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OS3 vs. DiFalco Controllers

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  • #16
    Ed. I know you do. With respect to the large cap. At least in HO you would not be allowed to use the controller. While the rules don't state it specifically I had a long chat with the three large HO organizations some time back on PWM controllers and caps.. PWM was on the fence depending on what was showing up on the board. Caps on the output were a no go as they store energy. It could be the same in 1/32. Plus the cap addition across the output would act as an anti-brake. You do have to slow down for the corner..

    The PWM output is not AC as the waveform does not go below zero. It is a bit like AC with a large DC offset. A PWM drive will cause the same heat issues with DC motors as will a half-wave rectifier. In addition every time the PWM output drops to zero the motor will experience inductive kickback as it switches from motor to generator. The kickback will generate a short duration high voltage spike. You might not notice whats going on but the drive will take a toll on motor brushes and armatures.
    Last edited by Maddman; 06-27-2020, 02:37 PM.

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    • #17
      My take on the Slot.it controllers versus the ones being discussed, is that in my own club, guys struggle with the concept of multiple braking modes, anti-spin etc. and with the light trigger feel - and to a lesser amount with the large handle when they have short fingers. - The same guys struggle with Prof M. controllers, preferring something with a Parma style of handle.
      I can get good results with a Slot.it, but prefer it with a stiffer trigger spring fitted, to get on the brakes faster.
      ( I have an SCP1 with standard cartridge, and and SCP2 with high current cartridge fitted)

      Numerous times in the past I watched a less able driver struggling with one on clubnights, and I could see from that, that his controller was set crazy.
      I could take one off a driver mid heat, driver his car two laps while tweaking the controller back to a sensible state, hand it back, and have them go "Wow that's way better"
      But by next week, they would have fiddled, and it would be in a crazy setup state again.
      Those same guys now have Difalcos, as they have less knobs to fiddle with !!!!
      - But I still occasionally say "Rod, you have it set way too hot for this class, dial back your sensitivity"
      "ah yeah, that's better"

      .....siggggghhhhhhh.....


      As Slab said, no maintenance of the wiper as it uses a Hall effect non contact system.

      I haven't ever seen evidence of load on motors causing heating or shorter motor life - not disputing the tech advice from Maddman, just saying, I haven't seen it being material in service.

      They are however the best bang for buck I have seen in the market, with the only competitor for that title being an entry level Truspeed transistor controller.

      Something else for HO to try as he works through his options

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      • #18
        While I hate to say it the decision is probably down to OS3 and Difalco. I was approached several times about building a 1/32 version of the Magic. I declined for various reasons. The largest being that I don't race the scale and would not be able to properly develop the controller. Toward that end I would suggest looking at the group you race (or intend to race) with and get their recommendations.

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        • #19
          Superslab.

          See, that is the engineer in me showing up again.

          I got enough Electrical Engineering exposure to learn that a square wave -- as in your pulse-width-modulated output -- is equivalent to an infinite series of sinusoidal waveforms, of different magnitudes, across all possible frequencies. There is a mathematical magic trick called a Fast Fourier Transform that can translate a particular square wave into its component frequencies. Mr. Fourier's transform actually has practical uses in the real world. Truly.

          Yeah. Me too. The more I try to wrap my head around that the more I get unwound. That's why I am a Mechanical Engineer, not an Electrical.

          As for the fact the waveform -- whether square or sinusoid -- never goes negative does imply a DC offset in the positive direction. Remember I said "unipolar"? Yeah, that.

          The gist of all that technical bafflegab, above, is that there is more energy in a pulse-width-modulated DC waveform than a permanent magnet DC motor can process. That excess energy ends up converted to heat inside the motor. Not useful and possibly damaging.

          And I won't mind it if you call the distinction between our arguments academic. It most certainly is.

          No apologies. I paid good money for those academics. I enjoy a good opportunity to prove I stayed aware in lecture hall.

          Ed Bianchi



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          • #20
            Originally posted by HO RacePro View Post

            The gist of all that technical bafflegab, above, is that there is more energy in a pulse-width-modulated DC waveform than a permanent magnet DC motor can process. That excess energy ends up converted to heat inside the motor. Not useful and possibly damaging.


            Ed Bianchi

            Well that hasn't been the prevailing thought for all the makers who are electronics engineers who have been making PWM controllers, nor has it been a complaint I have ever heard a user of PWM controllers make of the beasts.

            PWM is used for DC motor control pretty widely in industry

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            • #21
              Originally posted by HO RacePro View Post
              Superslab.

              See, that is the engineer in me showing up again.

              I got enough Electrical Engineering exposure to learn that a square wave -- as in your pulse-width-modulated output -- is equivalent to an infinite series of sinusoidal waveforms, of different magnitudes, across all possible frequencies. There is a mathematical magic trick called a Fast Fourier Transform that can translate a particular square wave into its component frequencies. Mr. Fourier's transform actually has practical uses in the real world. Truly.

              Yeah. Me too. The more I try to wrap my head around that the more I get unwound. That's why I am a Mechanical Engineer, not an Electrical.

              As for the fact the waveform -- whether square or sinusoid -- never goes negative does imply a DC offset in the positive direction. Remember I said "unipolar"? Yeah, that.

              The gist of all that technical bafflegab, above, is that there is more energy in a pulse-width-modulated DC waveform than a permanent magnet DC motor can process. That excess energy ends up converted to heat inside the motor. Not useful and possibly damaging.

              And I won't mind it if you call the distinction between our arguments academic. It most certainly is.

              No apologies. I paid good money for those academics. I enjoy a good opportunity to prove I stayed aware in lecture hall.

              Ed Bianchi
              FFT’s! O wow I believe you are grossly overthinking things! We are not looking at using complex mathematical models to “build” a desired wave form. PWM controllers in essence just have an electronic switch in the line from the power supply to the track. Dead simple. The controller just decides what percentage of the time the switch is on vs. off based on the trigger position and the settings and then turns the switch on and off as required. No more, no less. Certainly no from the ground up square wave creation!

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              • #22
                SuperSlab,

                You miss my point. There are no gremlins inside a SlotIt controller furiously assembling square waves out of sine waves. It is the opposite -- when you chop DC into pulses it creates waveforms that have the same effect as a DC signal with AC 'ringing' on top of it. That is a real phenomenon that does in fact create excess heat in a permanent magnet DC motor.

                The fact you can analyze this with different mathematics is interesting, perhaps, but has no real impact. Math tries to model reality. Reality doesn't give a darn about the math.

                Ed Bianchi



                Comment


                • #23
                  Originally posted by HO RacePro View Post
                  SuperSlab,

                  You miss my point. There are no gremlins inside a SlotIt controller furiously assembling square waves out of sine waves. It is the opposite -- when you chop DC into pulses it creates waveforms that have the same effect as a DC signal with AC 'ringing' on top of it. That is a real phenomenon that does in fact create excess heat in a permanent magnet DC motor.

                  The fact you can analyze this with different mathematics is interesting, perhaps, but has no real impact. Math tries to model reality. Reality doesn't give a darn about the math.

                  Ed Bianchi
                  Ed - take a read - PWM circuits don't just through a pulsed, unmodified DC straight at the motor


                  https://www.electronics-tutorials.ws...odulation.html

                  Pulse Width Modulation


                  There are many different ways to control the speed of DC motors but one very simple and easy way is to use Pulse Width Modulation.

                  ut before we start looking at the in’s and out’s of pulse width modulation we need to understand a little more about how a DC motor works.

                  Next to stepper motors, the Permanent Magnet DC Motor (PMDC) is the most commonly used type of small direct current motor available producing a continuous rotational speed that can be easily controlled. Small DC motors ideal for use in applications were speed control is required such as in small toys, models, robots and other such electronics circuits.

                  A DC motor consist basically of two parts, the stationary body of the motor called the “Stator” and the inner part which rotates producing the movement called the “Rotor”. For D.C. machines the rotor is commonly termed the “Armature”.

                  Generally in small light duty DC motors the stator consists of a pair of fixed permanent magnets producing a uniform and stationary magnetic flux inside the motor giving these types of motors their name of “permanent-magnet direct-current” (PMDC) motors.

                  The motors armature consists of individual electrical coils connected together in a circular configuration around its metallic body producing a North-Pole then a South-Pole then a North-Pole etc, type of field system configuration.

                  The current flowing within these rotor coils producing the necessary electromagnetic field. The circular magnetic field produced by the armatures windings produces both north and south poles around the armature which are repelled or attracted by the stator’s permanent magnets producing a rotational movement around the motors central axis as shown.
                  2-Pole Permanent Magnet Motor




                  As the armature rotates electrical current is passed from the motors terminals to the next set of armature windings via carbon brushes located around the commutator producing another magnetic field and each time the armature rotates a new set of armature windings are energised forcing the armature to rotate more and more and so on.

                  So the rotational speed of a DC motor depends upon the interaction between two magnetic fields, one set up by the stator’s stationary permanent magnets and the other by the armatures rotating electromagnets and by controlling this interaction we can control the speed of rotation.

                  The magnetic field produced by the stator’s permanent magnets is fixed and therefore can not be changed but if we change the strength of the armatures electromagnetic field by controlling the current flowing through the windings more or less magnetic flux will be produced resulting in a stronger or weaker interaction and therefore a faster or slower speed.

                  Then the rotational speed of a DC motor (N) is proportional to the back emf (Vb) of the motor divided by the magnetic flux (which for a permanent magnet is a constant) times an electromechanical constant depending upon the nature of the armatures windings (Ke) giving us the equation of: N ∝ V/KeΦ.

                  So how do we control the flow of current through the motor. Well many people attempt to control the speed of a DC motor using a large variable resistor (Rheostat) in series with the motor as shown.

                  While this may work, as it does with Scalextric slot car racing, it generates a lot of heat and wasted power in the resistance. One simple and easy way to control the speed of a motor is to regulate the amount of voltage across its terminals and this can be achieved using “Pulse Width Modulation” or PWM.

                  As its name suggests, pulse width modulation speed control works by driving the motor with a series of “ON-OFF” pulses and varying the duty cycle, the fraction of time that the output voltage is “ON” compared to when it is “OFF”, of the pulses while keeping the frequency constant.

                  The power applied to the motor can be controlled by varying the width of these applied pulses and thereby varying the average DC voltage applied to the motors terminals. By changing or modulating the timing of these pulses the speed of the motor can be controlled, ie, the longer the pulse is “ON”, the faster the motor will rotate and likewise, the shorter the pulse is “ON” the slower the motor will rotate.

                  In other words, the wider the pulse width, the more average voltage applied to the motor terminals, the stronger the magnetic flux inside the armature windings and the faster the motor will rotate and this is shown below.
                  Pulse Width Modulated Waveform




                  The use of pulse width modulation to control a small motor has the advantage in that the power loss in the switching transistor is small because the transistor is either fully “ON” or fully “OFF”. As a result the switching transistor has a much reduced power dissipation giving it a linear type of control which results in better speed stability.

                  Also the amplitude of the motor voltage remains constant so the motor is always at full strength. The result is that the motor can be rotated much more slowly without it stalling. So how can we produce a pulse width modulation signal to control the motor. Easy, use an Astable 555 Oscillator circuit as shown below.



                  This simple circuit based around the familiar NE555 or 7555 timer chip is used to produced the required pulse width modulation signal at a fixed frequency output. The timing capacitor C is charged and discharged by current flowing through the timing networks RA and RB as we looked at in the 555 Timer tutorial.

                  The output signal at pin 3 of the 555 is equal to the supply voltage switching the transistors fully “ON”. The time taken for C to charge or discharge depends upon the values of RA, RB.

                  The capacitor charges up through the network RA but is diverted around the resistive network RB and through diode D1. As soon as the capacitor is charged, it is immediately discharged through diode D2 and network RB into pin 7. During the discharging process the output at pin 3 is at 0 V and the transistor is switched “OFF”.

                  Then the time taken for capacitor, C to go through one complete charge-discharge cycle depends on the values of RA, RB and C with the time T for one complete cycle being given as:

                  The time, TH, for which the output is “ON” is: TH = 0.693(RA).C

                  The time, TL, for which the output is “OFF” is: TL = 0.693(RB).C

                  Total “ON”-“OFF” cycle time given as: T = TH + TL with the output frequency being ƒ = 1/T

                  With the component values shown, the duty cycle of the waveform can be adjusted from about 8.3% (0.5V) to about 91.7% (5.5V) using a 6.0V power supply. The Astable frequency is constant at about 256 Hz and the motor is switched “ON” and “OFF” at this rate.

                  Resistor R1 plus the “top” part of the potentiometer, VR1 represent the resistive network of RA. While the “bottom” part of the potentiometer plus R2 represent the resistive network of RB above.

                  These values can be changed to suite different applications and DC motors but providing that the 555 Astable circuit runs fast enough at a few hundred Hertz minimum, there should be no jerkiness in the rotation of the motor.

                  Diode D3 is our old favourite the flywheel diode used to protect the electronic circuit from the inductive loading of the motor. Also if the motor load is high put a heatsink on the switching transistor or MOSFET.

                  Pulse width modulation is a great method of controlling the amount of power delivered to a load without dissipating any wasted power. The above circuit can also be used to control the speed of a fan or to dim the brightness of DC lamps or LED’s. If you need to control it, then use Pulse Width Modulation to do it.





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                  • #24
                    So much for basic theory of how to make a PWM circuit. One of the biggest issues (and so far yet untouched in this thread) is controller feel and feedback. A resistor controller has a certain feedback from the car. This is caused by motor load, motor RPM, amount of power given to the car and other factors. Some electronic controllers mimic this. Some do not. It all depends on the controllers negative reference point.

                    Controllers such as the Ruddock, Lucky Bob and others use the red wire as the negative reference. Based on the circuit designs I have seen for a PWM controller it is safe to assume that they also use the red wire as the negative reference. With these controllers the voltage applied to the car will always be the same for a given trigger and sensitivity pot position.

                    There are other controllers such as the Difalco that use the black wire as the negative reference point. Such a controller will interact with the car’s motor and the voltage provided to the car will not be the same at all times for a given trigger and sensitivity pot position. This strangely enough may make the car easier to drive.

                    Case in point. When I transitioned from my highly modified Ruskit (Parma) resistor controllers with variable resistance, variable brake and choke to a Ruddock it took a solid season to get familiar with the controller and learn to drive a car on the limit. When I went from the Ruddock to my first generation electronic controller (which used the black wire as the negative reference) I won first time out and TQd.

                    It was a case of arrive late, pull a cold car and controller out of the box, hook up and race. I didn’t lead the first segment as the car had to warm up. By the end of the qualifying race I was in the lead and pulling away. Note that the others drivers in this race were not slow. They took the remaining sit outs in the A and B semis.

                    You need to consider how you want the controller to interact with you as part of the decision process. Some do just fine with a controller with no feel. Once I got used to my Ruddock it won many a race. Some do better with a controller that interacts with the car.

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