Member postings for J A Harvey-Smith

Thanks Muzzer,

I was briefly clutching at straws, thinking I might donate a new controller for a couple of hundred to make my working day a bit more interesting and productive, but I'm not going to fork out on positioners to talk to 30+ year old obsolete analogue servo drives on top. A proper job could be done replacing servos and motors, and power supply - expensive items to suit this big machine - and a couple of hundred soon turns in to thousands - but not from my pocket. As you suspect, a very large piece of scrap.

A very interesting thread, thanks to all contributors.

In my workshop LinuxCNC is the clear leader at the moment but yes, I do find it tedious getting the setup right and the keyboard and mouse are a pain. I ditched Mach3 the last time the KX3 mill caused grief, wasted a little time playing with Arduinos and Pis, but produced some seriously capable hardware using cheap ARM Cortex boards using the free mbed.org online compiler, along with Altera FPGAs. However, this was all for academic interest as I was never going to write software to create a full CNC controller, I don't have the years left and no desire to reinvent the wheel.

I work in an engineering workshop of a heritage railway. Last year we got a "new" (which in heritage railway parlance means post WW II) Harrison 500 CNC lathe. Lovely !! - for a week until the electronics failed. I'll steer clear of railway politics, but a year on it ain't fixed. The computer part of the control electronics was a DOS pc with obsolete proprietary plugin boards, this no longer talks to the considerably older servo drives which take +/- 10V signals to control axis velocity, position encoders feeding back to computer to close the loop. The spindle and axis encoders are good, the servos work (as proved using AA cell as signal source). I would not waste my time trying to persuade management to let me loose on the machine with a LinuxCNC system - nobody but me would ever use it (or feel confident with it), but I'll be reading through the manual for the 990TDB which (unless I've missed something obvious) just might be sellable to them.

If anyone can spot why this won't work, please advise. Thanks again.

Jon

Please keep the electronics coming.

Intended purely as constructive criticism, the case for a flyback, efficiency, catch (call it what you will) diode explained.

Motors are not the same as relays. As a simplified model, sufficient for this argument, the brushed dc motor may be modeled as a voltage source in series with an inductance - the voltage source represents the back emf and will be non-zero whenever the shaft is turning. The inductance is the inductance of the motor windings, denote this 'L'. Yes there is also resistance, inter-winding capacitance and other factors, but we only need to consider the voltage and inductance here.

This is what happens. Assume the motor is powered from a battery of 'Vbat' volts, and is rotating such that it is producing some back emf 'G' (for generated) volts. The switch TR3 closes (ignore on-resistance) and so applies a voltage of (Vbat - G) volts across the inductance L. Call this voltage 'E' such that E = Vbat - G

Rearranging the textbook inductance formula E = -L di/dt and ignoring the '-', the current in the inductance L will change at the rate (di/dt) Amps per second, which equals E/L.

The current flowing through the inductance increases linearly (ignoring effect of resistance) while switch TR3 is closed.

The flyback diode provides a path for this current when switch TR3 opens. The current flow in an inductance can not change instantaneously, and the voltage across the inductance will tend to oppose any change in current. This means that, when switch TR3 opens, the polarity of the voltage across the inductance reverses, and without some constraint, will soar off towards infinity. The only constraint in the circuit as drawn is the avalanche breakdown voltage of the TR3 device, as alluded to by 'Muzzer' above, and as he implies, this is safe enough for small loads. However, including a flyback diode, the current flow in the inductor is returned to the supply. This has the great benefit of reducing the average current drawn from the battery as energy from the inductor is returned to the supply rather than being wasted heating up TR3. Also, the current flowing in the inductor is the current producing torque. Returning this current via a flyback diode, the voltage across the inductor with TR3 switched off is far less meaning the rate of current fall is less, which is why this diode is sometimes called an 'efficiency' diode.

Jon Freeman

My best advice is to choose a project you are likely to see through to completion. This will do you more good than being over-ambitious and not reaching a satisfactory outcome. Show what you can do, not what you can't.

Take advice from your tutors. I've seen students get carried away with software and motors and wiring, leaving too little time at the end to write up the all important report. Be clear at the outset about relative importance of each element of the deliverables expected of you.

And good luck !

A long and interesting thread. The outgoing editor has taken a pasting for taking his holiday at the local branch of 'Sheds R Us', and sharing his many varied and interesting snaps with us. I wish him well for the future and welcome the new editor. Maybe I'll renew my subs and give him a chance.

Quite frankly I seldom find much of interest in MEW these days. Too many interminable series interspersed with 'How to make a Three-Eighths Gripley With a Knurled Flange' type articles by authors who have spent too many years surrounded by three-eighths gripleys when they should have been getting out more. If the title were ME Nineteenth Century Workshop then fair enough, but I'd like to see some fresh ideas, some novel new approaches to familiar problems. I'd like to see more use of low cost tools that never need grinding - computers and electronics, for example, and why is it that even the simplest of maths is so studiously avoided?

"intention to cover in depth the multiplicity of techniques...." indeed, a pull back towards this ideal would be welcome, recognising some of the exploding mutiplicity of techniques blossoming in the 21st century.