On Tuesday 24 November 2020 15:09:42 John Figie wrote: > emc-users, > > I acquired a Clausing 1450 NC lathe with the intention of LinuxCNC > retrofit about 6 years ago. I had all kinds of plans to make my own > servo drives but never really made much progress due to other > priorities. Now I am getting impatient and want to move forward. > Currently this machine has no controls. The machine is fitted with x > and z servo motors without tachometers and with no provisions for > tachs. The original control was NC by paper tape. I have none of the > original controls and I am apparently the 3rd owner intending to > retrofit this machine. The basic machine is early 70's vintage and > looks like an engine lathe. It is in remarkably good shape probably > because it is not a production style machine and it was probably a > real pain to program and run. The ball screws show no lost motion with > a 0.0001" reading indicator. The ways look good and are flame > hardened. The serial number is 155003 stamped on the bed. > > My plan: > I have an Asus M5A 78L-M LX MB with debian 10 and preempt-rt kernel. I > have LinuxCNC 2.8 installed. I also have installed an intel dual port > server NIC that is supported by the OS. > Max latency is 50,000 nsec average is about 25,000 nsec. with the > combination above. > > I am thinking that I want to use Mesa Etherent I/O cards and I want to > use PID servo control. > > About the motors: These are Peerless DC motors (8 brushes). 66V max > and 3.75 Amps Max. 800 RPM coupled directly to the ball screws with a > servo coupling. > > I will need to fit decent encoders to the ball screws. I have some > SICK hiperface SIN/COS encoders with 1024 cycles per rev. Even if I > just use these as incremental encoders without SIN/COS interpolations > I can get 4096 counts / rev and my ball screws are 0.2" / Rev so that > would yield 0.000048828" of linear travel per count. Max speed will > be 160 in/min @ 800 RPM. > > I am wondering what might be the easiest way to drive these motors and > get reasonable performance without spending too much money.
That last drops a pipe wrench in the gears, but here is what I would do. Pick a Mesa card that can supply 2 pwmgen modules, one for each axis that count=2 because with LCNC you don't need a springy compound under your toolpost, a block of cast iron sitting on the carriage, milled to the same height as the compound was will be many time more rigid and chatter-free. If this board has a spare pci socket, the Mesa 5i25 is a good candidate. You won't need the ethernet to run your machine. And the latency will be better I expect. Omron sells A/B/Z encoders for about $20/axis if what you have doesn't work. Up to 2048 per rev, that is 8192 edges per turn. Up to 6000 revs, so your 800 is a piece of cake. For motor drivers, the Pico Systems pwm-servo amplifier is a bit of overkill but is as bulletproof as they come. The only problem is that it needs a 12 volt signal and a tickle from a boot module to enable it. You may be able to find something smaller than its 160 volt, 20 amp maximum ratings. It is a full 4 quadrant controller, feeding the motors energy back into the power supply while stopping to reverse. But then it will use that energy to re-accelerate the motor in the other direction. But... Its around $120/copy. I actually use 2 of them, driving 1hp PMDC motors to around 2hp actual output as spindle drivers. Neither motor has complained in several years, but one of them has totalled the belt drive in a 7x12 lathe 6 or 7 times. That's how fast they can respond. I am in the process of motorizing a Chinese clone of a BS-1, and am using an estate gate motor rated at 100 watts at 24 volts, and have after quite a few false starts, finally found a dual H-Bridge driver with a pair of Infineon BTS 7960 chips on it, rated at 43 amps, but the max voltage is also 45 volts, below what your motors need for good performance. Running this motor from a 350 watt switcher putting out 24 volts, running overnight, the motor was not anywhere near too warm the next day, and this $13 driver was dead cold at room temp after about a 12 hour run. I have a higher voltage supply, a 42.5 volt, but haven't tried it yet. This driver needs the PWMGEN to be run in mode-2. This will be a common requirement for most of the H-Bridges out there. The problem is that most are designed to be used in cars for such as seat controls but will suffer destructive shoot thru at the higher voltages. So look around. Your 66 volt motors can probably take twice that for the few seconds they run to null the PID without damage unless the shaft twists in the coupling. They will make serious torque running that way. One of the reasons I like the Pico amplifier is that you can program the maximum current it will allow to reach the motor, thereby controlling the over torque. You'll hear the iron in the motor squeak when that happens. > This is not my first LinuxCNC retrofit. I had a lot of online help > back in about 2008 to retrofit a BP series II Interact. That machine > is still running LinuxCNC 2.3 and works fine for me. But LinuxCNC has grown many, many more capability's since 2.3. Watch your encoder's path back to the mesa card, if a std bob is in that path, it may not be fast enough. The symptoms are that motor speed can rise to where the encoders pulses no longer get thru the bob's input circuits, at which point the motor goes wide open until power is removed. Don't have expensive tools mounted in case of a runaway crash. There are ways around that so if you're hit by it, come back and see if (I am an old fart at 86, but also a CET) it can be fixed. > http://www.machineability.com/Bridgeport_series_II.html Thanks to guys > like John > Kasunich, Crhis Radek, Jeff Epler, Stephen Wille Padnos and others > typically one line in those days. But what really got my started was > attending an EMC-Fest in 2008 in Galesburg IL. > > Suggestions on Mesa + some kind of Servo Drive are welcome. I can > build stuff, I just don't want the project to be too big:-) > > Regards > > John Figie Stay safe and well, John. Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) If we desire respect for the law, we must first make the law respectable. - Louis D. Brandeis Genes Web page <http://geneslinuxbox.net:6309/gene> _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
