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High-Tech Rubber Motor Winders

From 32nd Annual Symposium 1999
National Free Flight Society

ABSTRACT

   F1C and R/C flyers have electric starters to get their engines going so why don't free flight rubber flyers have electric winders to more conveniently power up their models? Up until now, many types of electric motor-powered winding machines, like rechargeable drills and screwdrivers, have been adapted for the task. Their usefulness is limited because they lack feedback in the form of turn-count information and the knowledge or feel of the torque that many modelers rely on to wind rubber motors to the maximum power. This paper provides the science which will allow the experimenter to design and build a portable electric/electronic winding system for applications ranging from winders for delicate indoor models all the way up to "rope twisters" for Wakefields.
   A broad range of electric winders may be designed to provide basic features like continuous electronic torque

readings and turn counts, to more complex systems which can include programmable torque limiting and electronic data outputs. A plot of the Torque versus Turns graph in real time offers the serious free flight rubber flyer the ability to monitor, control and analyze the quality and character of a rubber motor. In most cases, the instrumentation to do this is not readily available. This paper describes a novel way of using the guts of a low-cost computer mouse along with the graphics feature in word processor programs to easily input and display the turns and torque data on a personal computer (PC). The results are surprisingly accurate if a computer aided design (CAD) program (even a very low-cost one) is used to plot and then calculate the area under the curve. A measure of the area is equal to the total energy stored in the rubber motor.

INTRODUCTION

    Winding a rubber motor is an art. Experts rely on years of accumulated experience to pack as much energy into the motor as is possible. The damage, in terms of equipment and contests lost from even the slightest miscalculation cannot be tolerated. Most practitioners of this art use manually-turned winders coupled to mechanical turns counters and spring-type torque measuring devices. Except for winding systems used by indoor duration flyers, most outdoor winders need to be stopped in order to read the torque meter, although the turns counter may, in most instances, be read continuously. Is it possible to more automate this process and at the same time, reduce the art to science by using electronic "smart" systems?

    A number of years ago, I worked with the current-to-torque relationships of permanent magnet (P-M) DC motors and was able to design, build and use a prototype of an electric rubber motor winder (shown in the photo) which featured continuous electronic torque readings. Although the system worked better than I had expected, it did not meet all of my requirements.

    With the benefit of experience, hindsight and some new ideas, I hope to inspire other experimenters to develop practical winders for specific applications.

ELECTRIC WINDER WITH ELECTRONIC TURN AND TORQUE METERS
Theory

    The basic electric/electronic winder shown in Fig. 1 consists of a battery-powered electric motor coupled to the output end-effector (hook or the like) through a mechanical motion convertor (gearbox or belt drive). The power supply (battery) is connected by a manually-activated motor starter to the motor through a sensor that provides an output analogous to the load torque on the end-effector. The number of turns that the end-effector makes is detected by another sensor which also provides output data.

The goal is to design the winder so that it twists the rubber motor rapidly at the beginning of the procedure, slowing down to a more manageable speed as the torque builds to allow greater control at the end when the winding procedure becomes very critical. Winding speed and the maximum torque available from the system is a function of the characteristics of the electric motor and the coupling ratio of the mechanical motion convertor.

Practice

    Drive System —

Electric Motor: The permanent magnet DC electric motor is the motor of choice for this application not only for its efficiency but primarily for the linear relationships of motor current and speed to output torque as is shown in Fig. 2. For a specific application, select a motor that will use the greater part of the motor's range of available torque. This will give the best torque-measuring accuracy along with the desired speed change for controlled winding. Fortunately, the hobby market offers a broad range of electric motors and battery packs, designed for R/C cars and aircraft, which are ideally suited for this application. The motors and batteries found in cordless tools such drills and screwdrivers may also be adapted for this task.

Mechanical Motion Convertor: Most usable electric motors rotate too fast, so some efficient means of reducing the speed is needed. All mechanisms that reduce rotational speed also proportionally increase the available output torque. This beneficial secondary feature allows for the use of a smaller, lower current motor for a specific application. Gear and timing belt drives with ratios greater than 3 to 1 can be used. The lower ratios are for designs that will be aimed at low-torque, high-turn applications (indoor duration), while the higher ratios are more useful for high-torque, low-turn needs (F1B, Coup).
     Many off-the-shelf motor and speed reduction units intended for electric R/C aircraft may be used with very little or no modification. A custom-built drive made from electric R/C car replacement gear and timing belt drive parts is

 

another option. (Make sure to use as many ball bearings as possible to reduce friction, particularly on the output shaft which must bear the tension load of the stretched rubber motor. The tension induced frictional load will be interpreted as torque by the winder's electronics, so it is important to keep this to a minimum.)

Power Supply: Rechargeable nickel cadmium batteries are best for most applications but alkaline units may be used for some low-powered winders. Again, battery packs intended for R/C cars and aircraft are most suitable if their capacity is sufficient to allow the required number of windings before recharge. For some high power applications, gel cells or motorcycle batteries may prove more dependable.

    Control System —

Brake: After the rubber motor is wound to full torque, turning off the current to the winder motor will result in immediate and rapid unwinding unless some sort of brake is in place to prevent this action. Although braking current circuits may be designed to lock up the electric motor, this technique is very hard on both the batteries and motor brushes. A simpler and more efficient solution is have a pin, located on one of the components of the drive train, come in contact with a pawl placed in its path when the electric motor stops.
     The pawl may work automatically like a ratchet so that it always locks up the drive in one direction or be manually or electromechanically (solenoid) controlled. Brakes on the

 

motor shaft are preferred because a lower stopping force is required.
     Another option is to use a worm gearbox which is inherently self-locking and offers higher ratios in a much smaller space; unfortunately it is less efficient and more difficult to make or buy.

Motor Starter: The motor starter is an electrical switch ergonomically located to provide manual control of the winder. If the winder is intended for applications that might require winding in both directions, an additional DPDT switch may be wired into the leads going to the motor to reverse the polarity.

    Instrumentation —

Torque Sensor: Sensing the output torque is done by measuring the motor operating current using a shunt resistor (Rs) placed in series with the motor and battery circuit. The current passing through this resistor produces a torque-sensitive voltage which is sent to the readout every time the motor starter switch is activated. The torque voltage output equals the motor current multiplied by the shunt resistance.

Torque Readout: The signal from the torque sensor Rs is first conditioned by R1, R2 and R3, to cancel out the voltage generated by the motor's no-load running current, and then fed to a voltmeter. Analog or digital readout meter types may be connected. Sensitive analog panel meters like the one used in the prototype work well but are getting expensive. Meters removed from low-cost VOM testers may be possible substitutes. Miniature digital panel meters are becoming reasonably priced, are more accurate and will fit into a much smaller space. Digital multimeters, although larger and not as easily housed in a portable winder, may often be purchased at remarkably low prices from auto and tool supply stores.
     Analog meters should have a sensitivity of one milliamp (mA) or less for full scale deflection while digital meters should have an input sensitivity of 200 millivolts (mV) full scale.

Turns Sensor: Sensing the number of times the winder's output shaft has made a complete revolution is the task of the turns sensor. Although light-chopped photo systems may be an option, magnetic detection based systems are often easier to build.
     A magnet is mounted on the output shaft such that it provides a polarity or large flux change at the location of the sensor for each revolution. The magnetic reed switch is a good choice for a flux change sensor in that it works very reliably without the need for any auxiliary electrical power and may be connected directly to the turns counter. Reed switch sensors though, are limited to low-speed winders. For both high and low count-rate applications, the Hall Effect sensor is the best choice.

 

Turn Count Readout: By far, the best counters are the miniature solid state units manufactured by a number of companies and sold through most electronics supply houses. Some units are available with two input options, typically, 30 Hz and 10 KHz maximum counting speed, while others are 100 Hz or 500 Hz. The 30 to 100 Hz units are designed to ignore switch bounce and may be used with reed switch sensors while the higher input frequency devices require a clean signal like that obtained from a Hall Effect sensor.

Mechanical Design: The mechanical configuration of a particular winder is subject to the application, the parts selected and the preferences of the builder. For the outdoor flyer, an integrated unit with the motor, drive train, torque and turn meters along with the batteries all located in a hand-held case, will provide both convenience and ease of operation. The indoor flyer may prefer to locate all of the components in a case which is designed to be clamped to a table. A remotely-located hand or foot switch would then free up one or both hands which could then be used to hold and manipulate the model.

DESIGN EXAMPLE

Problem: Design a winder for Mulvihill models which will be able to wind 1000 turns to a torque of 100 ounce inches in one minute.

Motor/Speed Reducer: The first task is to locate a suitable motor. A '540' size unit with characteristics shown on the graph in Fig. 2 shows promise. Next, determine the coupling ratio that will give an average output speed of 1000 rpm. The desired output torque of 100 ounce inches should also be obtained when the motor is providing the greater part of its available torque. If a good match cannot be found, use a larger motor operating on fewer cells to shift the operating curves of the motor so that a good design compromise may be obtained. In this example, a 10:1 speed reducer coupled to the motor shown in Fig. 2 will do the job.

Instrumentation: A digital panel meter with a sensitivity of 200 mV full scale will be used for the torque readout. If we desire the readout to indicate 1 mV per ounce inch, we could accurately tailor the value of Rs so that the current change between no-load and 100 ounce inches results in a sensor voltage change of 100 mV.

 

R1, R2 and R3 are then chosen to create a voltage at the negative terminal of the meter equal to the voltage on the positive terminal when the winder is operating with no output load. R1 should have an impedance less than 1/10th that of the meter and R3 should be adjustable in order to zero the meter. The value of Rs (.006 Ohms) equals the desired meter voltage (0.1 V) divided by the motor current difference (15 A) between no-load and full-load conditions. Another option is to make Rs larger than required and design an adjustable attenuator circuit for the meter to allow for precise calibration. Resistance values for Rs can be very low, making resistors hard to find. (Try using lengths of piano wire for the shunt resistor.)

Battery: Select a 6 cell R/C car pack with a 1700 mAh (1.7 Ah) capacity and calculate how many winding cycles we get. To make things simple, assume that each one-minute winding cycle requires an average of 8 Amps of motor current. The battery is able to supply 1.7 Amps for one hour or 8 Amps for 12 minutes. One should expect at least 12 winding cycles before recharge.

ADVANCED WINDER DESIGNS

Other Useful Features: Programmable torque limiting may be achieved by conditioning the sensor voltage and using it along with suitable transistor switching circuits to interrupt the current to the motor when the desired torque is obtained.
     Sample and hold meter circuits may also be designed to maintain the highest torque reading on the meter until purposely reset.

Connecting the Winder to a PC: Plotting the Torque versus Turns curve in real time offers exciting opportunities to really understand what is happening during the winding cycle and to optimize the performance of the rubber motor system. Fig. 4 shows a low-cost way of inputting the needed data to a PC which then functions as a "smart" data plotter. The printed circuit board and optical wheels of a low-cost, serial, wheel-type mouse may be built into the winder. The X and Y axis optical wheels, which were originally turned by the mouse ball, are coupled to the winder using rubber-band belts suitably arranged such that the cursor moves in the desired direction. The coupling ratios must be designed so that the cursor motion covers most of the monitor screen. The X or Turns axis requires the most conditioning. The X axis wheel should have all but one of its windows covered and the coupling ratio adjusted so that the cursor moves most of the way across the screen when the desired maximum number of winder turns is reached. It is possible to adjust the mouse sensitivity with software in Windows but unfortunately each axis cannot be independently calibrated this way. This is not all that critical because the X and Y axis scales may be suitably calibrated when initially drawing the "graph paper". It might be a good idea to engineer some way of manually turning the optical wheels so that the cursor may be set to zero from the winder. This would not necessary if the graphics

program will allow simultaneous control of the cursor from the keyboard.
     In order to signal the computer to start recording the data, the left mouse button switch must be activated during the winding cycle. This may be automatically done if an extra set of contacts on the motor starter switch are wired in parallel with the mouse button switch, or locate another switch such that it can be activated by the flyer's free hand to provide selective control of this function.

Plotting the Torque versus Turns Curve: Any software program that allows the drawing of freehand lines on the computer screen will turn the PC into a plotter. First draw the X and Y axis and calibrate each axis scale by simulating a load on the output shaft of the winder and using the torque and turns readout information as reference. Save this "graph paper" so that it can be used each time you wish to plot new data.
     To plot a Torque versus Turns graph, first call up the calibrated "graph paper", then select the Freehand or Scribble function from the program's line menu, zero the cursor and start the winder. A plot similar to the one shown in the photo should be generated.
     The best software to use for this application is a CAD program as it will calculate the area bounded by the curve. This area is a measure of the total energy stored in the rubber and the shape of the curve gives the torque characteristics of the motor. Comparing sequential winding-cycle plots of the same motor will show any increase or decrease in expected performance.

SUMMARY

     "Old habits die hard" so it is debatable whether this technology will ever replace the existing and entrenched manual system of winding rubber motors. It should be noted though, that the Torque versus Turns PC plotting feature would give the serious competition flyer a valuable tool to use for optimizing both performance and design.
     Adapting the plotting feature to manual winders presents some unique challenges which I'm sure creative experimenters will find interesting.
REFERENCES
1. "DC Motors, Speed Controls, Servo Systems" An Engineering Handbook by Electro-Craft Corporation, Third Edition
2. "RS-540RH-5045 Motor Data Sheet" Mabuchi Motor Corp.
3. "WordPerfect Presentations Version 3.0 for Windows" Corel Corporation Ltd.
  4. "Key Cad Complete (early version)" SoftKey Software Products Inc.
5. "Cub3 Miniature Electronic Counter Bulletin No. Cub 3/RC" Red Lion Controls

Published 1999 NFFS*

GizmoGeezer Products supports the NFFS and its efforts in keeping free flight modeling alive and well. Many great articles may be found in their publications. Order yours today from Robert McLinden at rmclinden@earthlink.net, 3903 West Temple Place, Denver CO 80236.
Or visit the web site at www.freeflight.org.

*The National Free Flight Society (NFFS) is a nonprofit corporation, operating in conjunction with the Academy of Model Aeronautics (AMA), the National Aeronautic Association (NAA), and the Federation Aeronautique Internationale (FAI).

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