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REMOTE CONTROL BY BOB YOUNG Large servo amplifiers for model yachts & machinery This month, we will move on to some special applications of large servo amplifier technology. These are used for the remote control of machinery and heavy vehicles, although their principles of operation are similar to those used in model aircraft. Many areas of model R/C require mechanical outputs which are quite different from the normal servo which delivers a rotary output restricted to approximately 100 degrees of movement and 1-3kg of thrust. These outputs include large servos for quarter scale aircraft (or bigger), robotic puppets and sail winches, plus speed con- trols for electric propulsion and remote switches. In one special case, where I radiocontrolled a full size Volkswagen 1600 TLE, I used servos which delivered 30kg of thrust. Thrust in this case is defined as the actual force measured on a spring balance from the output arm used (I find this a little more Fig.1: this servo is designed specifically to serve as a winch for model yachts & features a multi-turn output for hauling in the sheets. descriptive than the usual kg/cm). The servo motors in this last case were Valiant windscreen wiper motors which had to be forced cooled from a separate blower. As Hages would say, "now that's a servo". It was in the Volkswagen job that I discovered the problems ofunderpowered servos in which compensation is made by recourse to lower gear ratios. The result is a servo of adequate thrust, but uncomfortably long transit times. In a vehicle designed to travel at speed, long transit times on the steering and brakes make for some hair-raising experiences. I remember having a chuckle when the Apollo astronauts complained about the difficulties they had in steering their moon buggy. They had a seven second transit time on the steering from memory, on a vehicle designed for about 10km/h top speed. The script on the Volkswagen commercial called for a top speed of 80km/h so that it could overtake the · camera car and pull away into the distance. Because of the tight time allowed for the job , I found to my horror that I had to use a gearbox which resulted in a 4-second transit time on both the brakes and steering. The difficulties in steering at · this speed were indescribable. On one occasion, I cut in too early on the camera car after overtaking it and the driver braked too heavily. As a result I fell off my seat, dropped the transmitter and had to go looking for it. It the meantime, the Volkswagen, given its freedom, had taken off across the nearest paddock, dragging a barbed wire fence behind it. Volkswagen had that car back on the road the next morning, comp lete with a new set of AUGUST 1991 33 sistors. If these are poorly selected, it can result in downgraded servo power because some of the power is lost as heat. The photo of Fig.2 shows what is essentially the same servo, this time fitted with a double drum output. This is very useful in model yachts, as usually there are two sails which require trimming: the jib (or little sail close to the pointy end) and the mainsail (or large sail at the blunt end). One usually tries to sail the yacht with the pointy end heading more or less into wind. The degree to which the yacht can point into wind is a measure of its overall efficiency. To achieve this requires good winches of great power and accuracy. Circuit description Fig.2: this servo is basically the same as the one shown in Fig.1 but features a double drum output. This is very useful in model yachts, as usually there are two sails which require trimming: the jib and the mainsail. passenger side body panels, and the commercial went to air in due course. Such are the joys of commercial TV work. Winch servos The photo of Fig.1 shows a special type of servo designed specifically as a winch for model yachts. Note the unusually large size of this servo, which is not under weight restraint to the same degree as an aircraft servo. The main criteria for this type of winch is output power and multiple turns on the output drum. The multiple turn output is required because the drum is used to haul in the sheets (sheets being the ropes, just to confuse non-yachties). The number of turns is usually about 8-12, depending upon the yacht size and servo power. As a matter of interest, we used to test our winches by lifting a bucket off the ground. If the winch passed the bucket test, we felt it was OK. No big deal did I hear you say? Perhaps I should have mentioned that the bucket contained two house bricks, each weighing 4kg. This was the thrust necessary for an "A" class yacht working in heavy weather. There are also some very large servos designed for large model aircraft and these may be readily converted into a "BAR" type winch. By using a large output arm , the amount 34 SILICON CHIP of throw is sufficient to provide the necessary travel to position the sails in the correct location. This overcomes the problem of the sheets becoming tangled or slipping off the drum. Another consideration in setting up a winch is the transit time or time taken to haul in the full length of the sheet under heavy conditions. There is little point in using the drum as the major source of the mechanical advantage by the simple expedient of using a smaller diameter drum, because this only increases the time required to complete the increased number of revolutions. The speed of manoeuvring when rounding a buoy is dependent upon the speed with which the sails can be retrimmed and long transit times can be costly in terms of race times. Thus, we soon arrive at the conclusion that servo power is the all important factor in winch design and servo power begins with motor size, particularly armature diameter and winding resistance. In fact , the motor more or less dictates the servo size and the servo is thus designed around the servo motor. The servo illustrated in Fig.1 uses a 5-ohm 26mm motor and provides plenty of grunt when fitted with an amplifier capable of delivering the required amount of current. Another important consideration here is the voltage drop across the output tran- The diagram ofFig.3 shows the circuit of a typical winch amplifier, based on the popular NE544 servo amplifier IC. This is fitted in turn with a power amplifier to provide the current required by a 5-ohm motor. The NE544 can drive a small motor direct provided it draws less than 300mA but anything over this requires additional amplification. In the amplifier shown in Fig.3, VRl is a 5kQ feedback pot which is mechanically coupled to the output shaft of the final gear on the servo drive chain. R14 & R15 provide travel adjustment and wiper centring. This mechanical coupling can be in the form of a reduction gear set to provide the required number of turns on the output drum. Fine adjustment of the number of turns can then be obtained electronically by varying the values ofR14 & R15. A similar result can be obtained by adjusting Rl 0. However, this will also shift the neutral point and there is no easy way to readjust the neutral, although R14 & R15 can be used to some effect for this purpose. Input capacitor C7 provides DC isolation and C8 is the pulse stretcher. Be sure that only a good quality barrier ceramic is used for C8. Do not use TAG tantalum capacitors in this location. C4, C5 & C6 are all critical to temperature and therefore use only TAG tantalum capacitors here. Do not use low voltage ceramics. Transistors Q1-Q6 form a bridge driver amplifier for the IC output. Note that a separate battery may be used to power this amplifier if re- RX, MOTORV+ 04 11 2TX753 10 C9 0.1 12 MOTOR C7 1 IN~_ 4 IC1 NE544 Cl .022 R4 150k 14 R2 1M 5 3 03 2TX653 R15 W 2.2k owo R12 150k MOTORV- Sk R10 C5 .022 CB 0.22 VR1 C6 .022 R14 22k W2.2k W :WINCH OW : DOUBLE WINCH owo Rxv~ Fig.3: this diagram shows a typical winch circuit, based on the popular NE544 servo amplifier IC. In this circuit, IC1 drives a bridge amplifier circuit (Q1-Q6) to increase the current drive to the motor. The NE544 can also drive a small motor direct, provided it draws less than 300mA. quired. This is a good idea for, in many cases, sheets can tangle, stall the motor and thus flatten the battery. If the receiver is running from this battery, then control is lost and a long swim is called for. As an additional safety feature, a fuse may be installed in this secondary battery to protect the power amplifier and servo motor. These things sometimes take a long time to repair and can end up being quite expensive. There is also an element of flexibility added to the design as a result of using a separate battery, as the drive voltage to the motor can be increased without the need for voltage regulation on the servo amplifier. Feedback resistor R2 provides the damping required to prevent overshoot and oscillation around neutral and is a critical adjustment. Too much damping and the servo shuts down too early. Also, the neutral is broadened and thus the servo is less accurate. The worst case here is that the servo comes to rest before neutral and leaves a residual current which may be just below the motor start current. This can be quite high in some motors and overheating of the output transistors can occur. Conversely, too little damping and the servo overshoots and oscillates before corning to rest, again increasing current consumption and heating the transistors and motor. In the worst case, the servo never comes to rest and continues to oscillate, which will quickly destroy the output transistors and motor. The ideal situation is called "deE).d beat" and is difficult to obtain. The ------------------V+ 11 CS IN~l_ 4 01 BC327 U:1 N~S44 14 C4 0.1 C2 R3 150k VR1 Cl 0.1 Rl 18k 0.1 CJ 0.1 C6 0.22 ,. Fig.4: this simple circuit can be used to drive a relay for those odd jobs that require a "momentary on" contact but can also easily be modified for press-on press-off operation. most accurate situation is to allow the servo in its unloaded condition to overshoot once and then return to rest. When the servo is loaded mechanically, this arrangement will perform very close to dead beat. C9 is a suppression capacitor, to eliminate motor noise. Sometimes a resistor four to five times the motor resistance is used in parallel with the motor as well. The motor noise can be quite troublesome and difficult to get rid of with some brands of motor. In this case, use a capacitor from each brush terminal to the motor case. Do not ground the case for sometimes the armature winding can short out to the case and then bang goes the servo amplifier. As a last resort, RF chokes may be fitted in series with each motor lead but make sure they can carry the starting current of the motor. Fig 4 shows the circuit of a simple switch used to drive a relay for those odd jobs around the model which call for "momentary on" contact. Horns, whistles, dropping bombs and waving pilots all call for this type of circuit. By replacing RLl with a latching relay, a "press-on press-off" output can easily be obtained. And although I hav~ never tried it, I see no reason why RLl cannot be replaced with a bipolar relay and driven directly from the NE544 bridge, giving a true toggle switch output. Once again the old faithful NE544 is called into service. Note that there is no requirement for a gear driven feedback pot here and so VRl is now a standard trimpot. SC A UGUST 1991 35