Field coil reverser for Taycol motors...

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Dodgy Geezer
Posts: 12
Joined: 17 Jun 2018, 09:54

Field coil reverser for Taycol motors...

Post by Dodgy Geezer » 01 Dec 2018, 15:08

Field wound motors like Taycols can't be reversed by swapping input polarity - see here: http://taycol.tk/Reverse.html

So you can't use a standard ESC to drive them in reverse. There is a trick to manage this using a bridge rectifier - see here: http://taycol.tk/Rectifier.html - but it has a few disadvantages:

- it converts the series-wound motor into a shunt-wound one
- it loses a couple of volts
- if a connection or component fails, all the power may be routed through one of the coils, ruining a vintage motor.....
- it's not exactly original....

The 1950s approach to the problem was a double-pole crossover switch, as shown here: http://taycol.tk/tayins501a.jpg And we should be able to replicate that using a relay driven by a polarity detector...

I enclose a simple detection circuit which seems to do much of the work - but it's a little unstable. It uses an LM358 op-amp as a comparator connected to the ESC output lines, and just outputs an LED to show that it's working. Eventually, the LED would become a transistor driving a relay which would swop the field coil windings around to obtain reverse.

The LED lights up fine when driving the motor one way. It ought to turn off when driving the other. but in fact it seems to stay on for a while - until the voltage gets to about 1v. I suspect that it's the capacitors still holding a charge, or perhaps a back emf from the motor? If I knew about such things I am sure that I could sort it out....

What I really need is a solid, positive way of latching the output line one way when it's positive, and the other way when it's negative. Which works with a PWM signal. I'm sure there must be a way to do it, but I can't find anything obvious on the net....

Does anyone here know a way? I enclose a circuit diagram....
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Martin
Posts: 168
Joined: 16 Feb 2018, 14:11
Location: Warwickshire

Re: Field coil reverser for Taycol motors...

Post by Martin » 01 Dec 2018, 15:29

Safest way would be to use a microcontroller, like an Arduino, to read the input signal, and drive the output signal to the ESC and a separate output to drive the relay coil. This might seem complex but the Arduino is cheap and tiny, only costs a couple of pounds on Ebay, and can be powered by a normal 'servo - style' connector plugged into the receiver. It will end up covered in heat shrink looking a bit like an ESC with one or two cables to the receiver, a connector to plug your ESC to, and a couple of pins or terminals to connect to the relay coil.

An advantage would be that the Arduino could prevent switching of the relay when the motor is running - it would be easy to build into the program the facility that when the Arduino detects a 'reverse' signal, it would ramp the motor down to a standstill, allow a little time for it to stop, then switch the relay over and ramp the motor back up again.

I can write you a sketch for it if you're interested in trying this route. If you are, let me know if you want to use a single radio channel (with 'stop' in the middle and forward/reverse working either side of that) or two radio channels, one for 'power setting' and a separate 'switch' channel to control direction. I suppose I could make the same sketch operate in either mode with a little jumper to select whether to use one channel or two channel control.

Dodgy Geezer
Posts: 12
Joined: 17 Jun 2018, 09:54

Re: Field coil reverser for Taycol motors...

Post by Dodgy Geezer » 01 Dec 2018, 17:33

Hmm...I can see the argument - but somehow doing it in software grates a bit. I was trying to revive my lost electronics experience as well as provide a service for the Taycol board, so I was hoping that someone knew of a simple 'zero detection' circuit.

I hadn't thought of an Arduino. I suspect you would have to use a shield if you were connecting to power circuitry, which would make it a bit more complicated. It would run off a single proportional channel - that way you have to ramp down the revs before reversing, rather than run the risk of switching over at full throttle.

One problem with micros is the horribly dirty RFI environment around Taycols. All those copper coils induce massive voltages and currents everywhere. Action Electronics produced a Pic-based unit to do something like this job - it was actually intended for reversing steam boats or waterjets, both of which need a zero point detection and then a mechanical action. So there is a small but solid market for boxes providing a 'reverse' signal. I tried one on my Taycols (because some of them have a single mechanical reversing switch) but it jittered so much in spite of all the suppression I could manage that it was a non-starter.

It's very kind of you to offer Arduino support. I will need to learn how to run the things at some stage, as I am getting myself a CNC cutter kit for Christmas, and that will have an Arduino processing the G code. But I think I am so close with this simple circuit that I would like to make it work at a simple level, where I can see where all my electrons are going! If it turns out to be impossible, I might well take you up on it in the New Year...

I attach a scope shot of the problem area - at about 30% of the PWM duty cycle you can see the capacitor discharge coming after the pulse stops. That seems to be associated with the spurious LED output signal. I suspect that a diode would stop things, if only I knew where to put it. Perhaps the cap value is just too high. Or too low...
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Last edited by Dodgy Geezer on 01 Dec 2018, 17:55, edited 2 times in total.

Martin
Posts: 168
Joined: 16 Feb 2018, 14:11
Location: Warwickshire

Re: Field coil reverser for Taycol motors...

Post by Martin » 01 Dec 2018, 17:52

No worries. I'll get on to it if and when you decide you want to try.

An advantage of using an Arduino is that you could switch from full ahead to full astern (on the transmitter) and the Arduino would then.
  1. Ramp the 'ESC' output down to zero (over say half a second)
  2. Wait a second for the motor to stop
  3. Switch the relay output
  4. Wait half a second for the relay to change over
  5. Ramp the 'ESC' output up to 100% again (over say half a second)
You'd be able to set the actual ramp and delay times by editing the Arduino sketch - so you could set any delay you wanted from zero up to something ridiculous like a minute or more.

The Arduino is quite good at working in noisy environments - you wouldn't need a shield.

I understand the desire to use a 'simple' analogue detector - but it's likely to cost more than an off-the-shelf Pro Mini Arduino from Ebay. Sometimes it's worth using a sledgehammer to crack a nut if a sledgehammer only costs a pound, but a pair of nutcrackers costs ten pounds! :lol:

Dodgy Geezer
Posts: 12
Joined: 17 Jun 2018, 09:54

Re: Field coil reverser for Taycol motors...

Post by Dodgy Geezer » 01 Dec 2018, 19:08

What a world - where more computing power than Bletchley Park had costs less than an op-amp and a couple of caps/resistors! I see that they are available on ebay for between 3-4 pounds - but I suspect that you need a bit more to code them up. Can you really run an Arduino with direct connections to the motor input cables? You might get spikes of 50v or more.

It's actually the cost of the little bit of Veroboard which makes doing your own circuits that expensive. A lost art.....

Martin
Posts: 168
Joined: 16 Feb 2018, 14:11
Location: Warwickshire

Re: Field coil reverser for Taycol motors...

Post by Martin » 01 Dec 2018, 21:07

You'd connect the Arduino to the Radio receiver. Its only connections would be to the Radio receiver, the ESC (the connector from the ESC that would otherwise plug into the radio receiver) and the relay coil. If the relay coil is a 5V one that's all you'd need, if you want to run the relay coil at some higher voltage you'd also need a connection to the battery. If the relay coil is a 5V one that works on 40mA or less then it can connect direct to the Arduino. If the relay coil needs more volts or more current then we just have to add one transistor to drive it.

So there's no direct connection between the Arduino and the motor input cables. Bear in mind that any modern receiver and any modern ESC will already have inside it at least one chip that is comparable to the one on an Arduino Pro Mini - so you already rely on such chips being reliable in electrically noisy environments.

To program a Pro Mini, you either need a USB-to-5V serial interface, or a USBASP. Both are available on Ebay for similar prices to the Arduino itself. Probably best to get both as the USBASP is better for final programming, but the serial interface is better for debugging work when developing your own sketches. If you get an Arduino Nano, or some other types then you don't need anything other than a USB cable.

Best way to get started, if you've never used Arduinos before is to get either a UNO (the older simpler one without WiFi) or a Nano - for both you only need the USB cable. A Pro Mini is basically a NANO without the built-in USB interface. You can buy the 'genuine' ones if you want, but the clones on Ebay are cheaper and (in my experience) work just the same.

If you want me to program one for you, then you don't need anything! I can put a pre-programmed one in the post and you'll only need to wire it up and provide it with whatever physical protection it needs for installation in your boat or whatever you intend to use it in.

Dodgy Geezer
Posts: 12
Joined: 17 Jun 2018, 09:54

Re: Field coil reverser for Taycol motors...

Post by Dodgy Geezer » 03 Dec 2018, 12:17

You'd connect the Arduino to the Radio receiver. Its only connections would be to the Radio receiver, the ESC (the connector from the ESC that would otherwise plug into the radio receiver) and the relay coil.

Ah - I see. You'd use it as a switch to come on at the crossover point of the control pulses, rather than detecting the polarity at the motor.

In that case, you could simply use a standard R/C switch - like one of these - https://hobbyking.com/en_us/turnigy-rec ... tch-1.html I use these quite a lot - http://www.lutonmodelboat.co.uk/hints/R ... 20Iss3.pdf - you simply adjust the pot to determine when you will switch. Being able to adjust means that you can set the switch up for an ESC where the neutral point is not at 1.5ms....

I do like the elegance of directly detecting the polarity change at the motor, however - that would self-correct for any variation of centre. I have been doing a bit more experimentation, and have a circuit which seems to work. The problem was trying to get the right cap/resistor combination to change the PWM signal into DC. Without a formal electronics background I just did trial and error, and have found something that seems to fit the bill. The only issue now is the the radial capacitor is an old NATO-designated component from my box, and I'm not sure what it is. It's C1 in the diagram below...

It has markings "0.003/500" on it, which makes me think it is a 0.003mf at 500V. My cheap tester says it's 0.005uf, or 5.5nf. It would be nice to be able to do the calculation so that I can get the value right in the optimum rather than 'working on the bench'...
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