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  1. #11
    So you are electronic gearing at 10:1 (10,000/(10/1)=1000ppr
    -use common sense, if you lack it, there is no software to help that.

    Email: [email protected]

    Web site: www.jazzcnc.co.uk

  2. #12
    Yes, though within UCCNC I've settled at 1500ppr, which accounts for the 40:60 pulleys.

  3. #13
    ...which was to recover the 2000rpm spindle speed on a 3000rpm servo that replaced a 5000rpm BLDC. My head hurts.

  4. #14
    Okay, so here's something different.

    I've been driving this spindle servo using a UC300eth with step/dir signalling from UCCNC, with the two pins from the £5 Chinese BoB driving two separate TTL->RS485 adapters (£3 boards, based on MAX485 chips) - these to provide a differential drive to the PP+/PP- and PD+/PD- position inputs on the servo controller. This worked out of the box from first attempt, so many months ago. I should say there's ~10 ft of 4-core dual-twisted-pair signal cable between the control box and the servo controller.

    Nothing has changed, wiring wise.

    Today, no spindle. Nada.

    I hate working on this machine as the controller is built into the bench as a rack-unit (I'm never doing that again). But, stripped it all down, and noticed that when scoping the step/dir signalling to the servo controller, if I (accidentally) grounded the PP- with the scope ground on the probe, that the servo kicked into life. I've checked this all the way back to the MAX485's, so it's nothing to do with the wiring or connector, but that the differential drive output from the '485 can no longer drive the Opto-inputs on the servo controller. Annecdotally, this appears to be the same on the direction input as well as the step input.

    QUESTION: There's nothing other than wiring on the typical Chinese servo drivers to set Unipolar/Differential drive, is there? - pretty sure it's just a case of driving the opto-input accordingly.

    So, I have a quandary.

    1) Replace the 485 drivers with same (need to order some) - but I'm loathed to do that - there's no reason to doubt that the replacement will also fail at some stage.
    2) Replace the 485 drivers with an open-collector transistor drive - I've plenty of trannies to hand, and that allows the unipolar drive iaw the manual. But it's a strip-down of the control box.
    3) But the servo controller input is just an opto-isolator input configured for 0/5V signalling. The easiest solution is to drive the servo from the BoB output pin, and ground the PP- input at the servo controller input. This is the lazy solution.

    QUESTION: What signalling method do others use for reliably driving the step/pulse input on servo controllers?
    Last edited by Doddy; 04-10-2020 at 08:26 PM.

  5. #15
    Does your controller give a rough schematic of the input circuitry? The ones that I've seen had the "differential" input simply going to the diode side of the optoisolators with an antiparallel diode and a small series resistor. If so, driving it single ended is no problem, though if it were me (and since you already have it wired up with twisted pair) I'd do the grounding of PP- back at the BOB.

  6. #16
    Muzzer's Avatar
    Lives in Lytham St. Annes, United Kingdom. Last Activity: 2 Hours Ago Has been a member for 6-7 years. Has a total post count of 417. Received thanks 61 times, giving thanks to others 10 times.
    I expect the "backward" drive of the opto input through the antiparallel diode reduces the recovery time of the LED, or at least overcomes its capacitance. The propagation delay of a typical opto can be around 1us or so and the input capacitance may be as much as 200pF or more.

    You may have noticed that some drives show a higher PPR for differential mode than for single ended and this can only be accounted for by the LED being flipped more quickly.

    Have a look at what's inside any of the controllers you have to hand. Many of them are simple, single-ended (open collector) drivers and the rest are what you used. Not much help to you, I know....

    EDIT - my Yaskawa Sigma servo (4th axis) has SN75174 drivers https://www.ti.com/lit/ds/symlink/sn...oogle.com%252F

    These are probably a good "golden reference".
    Last edited by Muzzer; 05-10-2020 at 02:34 PM.

  7. #17
    Quote Originally Posted by Muzzer View Post
    You may have noticed that some drives show a higher PPR for differential mode than for single ended and this can only be accounted for by the LED being flipped more quickly.
    That's a good point, so if you are going to drive them single ended you want a driver that can both sink and source current, not an open collector type.

  8. #18
    Muzzer's Avatar
    Lives in Lytham St. Annes, United Kingdom. Last Activity: 2 Hours Ago Has been a member for 6-7 years. Has a total post count of 417. Received thanks 61 times, giving thanks to others 10 times.
    No, generally a single ended driver just sinks current. When you turn it off, the current into the driver stops. If there is a decent capacitance in / around the opto LED, it takes a few hundred nanoseconds to actually turn off completely.

    The differential line driver (RS485 etc) both sinks and sources current by swapping the polarity over using a totem pole output stage. So generally you have a choice between a sink-only single-ended driver or a full sink-source differential driver.

    It's a pity the current solution already uses differential drivers. Do both sides of the driver output still work? Or is the failure due to perhaps one of the lines no longer switching? It's possible that might explain why it bursts into life when the scope grounds one side.

  9. #19
    I scoped it last night with 2 channels in differential mode and the output from the 485 were sweet (common mode noise all gone), particularly compared against the single ended measure (full of noise). Also, last night, changed the kernel frequency of the uc300 to 50khz to increase the pulse width to 10us (at 100khz kernel the pw is a rather narrow 5us).

    The thing that gets me is that this was working fine. Going single ended feels an inferior move than differential driven)... unless one of the 485 outputs is starting to fail hi-z.

    Equivalent input circuit is as you describe, opto, reverse diode and series resistor -330R

  10. #20
    Quote Originally Posted by Muzzer View Post
    No, generally a single ended driver just sinks current. When you turn it off, the current into the driver stops. If there is a decent capacitance in / around the opto LED, it takes a few hundred nanoseconds to actually turn off completely.
    Depends whether it's a bipolar/TTL or CMOS driver. The former tend to be MUCH better at sinking than sourcing, whilst the latter are pretty symmetrical due to the complementary output stage, e.g. the datasheet for the 74HC245 which is quite popular on cheapo BOBs shows 0.18V droop when sourcing 6mA and 0.15V when sinking 6mA: 30mV difference in the ~3V across the series resistor. Differential would still be a bit better as you potentially have 2x the voltage discharging aforementioned capacitance, but it's a darn sight better tah open collector drive.
    Last edited by Voicecoil; 05-10-2020 at 10:35 PM.

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