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Ein Servo Mit Most hobbyists would be familiar with the little servos used to control model planes, boats and cars. They’re fine if that’s all you want to control. But what if your application calls for a servo with industrial-strength muscle? That’s when you need our new, B-I-G, powerful, industrial-strength, Jumbo Servo. Y our typical model servo is capable of very fine adjustment over a range of about 90° or so. It measures about 40 x 20 x 35mm, weighs about 50g and has a torque somewhere around 5kg-cm (some a bit more, some a bit less). Our new “Jumbo” servo is also capable of very fine adjustment over a 90° range. It comes in at 180 x 110 x 110mm, weighs about 1300g and has a torque somewhere in the kg-m range (no, we couldn’t measure it!). Suffice to say it’s a tad more than “typical” model servos! Possible applications What on earth would you want that sort of muscle for? Here are just a few applications that we thought of – you can probably think of many more (in fact, right now there are readers throughout the South Pacific thinking “at last! Now I can…..”). •  Robotics – no longer are you limited to piddly little designs. Build a monster! •  Radio control of large (eg, 1/4-scale or even bigger) models – steering, brakes, etc which require some real power. •  Remote (as distinct from radio) control (ie “fly by wire”) in real boats, cars, etc – eg, the rudder, trim or even throttle control without the usual mechanical linkages. •  Rotator for a radio or TV antenna; even a satellite dish azimuth/elevation positioner. •  Remote gate or door controller. 68  Silicon Chip •  Heavy duty pan or tilt controller for a remote camera or camcorder – eg, unattended wildlife photography or surveillance work. • Remote (or even local) electronic control of valves or flow control devices, especially if they are in hazardous areas. •  Flue, vent or high hopper window openers/closers. • Remote winch or sail furling on a real yacht (you add the hardware!) •  Perhaps (obviously with additional electronics) even navigation control with feedback from a GPS unit (as published last month in SILICON CHIP). We’re sure we have merely scratched the surface of ideas for this one. It’s one of those projects that is a solution waiting for an application – and there are literally countless applications. Servo control The vast majority of servos sold today are designed to operate to a somewhat standard 1.0-2.0ms pulse width on a 20ms (+/-) frame rate. At centre, the pulse width should be 1.5ms. Increase the pulse width and the servo turns “forward”, proportionally all the way up to 2.0ms where it is at full forward. Similarly, decrease the pulse width and the servo turns “reverse”, with the servo in full reverse at 1.0ms. The frame rate, or time between pulses, is usually quoted as 20ms (or 50Hz) but this does not appear to be crucial. The pulse width, though, is – for obvious reasons. The Jumbo Servo also uses this 1.0-2.0ms/50Hz standard, so it is compatible with the vast majority of radio control equipment sold. Radio control units tend to use a standard colour coding in their output leads – red, yellow and black. Red and black are + and – power respectively, while the yellow is the pulse train (normally referenced to the black lead). For convenience, we often use red, brown and black wires in hobby radio control wiring because these are the first three colours in a rainbow cable – very handy because the three wires can be stripped off together. (In fact, in most rainbow cables you get two lots of black, brown, red wires – most rainbow cables have 15 or more conductors). Inside a “normal” servo is a tiny electric motor/gearbox, which is driven one way to send the servo actuator forward and the opposite way to go reverse. Outside our jumbo servo is a much larger electric motor/gearbox which works in exactly the same way. While we used a particular motor/gearbox combination in the prototype, you could choose from a huge range of motors and gearboxes, depending on the amount of grunt you need. The motor/gearbox we used is actually a powerful little German unit (aha! so that’s the association in the title!) from Oatley Electronics but others which you could use include a variety of automotive models – wind- Gerrunttt! screen wiper motors, auto headlight motors, electric antenna motors and so on). You can also obtain a variety of motors and gearboxes from hobby and electronics stores. Bear in mind, though, that a too-high gear ratio (say 100:1 or more) may result in a particular servo position being difficult to accurately or consistently reproduce. This is because of the latency of the motor/gearbox – the motor might make several turns before the geared output starts to turn. Of course, the higher the ratio, the more torque you’ll get from a given motor so it’s something of a tradeoff. In a lot of cases, this won’t matter. The prototype had also slightly lower than the normal 90° travel – it was about 85°. This is because of component tolerance spreads and could be corrected by closer component selection. However, this may or may not be important to you – some applications may only need half this travel, or even less, so less wouldn’t matter. Fly-by-wire If you don’t have (or don’t want) a radio-control unit with receiver, you won’t have a source of the 1.0-2.0ms/50Hz pulses required to control the servo. Fortunately, that’s easy to solve. You can quite simply synthesize such a pulse stream with just a few components. As we mentioned before, the frame or pulse rate (50Hz) is not particularly critical but the 1.0-2.0ms pulse width is. For those who want to use a wired controller for the servo, we show details of a small variable pulse generator which creates those 1.0-2.0ms pulses at about 50Hz. The pulse width is controlled by a pot; centre is off, full anti-clockwise is full reverse and full clockwise is full forward. This can be connected as far as you like (within reason!) from the servo unit itself. Mechanicals We show the details of our prototype in the drawings and photographs which accompany this article. Needless to say, there are many ways to skin a cat – and your servo mounting arrangements could obviously be very different if you use a different motor. The two basic requirements are: (a) some means of mounting the servo Article by Ross Tester actuator “arm” to the motor (gearbox) shaft, and (b) some means of connecting the positioning sensing, or feedback, pot­entiometer to the motor (gearbox) shaft. The photographs and drawings show how we accomplished this in the prototype – again, yours will depend on the motor/gearbox used. Our servo actuator arm was a 250 x 15mm strip of 10 gauge aluminium, bent over on itself but with a 10mm “bell” at the midpoint. A hole was drilled into this to accommodate a screw and locknut which in turn fastened on the gearbox shaft. Of course, holes were also drilled in the arm to allow the shaft to pass through (a fairly tight, or “friction” fit). One or two 3mm screw(s) and lockMAY 2001  69 The circuit of the servo controller section. The input can be from a radio control receiver or, as we explain later, a purpose-built oscillator. nut(s) prevented the two halves from “opening up”. This screw could also be a connection point for whatever the servo arm was actuating, if necessary. A shallow “U”-shaped bracket was made up to support the feedback pot and, for convenience, the servo controller electronics housed in a small zippy box. (The electronics can be mounted remotely if desired). The bracket was glued, not screwed, to the electronics box, again more for convenience than anything else. The pot shaft was connected to the motor shaft with a short length of heatshrink tubing, shrunk into position once the pot was mounted and the two shafts aligned. Circuit description There’s not a great deal to the circuit. It basically consists of two sections: the pulse detection, shaft position and driving circuitry based on the ZN409 70  Silicon Chip Servo Driver IC and the “H-bridge” motor driver (Q1-Q8). The circuit is in fact very similar to one contributed by Nicholas Baroni in “Circuit Notebook”, SILICON CHIP December 1997. The 50Hz pulse stream is fed into pin 14 of IC1. This chip has its own reference oscillator, producing 1.5mswide pulses every 20ms (ie, 50Hz). The incoming pulse stream is compared to this reference. Usually, a trimpot would be used to adjust the reference oscillator to account for variations in receiver outputs but in this case there is a pot, rather than a trimpot, and it is connected rather differently. The potentiomenter is now physically connected to the gearbox shaft and varies as the servo position varies. This gives the IC feedback, letting it know where the shaft is at that time. More on this shortly. Also, most ZN409 motor-driver circuits have the outputs from pin 5 and 9 – as you can see from the circuit, our outputs are pins 7 and 8. If the incoming receiver pulses are longer than the reference oscillator pulses, the pin 7 output is taken high and pin 8 output taken low. Conversely, if shorter, pin 7 goes low and pin 8 high. If the pulses are the same length, both pin 7 and pin 8 are high. As Q1 and Q2 are PNP devices, a logic “high” on their bases will turn them off and a “low” will turn them on. Therefore, unless the IC sends both pins 9 and 5 high, when Q1 is on Q2 must be off and vice versa. If the pulses are long and Q1 is off, Q3 and Q5 will also be off. At the same time the base of Q2 is taken low, turning it on. Q6, Q8 and Q4 are also turned on. Current can therefore flow from positive, through Q4, the motor, Q8 and back to negative. The PC board component overlay shows where everything fits. There are two additional 0.1µF capacitors not shown on this overlay; they are for motor noise supression and are wired directly between the motor terminals and the earthed motor, with leads as short as possible. The wiring on the right side of the PC board should be heavy duty, able to handle the heavy motor current. The wiring on the left is ideally made from ribbon cable. Close-up of the servo controller, removed from its case. Compare this to the PC board overlay above. Therefore, the motor will turn in one direction, turning the servo actuator arm attached to its shaft (or more correctly, its gearbox shaft). But remember that feedback pot we mentioned before? As its resistance varies, it changes the width of the pulses from the reference oscillator in IC1. At a certain point, the comparator will register that the reference pulses and the incoming receiver pulses are identical and send both pins 9 and 5 high. When this happens Q1 and Q2 are both turned off, in turn turning Q3 and Q6 off. Q5 and Q8 turn off when this happens, so current cannot pass to the negative supply and therefore the motor cannot turn. If the incoming pulses become shorter than the reference, the whole operation above reverses; the net result is that current can flow from positive to negative via Q7, the motor and Q5. But this current flow is in the oppo- site direction as far as the motor is concerned, therefore it turns the opposite way – that is, until equilibrium is once again reached, with the feedback pot fooling the comparator into believing that the pulse widths are equal. Power supplies The circuit requires two supplies, +12V (or the voltage at which your motor operates) and +5V. The 5V is usually supplied by the radio-control receiver (via the 3-wire cable which also supplies pulses); if you build the servo oscillator/controller unit there is also a 5V regulated supply built into that. Otherwise you may need to lash together a similar 7805 regulator circuit which can derive its input from the 12V DC source for the motor supply. While we are specifying 12V for the motor supply, there may be users Viewed from the underside, this pic shows the electronics of the Jumbo Servo with the case cover removed for clarity. Note the position feedback pot mounted on the bracket which also holds the case and PC board. MAY 2001  71 These two close-up shots show the servo controller arm and its method of mounting on the gearbox shaft. The position feedback potentiometer must be aligned with this shaft and connected to it – we found the easiest way was with heatshrink. who want to run higher voltage motor/ gearboxes. One advantage of this is that for the same torque, a higher voltage motor will normally draw less current. With the transistors specified, higher voltage motors are a possibility (eg, 24V truck wipers) but we must emphasise that these have not been tried. You may also need to supply heatsinking for the power transistors. Inertia and dead band The ZN409 has a built-in “deadband” which stops it trying to adjust the servo over too close a range. Without the deadband, the servo motor would continually “hunt” or chatter as it tried to correct its position. This is caused by the mechanical inertia of the motor/gearbox assembly. The circuit tells the motor to spin for so long then, when the circuit senses that it has reached the right point, motor current is cut off. But the motor cannot stop spinning immediately – it slows to a stop. This takes the servo slightly beyond where it should be. So the circuit tries to correct this and spins the motor back the other way – woops, too far, so it corrects this and... The dead band stops this happening. It won’t let the controller supply power to the motor if the servo is within a certain band or percentage of where it should be. The capacitor connected to pin 13 slightly extends the ZN409 normal deadband to take into account the longer inertia of the larger motors used in this servo. With the .022µF capacitor shown, the dead band is about 14% of the servo travel – fairly normal for a servo but if unacceptably large, you could reduce this capacitor somewhat. See what works for your application. 72  Silicon Chip The two 2.2MΩ resistors serve a related function, albeit inverse, in the “stick” of the radio control unit. They give the stick more control, without a lot of dead stick (ie, the amount the stick must be moved before there’s any reaction from the servo). If necessary, these resistors can be reduced but don’t go below about 560kΩ. Lastly, the two 22µF capacitors between these resistors really are connected “back-to-back” as shown, as the polarity across them can (and does!) reverse. Pulse source We’ve already mentioned that this controller is compatible with the vast Parts List – Jumbo Servo (Actuator) 1 PC board, 52 x 77mm, code K165 1 12V motor/gearbox assembly (see text) 1 14-pin DIL IC socket 8 PC stakes 3 lengths black-brown-red ribbon cable (to suit) 1 length 3-conductor ribbon cable (to suit) 2 lengths heavy-duty red hookup wire 2 lengths heavy-duty black hookup wire 1 aluminium bracket to hold feedback pot (see text) 1 aluminium servo actuator arm, captive to shaft (see text) 1 length heatshrink tubing to suit gearbox shaft & potentiometer Semiconductors 1 ZN409 servo controller IC (IC1) 2 C8550 PNP transistors (Q1, Q2) 2 BC639 NPN transistors (Q3, Q6) 2 MJE2955 PNP power transistors (Q4, Q7) 2 MJE3055 NPN power transistors (Q5, Q8) Capacitors 1 470µF 35VW electrolytic, radial type (C2) 2 22µF 25VW electrolytics, PC mounting (C3, C4) 1 2.2µF 25VW electrolytic, PC mounting (C9) 1 0.47µF polyester (C7) 3 0.1µF polyester or MKT (C1, C5, C6) 3 0.1µF ceramic (C10, C11*) 1 0.022µF polyester (C8) Resistors 2 2.2MΩ     2 100kΩ     2 10kΩ    1 12kΩ    1 5.6kΩ    1 1.2kΩ 8 470Ω      2 68W 1Ω 1 10kΩ linear potentiometer * solder between motor terminals and earthed motor case­ It’s not so much a Jumbo Servo Controller as a Jumbo Servo Controller Controller. It contains two oscillators whose pulse width is variable between one and two milliseconds; ie, perfect for “driving” the Jumbo Servo. majority of radio control receiver outputs, with their 1.5ms-wide output pulses (±0.5ms) on a 50Hz square wave. Connect the output of the radio control receiver to this circuit and you should find the combination works perfectly. However, if you don’t have an R/C receiver (or want to wire the controller direct) it’s very easy to build an oscillator which simulates this waveform. That’s what the other box in our photographs does. In fact, built into this box, with oodles of room to spare, are two such oscillators (obviously for controlling two Jumbo Servos). If you want to control more, you could arguably fit four or even six oscillators in the disposals box we used. This box was once a 110V power supply – not exactly usable in Oz or NZ, so we threw away the transformer (OK, we lied – it’s a great paper weight!). We did keep and use the small rectifier PC board, though – it provides some useful filtering and also protects against reverse polarity supply. This board also fits into the box – still with plenty of room. The oscillator is based on a 555 Inside the box looks like a dog’s breakfast (’cos it is!). The vertical PC board contains two oscillators (hence the two pots on the front) while the other PC board is a rectifier board retrieved from a 110V supply and “crammed in”. timer, running at around 50Hz. This circuit is a little different from most 555 timer circuits in that it is effectively “back to front”. Normally, pin 3 of a 555 is its output pin but we use pin 3 to charge and discharge the timing capacitor, taking the output pulses from what would normally be the discharge pin (pin 7). The 555 output can both source and sink current. When its output is low, C3 discharges through the IC and when high, it charges C3, with both the charge and discharge times dependent on the setting of VR1. Note, though, the large discrepancy in series resistors between the charge and discharge cycles: these set up the oscillator to provide the one-to-two millisecond-wide output pulses, taken from pin 7 . Construction Start, as always, by examining the PC board(s) to ensure it (they) is (are) free from defects. We’ll assemble the main PC board first. Mount and solder the lowest-profile, non-polarised components first –ie, the resistors and ceramic or polyester capacitors. Use the colour code in the table or check their value with a digital multimeter if you aren’t sure. Next solder in the electrolytic capacitors. The large electro near the power transistors is a little unusual these days – it is an axial type rather than a PC board mounting type. Detail of our servo arm. Exact size is not important – this size was chosen because it is easily made from a 250mm length of 20mm x 3mm strip aluminium, commonly available at hardware stores. MAY 2001  73 Here’s what the contents of the controller oscillator box reveal: the two oscillators (on one PC board) at left, while the board in the background is the one recovered from a 110V supply. It contains a bridge rectifier along with a nice big smoothing capacitor and a fuse, so it doesn’t matter which way around you connect power (low voltage AC, even!). The ICs at the back of the oscillators are 7805 regulators to give a 5V supply. If for some reason you cannot get an axial, a PC board type can be used but you’ll have to run one of its leads back along the body in order to lie it flat on the board. (Standing up it would be too high to fit in the case). Now solder in the small transistors, taking care that you don’t mix ’em up. All look much the same but they aren’t! Solder in the IC socket, making sure its notch goes the same way as shown on the PC board overlay. And finally, solder the four power transistors in place. Again, they are not all the same. They mount down close to, but not right on, the PC board – allow say 3mm space under them. Try to mount them all at exactly the same height – just because they look neater that way. Plug the IC into its socket, again ensuring the notch lines up with the notch on the socket. Apart from soldering on the various connecting wires, this PC board is now complete. Note that one resistor and the pot should be left over – the resistor solders direct to the pot terminals. In like manner to the controller board, solder the components to the smaller PC board (the oscillator board). If you are only going to control one servo, you only need to place one set of components (the board contains two identical halves for two oscillators in case you want to control two Jumbo 74  Silicon Chip Servos – eg, steering and brakes on a big model car). Connecting cables Most of the connecting cables can be trios (ie, 3 wires in one strip) peeled off a length of ribbon cable. Bearing in mind what we said above about blackbrown-red colours, remove suitable lengths of cable and connect as shown in the diagrams. Wires to the remote pot can also be a trio from ribbon cable – colours here aren’t at all important; use what you have the most of. Just remember to connect the right one to the right point on the PC board! Cables which connect to the motor and to the battery or power source should be considerably heavier than ribbon cable. For a motor which draws, say, 5A continuous, we would be inclined to use 10A cable to minimise voltage drop (I2R losses) – especially if the motor is mounted any distance away. You can buy “auto” cable rated at 20A or more which is even better. We would normally always use red and black cable for polarised (ie, power) connections – it minimises the chance of a mistake. Having said that, you may note from the photographs we used red and green for the motor because that’s what the motor was supplied with. Oh well, 50% right is better than 100% rong! You may also have noticed that we used a trio of black-brown-red ribbon cable to connect power to the oscillator board (it’s more than thick enough for this purpose). In this case, we simply chopped off the brown in the middle but kept to the red and black convention for power. In this demonstration prototype, too, we have used much thinner red and black cable for the power connection to the PC board than we would have preferred. It’s just that we had some of this on hand and the lolly shop was closed and… Firing it up You might find it easier to check it all out without the servo actuator arm The Servo Oscillator is based around an old friend, the 555 timer. This circuit also includes a regulated 5V supply for the servo driver chip on the other PC board. The component overlay for one of the oscillators and 5V supplies. One is needed for each servo. At right are two such circuits on one PC board. in place, or at least not yet captive (ie, loosen the grub screw!). The arm has this annoying habit of getting caught in other things while flailing back on forth when spread out on the bench. Connect the feedback pot to the main PC board (remember that resistor across it!) and set it to roughly its midpoint. Apply power. You’ll probably find that nothing happens. That’s good, because without input pulses, the servo doesn’t know where it should be. Disconnect from power. Now’s the time to align the pot to the shaft – as we said, we used heatshrink for simplicity and ease; you might have other ideas. Now you’ll need either an R/C receiver with servo output or the oscillator. Connect either up to the “receiver” terminals on the PC board, observing the polarity of the power leads and the position of the signal lead (it goes to the centre). Apply power to the servo and oscillator (or turn on your R/C receiver and transmitter). Turning the pot (or moving the transmitter joystick) one way should make the servo turn one way, the opposite way should make it go back the other direction. If so, all you have to do is secure the Parts List – Servo Controller Oscillator (1 unit) 1 PC Board, 40 x 63mm, code K166 1 recovered PC board with components (see text) 1 8-pin DIL IC socket 8 PC stakes 2 lengths black-brown-red ribbon cable (to suit) 1 length 3-conductor ribbon cable (to suit) servo arm to the appropriate place on the gearbox shaft, mount the electronics in the appropriate boxes, run any necessary cables – and you’re done! If it doesn’t work There’s a snaffu somewhere, eh? Eliminate the radio control side by plugging in a standard servo (eg, from a model plane, car, etc) to the radio control receiver and make sure it works as intended. If you’ve built the oscillator, it can be plugged into a standard servo and checked. If everything works, there’s something wrong on the PC board – a component back to front or misplaced, a solder bridge or dry joint – or maybe you have simply forgotten to connect something to something else (the motor, maybe?) The board is quite simple, so if a check and double check finds nothing wrong, start checking voltages, for example: • power (from the R/C receiver or oscillator) at pin 10 of IC1 and also the emitters of both Q1 and Q2. • power (the same voltage as the battery) between the sources of Q5/Q7 and Q6/Q8. If you have access to an oscilloscope, Resistor Colour Codes                                       Value     2.2MΩ  100kΩ  12kΩ  10kΩ  5.6kΩ   1.2kΩ   470Ω   68Ω 4-Band Code (1%)  5-Band Code (1%) red red green brown  red red black yellow brown brown black yellow brown  brown black black orange brown brown red orange brown  brown red black red brown brown black orange brown  brown black black red brown green blue red brown  green blue black brown brown brown red red brown  brown red black brown brown yellow violet brown brown  yellow violet black black brown blue grey black brown  blue grey black black gold Semiconductors 1 7805 3-terminal regulator (IC1) 1 555 timer IC (IC2) 1 1N4004 power diode (D1) 2 1N4148 signal diodes (D2, D3) Capacitors 1 1000µF 35VW electrolytic, PC mounting (C1) 1 10µF 16VW electrolytic, PC mounting (C2) 2 .047µF polyester or MKT (C3, C4) Resistors (0.25W, 1%) 1 1MΩ    2 10kΩ 1 20kΩ linear potentiometer, PC board mounting you might check that there is indeed a 50Hz (ish) squarewave coming into pin 14 of IC1 and that pins 7 and 8 go high and low as they should. Wheredyageddit? Various kits are available from Oatley Electronics, who hold the copyright on the PC board patterns. They have the servo kit (all electronics, PC board and a case) for $35.00; a dual oscillator/controller kit (electronics, PC board and case) for $14.00; a power supply (including the 110V supply suitable for ratting) for $24 and, most importantly, they have the German Motor/Gearbox for $20.00 each. Contact Oatley Electronics on (02) 9584 3563, fax (02) 9584 3561 or via www.oatleyelectronics.com SC Capacitor Codes       Value    IEC code    EIA code   0.47uF 470n 474   0.1uF 100n 104   .047uF 47n 471   .022uF 22n 221 MAY 2001  75