Fullscreen HTML5Med Res (??MB)HTML4

RADIO CONTROL BY BOB YOUNG How does a servo work? This month we take a look at the principles underlying the operation of servos designed for use with R/C systems. You move the stick on your transmitter and the servo moves to a new position. Why? We shall find out. in a high level of interchangeability. Fig.2 shows an exploded view of a typical servo. Item 19 is the potentiometer which in this servo is a replaceable ceramic element which screws into a housing moulded into the servo case. Modern servos use a miniature sealed potentiometer. Pulse width modulation Not only is the design of the typical R/C servo an elegant example of modern mass production but the system whereby six, seven or more channels of data are modulated onto the radio carrier is elegant as well. It is here that the great mystery begins for the average electronics buff. Just what is the system of modulation and how does it result in such precise clockwise (CW) and counter-clock-wise (CCW) control of an electric motor? The basic servo is best defined as a closed loop, error cancelling system in which some of the output is fed back into the input in such a way that the system automatically seeks to come to rest in a state of zero error. In this null or neutral position it should draw negligible current. Fig.1 is the block diagram of such a system. A typical modern R/C servo has the following components: a plastic housing and gear train, electric motor, feedback poten­tiometer and a servo amp­lifier. The feedback potentiometer is mechanically linked to the servo output arm either directly or indirectly via a gear train. The indirect drive servo minimises the vibrational wear on the potentiometer but is more expensive. A three-pin plug is fitted as standard to most servos, resulting Fig.1: block diagram of a typical servo motor. A posi­tive-going input pulse is compared with an internally generated negative going reference pulse in the error amplifier and used to drive the motor. 66  Silicon Chip The input signal to the servo amplifier is a variable width pulse and it is here that the magic begins. The position of the servo output arm is slaved (or proportional) to the width of this input pulse. Thus it can be described as a pulse width modulation (PWM) system. Modern PWM systems have a virtually universal standard positive input pulse of 3-5V amplitude with a neutral of 1.5ms and varying between 1-2ms. The repetition rate of this pulse (Frame Rate) is typically between 14-25ms (70Hz to 40Hz) de­pending upon the number of channels transmitted. Don’t worry if this terminology is all Greek to you at the moment. We will explain it. The diagram of Fig.3 shows typical input pulse parameters. This pulse signal comes from the decoder which produces separate pulse signals for each servo. We will discuss encoders and decod­ers next month. While the basic elements of the modern servos differ little from their early counterparts, the same cannot be said about the servo amplifier which is now just an integrated circuit with a few external components, taking up little space inside the servo case. Example circuit As the modern IC servo amplifier is difficult to analyse, it is easier for us to look at a discrete servo amplifier devel­oped before the IC took over. Fig.4 shows the circuit of an old Silvertone servo. RV is the feedback potentiometer which is coupled to the motor. A positive-going pulse of 4.8V amplitude is fed from the receiver decoder into the base of transistor Q1 which operates as an emitter follower. The pulse signal appears across R1 in the same phase but with the base/emitter voltage drop of about 0.6V subtracted. This positive-going pulse is then fed via R6 to the summing junction and via capacitor C2 and R2 to the input of IC1, a UL914 dual OR gate. IC1 is configured as a one-shot multivibrator with a time constant set by C3, R3 and RV. This one-shot generates a nega­tive-going reference pulse of about 4.2V amplitude which is then fed via R7 to the summing junction R8, R9, C4, C5. The values of R5, R6 and R7 are chosen to deliver pulses of equal amplitude but opposite phase to the summing junction. R5 along with C1 also forms the supply decoupling network for the one-shot IC1. Timing diagrams Now we need to look at some timing diagrams which show how the input pulse and the reference pulse are summed to produce a drive signal to the servo motor. Fig.5 shows the first condition. The top trace (a) is the positive-going input pulse while the second trace (b) is the negative-going reference pulse. When these two pulses are applied to the summing junction the result is trace (c). As you can see, the pulses have exactly cancelled out since they have equal amplitude and duration. The result is zero output, the condition required for neutral or rest position. Fig.6 shows the conditions for clockwise drive (CW) of the servo motor. Here the positive pulse (a) is of longer duration than the negative reference pulse (b). The output of the summing junction is a positive pulse, the duration of which equals the difference between the input (positive) and reference generator (negative) pulses. This positive pulse is transferred to the bases of Q2 and Q3 via capacitors C4 and C5. As Q2 is a PNP transistor it will not respond to this Fig.2: exploded view of a typical servo. Item 19 is the poten­tiometer which in this servo is a replaceable ceramic element which screws into a housing moulded into the servo case. Modern servos use a miniature sealed potentiometer. November 1997  67 Fig.3: typical input pulse parameters for an R/C servo. This pulse signal comes from the decoder which produces separate pulse signals for each servo. positive-going pulse but NPN transistor Q3 will. Capacitor C6 is a pulse stretcher and provides smoothing until the next pulse arrives 20ms later. The drive circuit for the motor is unusual in that it is the old centre-tapped 4.8V system (four wire system). Modern IC servo amplifiers use a bridge drive circuit which will give bidirectional drive from a single 4.8V battery (three wire sys­tem). With Q3 now conducting, transistors Q5 & Q7 will also con­duct and drive the motor in a clockwise direction. When we have the conditions shown in Fig.7, where the input pulse is shorter the than the reference pulse, the output of the summing junction Fig.4 (below): the circuit of an old Silvertone servo using discrete components. RV is the feedback potentiometer built into the servo mechanics. It adjusts the reference pulse width as the motor is driven to the desired position. 68  Silicon Chip is a negative pulse (c). This causes transistors Q2, Q4 & Q6 to conduct, driving the motor in the counter clockwise direction. Feedback seeks the neutral Now we come to the clever part. The feedback potentiometer RV is connected to the output shaft of the servo mechanics and is wired in such a manner that the servo motor always moves to reduce the error (difference) signal to zero by changing the width of the reference generator pulse. You can visualise this happening. Say, we have the condi­tion shown in the waveforms of Fig.7 and the input pulse is wider than the reference pulse. The motor will be driven clockwise and at the same time the setting of RV changes to widen the reference pulse. This narrows the pulse from the summing junction until ultimately the input pulse and reference pulse cancel each other exactly and the result is zero output to the motor. The servo is now in the null or neutral position and will stay that way until the input pulse changes. The same thing happens when we have the conditions shown in Fig.8. Here the input pulse is narrower than the reference pulse and the motor is driven anticlockwise. This changes the setting of RV to reduce the duration of the reference pulse until again, the input pulse cancels out the reference pulse and the motor arrives at the neutral position. To sum up, if the input pulse is narrow, the servo will move until the reference pulse is also narrow. If the input pulse is wide, the servo will move until the reference pulse is equally wide. Relating this back to the beginning of the article when we said that the neutral pulse width is typically 1.5ms, this means that when the input pulse width is also 1.5ms, the servo seeks the neutral or null position which is usually in the centre of its travel. If the input pulse is 2ms wide, the servo will move clock­ wise. If the input pulse is 1ms wide, the servo will move anticlockwise. Servo phasing In case you are wondering how to work out the correct sense for the potentiometer wiring let me tell you a simple way. You wire the positive and negative leads to the two outside tabs on the pot and the wiper to the lead coming from R14. When you plug the servo in, if it races down to Damping R4 is the main damping resistor, advancing or retarding the reference pulse generator slightly according to the direction of rotation. In this way the motor drive can be shut down just before the null point is reached, allowing the servo to coast smoothly to a stop at the correct position. In a feedback system there are three types of damping conditions possible: under-damped, over-damped and dead-beat. An under-damped servo will swing past the neutral point and then kick back past neutral and kick back again in increasingly small oscillations until the zero error point is reached. An over-damped servo will shut down well before the zero error point is reached and slowly creep back to neutral. The dead-beat servo will come straight back to the correct neutral with no over or undershoot. By adjusting R4 the correct amount of ref- SILICON CHIP SOFTWARE Now available: the complete index to all SILICON CHIP articles since the first issue in November 1987. The Floppy Index comes with a handy file viewer that lets you look at the index line by line or page by page for quick browsing, or you can use the search function. All commands are listed on the screen, so you’ll always know what to do next. Notes & Errata also now available: this file lets you quickly check out the Notes & Errata (if any) for all articles published in SILICON CHIP. Not an index but a complete copy of all Notes & Errata text (diagrams not included). The file viewer is included in the price, so that you can quickly locate the item of interest. The Floppy Index and Notes & Errata files are supplied in ASCII format on a 3.5-inch or 5.25-inch floppy disc to suit PC-compatible computers. Note: the File Viewer requires MSDOS 3.3 or above. ORDER FORM PRICE ❏ Floppy Index (incl. file viewer): $A7 ❏ Notes & Errata (incl. file viewer): $A7 ❏ Alphanumeric LCD Demo Board Software (May 1993): $A7 ❏ Stepper Motor Controller Software (January 1994): $A7 ❏ Gamesbvm.bas /obj /exe (Nicad Battery Monitor, June 1994): $A7 ❏ Diskinfo.exe (Identifies IDE Hard Disc Parameters, August 1995): $A7 ❏ Computer Controlled Power Supply Software (Jan/Feb. 1997): $A7 ❏ Spacewri.exe & Spacewri.bas (for Spacewriter, May 1997): $A7 ❏ I/O Card (July 1997) + Stepper Motor Software (1997 series): $A7 POSTAGE & PACKING: Aust. & NZ add $A3 per order; elsewhere $A5 Disc size required:    ❏  3.5-inch disc   ❏ 5.25-inch disc TOTAL $A Enclosed is my cheque/money order for $­A__________ or please debit my ❏ Bankcard   ❏  Visa Card   ❏ MasterCard Card No. Signature­­­­­­­­­­­­_______________________________  Card expiry date______/______ Name ___________________________________________________________ PLEASE PRINT Street ___________________________________________________________ Suburb/town ________________________________ Postcode______________ Send your order to: SILICON CHIP, PO Box 139, Collaroy, NSW 2097; or fax your order to (02) 9979 6503; or ring (02) 9979 5644 and quote your credit card number (Bankcard, Visa Card or MasterCard). ✂ one end of the throw, tearing the teeth off the output gears, you know you got it wrong. You then reverse either the two outside wires on the pot or reverse the two motor wires but not both and the servo operates normally. These days the servo manufacturers usually wire the motor and pot leads directly into the amplifier PC board and servo reversing is no longer possible. Servo repairing is no longer possible or cost effective in most cases, thereby increasing the pressure on transmitter designers to provide servo reversing at the transmitter end. To tidy up the remaining parts of the amplifier descrip­tion, R14 is the feedback voltage set resistor, setting the throw of the servo. The higher the value of R14, the more travel re­quired before sufficient control voltage was available to null the error. R3 will also provide throw adjustment. Throw is de­fined as the amount of angular displacement on the output arm for any given pulse width variation. D1 is an isolation diode. R8, R9 & R10 also act as base tie-down resistors for thermal stability. R13 is a current limiting resistor. Capacitors C7 & C8 are connected from each motor termi­ nal to the case. These capacitors must be mounted on the servo motor and form the noise suppression network for the motor. R15 prevents both sides of the amplifier switching on simultaneously. November 1997  69 Fig.5: the top trace (a) is the positive-going input pulse while the second trace (b) is the negative-going reference pulse. When these two pulses are applied to the summing junction the result is zero output (c), the condition required for neutral or rest position. Fig.6: conditions for clockwise drive (CW) of the servo motor. Here the positive pulse (a) is of longer duration than the nega­tive reference pulse (b). The output of the summing junction is a positive pulse, the duration of which equals the difference between the input (positive) and reference generator (negative) pulses. Fig.7: conditions for CCW drive. The input pulse is shorter the than the reference pulse, so the output of the summing junction is a negative pulse (c) which drives the motor anticlockwise. erence generator adjustment may be achieved. A slightly under-damped servo (one kickback) is the best compromise for heavily loaded servos. Setting the damping on any servo 70  Silicon Chip is the most difficult part of the servo design. The problem begins with the pulse stretching network and encompasses such factors as servo power, slew rate, operational load, dead band This is a Silvertone servo, circa 1973, showing the double deck amplifier board complete with 11 transistors. Also visible is the drive motor and feedback potentiometer. and most importantly the minimum impulse power of the servo amplifier. The dead band is the notch that the servo comes to rest in. If this notch is too wide, then centring inaccuracies occur; if too narrow, the servo chatters away because it cannot find a spot to come to rest. This results in excess current being drawn by the servo, overheating of the amplifier and brush wear on the motor. The minimum impulse power of the amplifier is the ability of the amplifier to obtain the maximum torque from the motor on the minimum error pulse. The higher the minimum impulse power the better the resolution of the servo and the less demands on the damping network. As you can imagine, if the servo is over-damped and it shuts down too early it must rely on the minimum impulse power to creep it back to the correct neutral. If the servo is heavily loaded and with too high a dead band, then the servo may sit just short of the correct neutral, introducing a control error which is annoying to the operator of the model. Worse still the servo is drawing excessive current, reducing battery life and overheating the transistors. Four or more servos doing this could reduce battery life to half and possibly result in a crash. So there you have it! Now you should have good understand­ing of the theory of servo operation. Just coincidentally, next month’s Circuit Notebook will include a servo based on a windscreen wiper motor. The operating principles are the same. Next month we will look at how the input pulse arrives via a remote SC or local link.