The Craftmans Shop > New from Old
The Sequel - Oh Blimey I bought a CNC Lathe (Beaver TC 20)
awemawson:
Good thought but there is no sign of belt slippage or squeals. It's a very wide multi-groove affair.
Got the 'scope on the analogue speed signal to the Mentor, triggered from the Servo Enable signal, and was horrified how much noise is superimposed on the base DC voltage. Not sure at this point if it is being picked up on the leads I'm using or if it is genuinely on the signal but I need to bottom it out. A 1uF capacitor across the scope terminals got it down to a level where I could see the signal but I'm still interpreting what I'm seeing.
Certainly it's looking vaguely like the diagram that I posted from the manual, but I still have work to do to see it properly.
Meanwhile if anyone wants my old Tektronix 453 I'm putting it up for grabs on the cork board, basically free, but make a donation to Eric to help with forum running costs:
https://madmodder.net/index.php/topic,12761.new.html#new
awemawson:
Well the usual running round in circles and chasing my tail has produced results.
Firstly I wired up a break out box to allow me safely to connect to the Servo Enable and Analogue signals for the DC Spindle servo system. As the X & Z axis signals and also those I wanted to hook into were all on the same 25 pin 'sub D type' the break out board was necessary to stop the X & Z servos slamming all over the place as I probed.
This allowed me to 'scope the signals and observe that servo action ceases at close to but not at set point - why ? Also the signal to noise ratio was pretty dire - for the small error signals down at the small increments involved ( about 45 mV) the noise was of the same magnitude. Some but not all this noise was being picked up on the twisted pair wires I was using and had a 165 uSec periodicy.
Then I turned my attention to the issue of the drive to the field coils. Plate on the motor says it wants 170 volts but I've never managed to achieve that without other problems ensuing. One big issue was not being able to measure current and voltage simultaneously and also have the fan mounted on the FXM-3 supply.
So I've added a DIN rail and brought the field coil connection out and via it, it such a way that they are both accessible and also there is a loop that I can get my clamp ammeter round. As an aside it has been suggested that the clamp ammeter, although being a true RMS one, won't show a correct current due to the chopped thyristor nature of the current wave form. Well I've disproved this, as it EXACTLY corresponds to my AVO 8 Mk 6 when both are measuring at the same time.
This also allowed me to see the 'field weakening' characteristics of the FXM-3 - to achieve high speeds the field current is drastically reduced over about 1500 rpm
So with the field set to 5.8 amps and resulting in 160.5 volts the motor characteristics were definitely different, but still it would not turn at the desired 5 RPM. Incidentally, this is 5 RPM at the spindle, the motor at this spindle speed in low gear needs to turn at about 22.5 rpm due to the 3.2:1 gear box, and belt ratios.
. . .why, oh why oh why ? A question that has had me racking my brain for months.
awemawson:
So I decided to read the Mentor-1 DC spindle drive manual from end to end leaving no commas and full stops unmolested :bugeye:
. . . and guess what - I found that there was a parameter (#161) to 'Enable Standstill Logic' which if the speed is very low, backs off the thyristor firing angle all the way to prevent creep. BUT there was also a parameter (#162) that if #161 was set allowed creep speeds for shaft orientation well I never.
So quickly setting this parameter to '1' guess what - M19 works :ddb: :ddb: :ddb: :ddb:
Now it's not all plain sailing, as, although there is a method of saving the new parameter values to EEPROM, try as I may it isn't working, and the parameter reverts to it's old value on a power down / up cycle.
...BUT M19 IS WORKING :clap: :clap:
In the video the program is stepping round in 20 degree segments and I've set the positional accuracy to 2/11 ths of a degree as per the original parameters that I obtained
RussellT:
I knew you'd find it! :nrocks: :nrocks: :nrocks:
awemawson:
And now the new parameters are saved :ddb:
Turns out BOTH security codes need entering despite the flag saying one of them hasn't been set :scratch: Anyway, they are saved, AND I narrowed the location down to its finest setting of 1/11 th degree and sure enough it servo / homes in on it with the accuracy asked for.
Just need to find out how PK is doing 'down under' with that probe - he's gone ominously quiet for a few weeks, I hope he's OK.
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