v129raaq@ubvmsd.cc.buffalo.edu (Richard T Ciesla) (04/02/91)
Hi everybody. I am trying to build an electronic speed controller for a radio controlled car's motor. The radio receiver that I have has output plugs with three wires that normally go to a servo. I would like to use the output pulses converted from the radio signal to control a 555 timer, which would in turn switch on and off transistors to control the motor. I want the timer IC to send out a variable high frequency determined by the output from the radio control system. This high frequency would switch the transistors on and off so fast that the motor will keep turning, with faster switching making the motor go faster. I would like to have the transistors bypassed when the motor travels at full speed. Also, I am using transistors rated at 15 amps each. If I wire them up in parallel, will the amp rating increase (ie 3 transistors@15 amps = 45 amps?) And if anyone out there is really familiar with R/C car electronics, I would like to hear from you, because I have other questions of a more detailed nature. Any replies would be appreciated. Thanks. Rich Ciesla v129raaq@ubvmsd.cc.buffalo.edu
pophal@nicmad.UUCP (Gerry Pophal) (04/02/91)
Check out this article for a schematic;
rec.models.rc
Subject: electronic servo switch-WARNING-GIF FILE
Message-ID: <1991Mar29.215508.29359@kuhub.cc.ukans.edu>
Date: 30 Mar 91 03:55:07 GMT
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ames--/ decvax-/ | Madison, Wis 53711-0451 tonya@hpldsla.sid.hp.com (Tony Arnerich) (04/04/91)
The motors won't go faster with higher 555 output frequency, if the duty cycle is the same. It is even possible that *less* power gets to the motor at high frequency, due to greater inductive losses. Set the chopping fre- quency and vary the duty cycle. A good frequency to use is at least 500 Hz, and not so much that all you do is fight motor winding inductance (<30 KHz for a small swiss motor I have). For variable speed on the car, it might be best to think about varying the motor *torque* as opposed to the free-running speed. In that mode, you get constant speed operation when the motor power equals the power requirements due to drag losses. Want to accelerate? Put in more power. Braking of a sort is accomplished by cutting power. Real braking comes from shorting the motor windings (not the power supply, though!). You can use a 555 timer as a pulse-width modulator. I haven't tried that personally, but I have driven DC motors using an H-bridge of power MosFets controlled by a 3524 PWM chip. Works like a champ. I noticed that motor speed was proportional to pulse width when there was a significant drag on the motor. If left to run free, motor speed was essentially constant for a wide range of duty cycle. You can get power MosFets from International Rectifier (such as the IRF530 and IRF9530) that have only 0.6 to 0.3 ohms on-resistance. That's bypassed for just about all intents and purposes. Not bad for something that can be driven directly by CMOS. Just be sure you drive the gates with at least 9V; 12V works extremely well. 4V is just barely on (high resistance). Which diagonal pair of transistors in the H-configuration is on determines motor drive direction. You can gate the on-signals (transistor gate drive) of either the P-channels or the N-channels from the PWM control signal to set the duty cycle. I'm putting the finishing touches on a synthesized sinewave power inverter that uses exactly this power output stage. The only difference is that in my application, the "motor" (transformer winding) changes direction 60 times a second, and the power output varies sinusoidally all the time. The circuit worked out very well. Yes, you can parallel some transistors. It's a real pain with bipolars, because when they get hot they go all the way and fry. MosFets, at least those I am familiar with (IR), are self-regulating and are inherently safe to parallel. No external circuitry is required, just bus them. That's exactly what is done in commercial motor drivers for RC - look for the gang of TO-220 packages. I understand from a RC-enthusiast acquaintance that they can handle 400 amps. You need that when you have inductive loads at high power. Don't forget the protection diodes! For more info, get IR's Power MosFet (HexFet is their copyrighted name) data book, and Motorola has an application note on the 3524 PWM controller. tonya@hpldsla.sid.hp.com
stevek@locus.com (Steve Krattiger) (04/05/91)
In article <68137@eerie.acsu.Buffalo.EDU> v129raaq@ubvmsd.cc.buffalo.edu writes: > > Hi everybody. I am trying to build an electronic speed controller >for a radio controlled car's motor. The radio receiver that I have has output >plugs with three wires that normally go to a servo. I would like to use the >output pulses converted from the radio signal to control a 555 timer, which >would in turn switch on and off transistors to control the motor. I want the It would be much better to plug in a servo, and hook the output to a slide potentiometer for variability of the speed. Wouldn't want to blow out the receiver module, they can be pretty expensive... >timer IC to send out a variable high frequency determined by the output from >the radio control system. This high frequency would switch the transistors on >and off so fast that the motor will keep turning, with faster switching making >the motor go faster. I would like to have the transistors bypassed when the >motor travels at full speed. Increasing the switching frequency won't increase the speed of the motor. What you want to use is 'pulse width modulation'. If you look at a standard square wave, half the time the power is on, half the time it is off. _____ _____ _____ _____ _____ | | | | | | | | | | | |____| |____| |____| |____| |____ At a high enough frequency (1KHz - 2KHz) you can apply this to a motor, and it will run smooth. Play with the frequency a bit, it will affect your power output, I've used frequencies all the way down to 200Hz, it depends primarily on the motor speeds you are trying to achieve. Anyways, looking back at the square wave, the period that the wave is high is called the duty cycle. For this wave, the duty cycle is 50%. The average power applied to the motor is approx Vin/2. __ __ __ __ __ | | | | | | | | | | | |_______| |_______| |_______| |_______| |_______ With the above duty cycle, the power output is approx. Vin/5 so obviously the motor will run signifigantly slower. If you adjust the duty cycle from 0% up to and including 100%, you have complete control over the speed of the motor, and by linking a slide potentiometer to a servo, you can control the motor speed directly from the remote control. Using a very simple circuit with a single LM339 OP-amp, I was able to construct exactly what you are looking for. However I was running a motorized boat towbar I made so it was drawing between 30 to 50 amps, which kinda leads to your next question: > Also, I am using transistors rated at 15 amps each. If I wire them up > in parallel... With regular transistors, this will not work well. But I found that it does work using FET's, however, you must use an individual gate resistor on each FET. You can just hook all the source leads together, and all the drain leads together to increase the amperage. > >Rich Ciesla >v129raaq@ubvmsd.cc.buffalo.edu I don't think that I could properly draw the schematic using ascii characters however I'll give it a try if you or anyone else would like to see how I built this circuit. Basically, all you need is a triangle or sawtooth wave- form fed into one input of an OP-amp, and a slide-potentiometer tied between Vcc and Gnd with the wiper attached to the other OP-amp input. Set the gain of the amp at 1000 or so, then adjust the potentiometer somewhere between Vcc and Gnd, and as the triangle waveform crosses the steady voltage line, it will produce a square waveform at the output of the amp. Vary the pot and you vary the duty cycle of the output waveform. Feed this into the gate of a high power MOSFET, and run the high power stuff of the source and drain of the MOSFET. Voila! Heavy duty motor speed control! simple. e-mail me if you have any more questions. +-----/\/\/\/--------||--+---->|---|---/\/\/\/--------/\/\/\/-------|<--------+ | Steve Krattiger |-/\/\/\--| Locus Computing Corp. .. | | stevek@locus.com === Los Angeles, California \/ | +----|(---------/\/\/\/----|<------|------|+|+|+-------)|-----/\/\/\/----||---+