motor loading and a host of other factors, the motor will start to turn slowly. From that point on, the throttle response is very smooth and linear, giving excellent control over motor revs. Throttle sensitivity Now push the throttle all the way forward, listening to the prop noise or observing the instruments to ensure that after about 7/8ths of the stick travel, there is no further increase in motor RPM or voltage at the gates. If a scope is being used, the gate pulse width should increase to the point where it is pure DC (about 14V) by 7/8th throttle. If this is not the case, and full stick travel still leaves the gate voltage in the switching mode, VR3 must be adjusted to increase the sensitivity of the throttle range control. Alternatively, if the throttle range control is too sensitive, VR3 must be adjusted to soften the range control. By the way, while VR3 is shown on the overlay diagram in last month's issue, it did not appear on the circuit diagram in the November 1992 issue. This was_a late change, made in response to requests from a number of enthusiasts, and has been included to give optimum throttle response. The change has been made by altering the value ofR2 to lOkQ and then connecting VR3 (50kQ) in series with it. The smoothest results are obtained by using the full stick travel to achieve full throttle. Using VRl and VR3 in JANUARY 1993 67 Another view of the twin-engine model (shown here partially completed) that was used as a test bed for the Speed 1B speed controller. A critical function in twin-engine models is that the prop speeds must be matched over a wide range. conjunction with each other will give end point adjustment. Switchmode frequency Having completed the above, we are now ready .for the fine tuning of the speed control switching rate. This is the most important part of the whole project. As you may recall, the rationale behind the design of Speed 1B was to allow it to be matched to the motor parameters. It was my belief that maximum efficiency would be obtained at a particular switching rate, depending on a myriad of variables in the motor design and application requirements. Such factors as winding inductan"Ce, motor capacitance and commutatio.g speed (which in turn is influenced by prop or gearbox loading) all contribµte to a complex and highly interactive chain that determines system efficiency. At the time, this was only a hunch and I had seen nothing in previously published work to confirm this feeling. After extensive testing, I can now publish the results confirming this premise. In this regard I owe a very large debt of gratitude to Barry Younger who worked flat out to provide the test facilities and who did much of the actual testing and tabulation of the final results. Put briefly, improvements of up to 25% in efficiency can be obtained by careful selection of the operating RPM and switching rate. This result will vary from motor to motor and with RPM on any one motor. The process of selecting a switching rate is divided into two distinct phases: (1) full throttle (DC mode); and (2) partial throttle (switchmode). The first step is to push the throttle to full power and monitor the RPM while swinging VR2 through its full range. According to theory, the RPM should not vary over the full range of VR2, as at full throttle the Mosfet gates are being fed pure DC. However some Table 1 68 FETType Switching R,ate Gate Peak Volts Gate DC BUK455-60A 540Hz 9.5V 13.SV J3UK455-60A 825Hz 9.5V 13.SV BUK455-60A 1.1kHz 9.5V 13.SV BUK455-60A 2.7kHz 6.8V 13.2V BUK455-60A 12.6kHz 3.1V 12.8V SILICON CHIP speed control/motor/radio combinations we have tested have exhibited a best switching rate at full throttle and this is a puzzle. The only explanation I can offer is that the voltage tripler is peaking in output at some frequency, yet the gate voltage does not appear to vary to any appreciable level that I can detect. As stated many times in this series of articles, the subtlety of the various interactions between system components is quite confusing. The second and more understandable phase is tuning the mid-throttle ranges. Here the switching rate, at least according to theory, will have a very definite effect. There is a trap to be careful to avoid in the partial throttle test and it threw me off-track for some time during early testing. Part of the reason for the long delay in this project was that I could not decide what was the best frequency to use and even now I still cannot give a definitive answer to this question, for the simple reason that there is no best frequency for all applications. Each application has its own ideal rate, hence the need for a variable switching rate. What we are trying to establish here is the point of maximum efficiency of the motor/speed control/radio combination in the mid-throttle ranges; ie, in the switching mode. Once again I must point out that once the throttle is fully advanced, the whole argument about switching rates becomes academic, because the speed controller moves out of switching mode and into DC mode. The switching rate then has absolutely no bearing, or at least it should have no bearing, on gate voltage :-- apart from the anomaly noted above. For those who missed the earlier articles, there is a long-standing argument in electric power modelling circles over the merits of 50Hz or 2.5kHz switching rates. The argument runs that controllers using the 2.5kHz switching rates are less damaging to the motor magnets, use less current and run cooler. They are also much smoother in use and the 50Hz controllers are disparagingly referred to as "rattlers". The main problem with the 2.5kHz controller is the component count. This is much higher than in a 50Hz controller which uses the receiver input pulse as the master clock. Thus, a Kit prices & availability Kits for the Speed1 B Speed Controller are available in a number of configurations and prices, as follows: (1 ). Surface mount PC board with all components installed, including trimpots but without Mosfets, together with an' unpunched case ..... $69.50 (2). Complete unit, assembled and tested, includes servo lead, punched case and with 8 SMP60N06-18 FETs fitted .. ................................. $175.00 (3) . Complete unit, assembled and tested, includes servo lead, punched case and with 4 BUK456-60A FETs fitted ... ................................. $129.00. Servo lead (depending upon brand) .................................................. $5-7. SMP60N06-18 FETs ................................... ........ .. ...... .. ... .. .. ............. $9.50 BUK456-60A FETs ................ .. ................................ ............... .. ......... $6.50 Graupner ECO 600 non ball-race motors .. ..................................... $28.00 Graupner ECO 600 88 ball-race motors .......... .. ... .. ...... .. ............... $49.00 Post and packing for all the above kits is $2.50. Payment may be made by Bankcard authorisation of by cheque or money order payable to Silvertone Electronics. Post orders to Silivertone Electronics, PO Box 580, Riverwood, NSW 2210. 2.5kHz controller is more bulky and more expensive and so the argument rages on, with both schools quite vocal about their point of view. To complicate matters, the microprocessor controller arrived, reducing the component count but introducing software and service problems. After months of testing in the early days of researching this project, I became more and more confused as I went deeper and deeper into the argument. To make matters worse, I could never seem to obtain a repeatable set of results. What I did find was that the inter- $ 5 99 For many years you have probably looked at satellite TV systems and thought "one day". You can now purchase the following K-band system for only: actions between the switching rates, FET gate input capacitance, motor commutation rate, motor loads and a host of other factors were so complicated that a logical analysis was almost impossible. In the end, I decided that the answer was a variable switching rate design which would allow the user to fine tune the unit to his combination. Table 1 gives some idea of the parameters of the Speed 1B with various switching rates. Notice that the peak gate voltage is starting to roll off at 2.2kHz with this particular set of FETs. By 12.6kHz, the gate input waveform was a virtual Here's what you get: • A 1.6 metre prime focus dish antenna, complete with all the mounting hardware. • One super low-noise LNB (1.4dB or better). • One Ku-band feedhorn and a magnetic signal polariser. • 30 metres of low-loss coaxial cable with a single pair control line. • lnfrared remote control pre programmed satellite receiver with selectable IF & audio bandwidth, polarity & digital readout. Your receiver is pre-programmed to the popular OPTUS transponders via the internal memory. triangle of 3.1 V and the FETs were starting to heat badly at 3/4 throttle they weren't being turned on hard enough. Interestingly enough, the unit still worked well, with a very smooth throttle response. There was no heating in the tests conducted up to 3.5kHz, despite the fact that the peak gate volts had dropped below 6V at this point. To return now to the trap mentioned above, it involves the method of testing. If we regard the real measure of efficiency as the ability to move a given load with th8 minimum of energy, then testing should proceed as follows. Set the throttle at some given point (half way, for example) and measure the RPM and note this figure. Now, while monitoring RPM , tune VR2 through its complete range, looking for the lowest possible source/drain current that will deliver this RPM. In fact, at some point the RPM may increase at the same current draw, and this is what makes this test so confusing. In this case, back off the throttle until the target RPM is once again obtained and note the new current, which will of course be lower due to the reduction in duty cycle. On one unit tested, this point occurred at about 1200Hz. Most units tested exhibited a minimum 5% increase in efficiency at the best frequency, a useful figure. Finally, do a range test before flying the plane. The radio should not be significantly affected by running the motor at any speed. SC AV-COMM Pty Ltd, PO Box 225, Balgowlah NSW 2093. Pb: (02) 949 7417. Fax: (02) 949 7095. All items are available separately. Ask about our C-band LNBs, NTSC-to-PAL 9onverters, video time date generators, FM2 & EPAL & Pay TV hardware. r,--------------~ I YES GARRY, please send me more information on K-band I satellite systems. I Name: _ _ _ __ _ _ _ _ _ _ __ I Address: _ _ _ _ _ _ _ _ _ _ _ __ : _ _ _ _ _ _ _ _ _ P'code: _ _ __ I Phone: _ _ _ _ _ _ _ _ _ _ _ __ 10/91 I ACN 002 174 478 ]ANUARY 1993 69