mbalentine
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cfordyce started following mbalentine
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The implications of this are interesting. Are you able to comment on available drive capabilities/limitations to operate in current mode at this update rate? Will there be ODT drive products to create a 'single source' solution based on higher speed eCat?
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You may want more details than I am going to offer... 1) Why do you want to do such a thing? 1a) What type of amplifier interface are you using? I would think that a direct PWM such as a Brick would offer the best level of control for the transitions (but I am not that familiar with Ethercat) 2) Can you give some info on the difference of Force Constant and Pole Pitch between the two motors? 3) Obviously you are wanting to point both motors to the same feedback register in the ECT. The setup parameters for each motor will then be determined by the specific motor characteristics. 4) I would think that you do not not want to 'kill' either motor, but rather change between open loop and closed loop mode so that both motors are always active and commutating. I believe this will also eliminate the need for a 'pmatch' command and the delay required for it to execute. 5) You will need to calculate an initial output value for the motor transitioning into closed loop mode. This will need to be based on the output of the motor going out of closed loop and scaled appropriately based on the ratio of force constants. 6) It may be best to maintain a small open loop output on the motor transitioning into open loop mode. Think of it as an assist to the motor in closed loop. This will be more difficult if the load is noisy.
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PPMAC is a little more flexible than the conventional looping practiced in traditional PLC hardware from other suppliers. A good overview can be seen in two sections of the May 2019 PPMAC Users Manual: - 'POWER PMAC COMPUTATIONAL FEATURES' [pp 516] opens with a section titled 'Computational Priorities'. This deals with repetitive (looped, sort of) tasks such as motion and plc programs. Page 65 lists tasks based on the various clock interrupts. - 'WRITING C FUNCTIONS AND PROGRAMS IN POWER PMAC' [pp 807] deals with executing compiled C code within PMAC. C code executes much faster than interpreted script and can be executed from a hardware capture interrupt. Danger in hogging too much of the available resources if you do too much, too often. 'Data Gathering' is another option for collecting data (no logic execution), but may not trigger often enough for your needs. Max frequency is Phase clock. See pp 33 in User Manual.
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Just a quick FYI on phase direction. The problem you cite should have shown up in the Open Loop Test where the motor is phased forward and the position is displayed graphically. You can also observe your mechanism to see if it is moving in a positive direction. This is the point in commissioning where feedback, phasing, and mechanism direction are confirmed to all be in agreement. Very important to confirm agreement of motor & feedback to prevent a potential runaway condition. (phase algorithms will also fail since the movement is not what the algorithm anticipates) Once the motor & feedback are in agreement, the mechanism direction can be reversed by changing both motor phase AND encoder direction sense. Eric Hotchkiss pointed out a method available in PPMAC of reversing motor phasing without swapping wires here: http://forums.deltatau.com/showthread.php?tid=2928&highlight=phase Note that in these instructions the encoder direction sense is being changed along with motor phase direction. It is assumed these are already in agreement.
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RESOLVED - Description of Biss-C Setup on PowerBrickLV
mbalentine replied to hannsx's topic in Power PMAC
http://forums.deltatau.com/showthread.php?tid=842&pid=3169&highlight=biss-c#pid3169 Comment from 2012 regarding Biss-C I think is still relevant. Not in Gate-3, but can be accessed in ACC-84E Unless your Brick has the ACC-84 option, you may need a different feedback (???) Acc-84E manual Biss-C starts on page 125 (unfortunately for Turbo) http://www.deltatau.com/manuals/pdfs/ACC-84E.pdf?id=635787783277365390 Setup for the ACC-84E option on a P-Brick (with ACC-84E option) is covered in the Power PMAC Users Manual beginning page 128 -
In a recent posting you refer to (I assume same mechanism) needing more motor current, in part due to stiff bearings. This can affect the success of the two step phase finding algorithm. The algorithm looks at rotor (encoder) position and assumes magnetic alignment when a phase is excited, and then validates by checking if the motor moves an appropriate distance when a different phase is excited. If the delta is too big/small, the alignment cannot be trusted and the test fails. Causes are generally twofold: - physical obstruction, stiff mechanism, bias due to gravity. - Magnetic pole irregularities in the motor. This is always present to some degree and can cause random pass/fail results depending on which pole/magnet happens to be in position at start-up. Motor[x].PhasePosSf is used to determine the 'correct' movement of the rotor during phase finding. It may be possible to alter this value during phase finding to loosen up the pass/fail criteria but it must be returned to the proper value. DT should be consulted.
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Thank You I should be good on inductance. Still some work to do, but I anticipate 5-15 mH. Is there any meaningful overhead penalty for doing a circle move vs sine calc?
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I would like to use a Direct PWM drive as a single phase, variable frequency( 30~250 hz), current controlled supply. First thought is to use a 'virtual axis' with commutation as a brushed DC motor and write a sinusoid function to Motor[x].IaBias from a Motion prog, PLC©, or C. Would using a realtime PLC© have less overhead than a Motion prog? Is this the right overall approach? Any comments on how best to do this and/or considerations to keep in mind? Thank You
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Please recall when using the setup software to commission the motor there was a test (Open Loop test) to confirm that the direction sense of feedback matched the physical rotation of the motor. This is very important because if the sense of direction for feedback and motor do not match, your system will be unstable. A stable system uses 'negative' feedback. If the direction sense of either motor or feedback is changed, then the system has 'positive' feedback and becomes unstable. This is why your motor is vibrating - also very possible is a dangerous runaway condition. To change whether you consider cw or ccw as forward (mechanically speaking), you must do two things: 1) Change the physical direction of motor rotation 2) Tell the feedback mathematics that you have done this, so that stability is preserved. To effect #1, swap two of the 3 phase motor wire connections. ABC -> ACB To effect #2, change the value in Gate3[0].chan[2].EncCtrl, as you have already done. Once the correct relationship is established (Open Loop test), never change just one, always change both together. .OR. You can leave the motor wiring and encoder direction definition alone and change the way your software defines forward & reverse. This can be a little confusing because you would be using a negative software command (X-2.34) to move mechanically forward. Same result, but probably better to do it the way you have begun.
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Curiosity: If there were three motors configured in this way with 30k PWM, roughly what level of processor utilization (load) would that impose? 465? ARM?
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Something I have done before for battery charge and other DC power supply applications (should have mentioned this before) is to use a good old fashioned SCR, brush motor, 4Quadrant, DC drive. - Cheap - Single phase at standard line voltages with fully adjustable output. - Three phase for more $$, less ripple - Line regen. No resistor / no heat / no 'chopper' / 100% regen capacity - Tachometer input can be used as voltage sense on PMAC side of bus filter, allowing fairly tight regulation (0.1-0.25%) - Can use accel/decel settings for controlled power-up & bleed-down of bus. - DIN rail mounting - Example would be a Bardac K680XRi for ~US$800. Fully digital, ~7A @ 0-100vdc. Should be avail in EU, but there are many manufacturers. This is just one example. https://bardac.com/dc-drives/k-series/ - BUT! You still have to add bus filtering. The notching that the SCR's deliver (and to protect against dI/dt faults) will require a series inductor (+R?) followed by R||C. I would be tempted to add a zener in series with the R. - The filtering is pretty simple and could be done on a terminal strip with standard components. - The drive provides a regulated 10vdc output that can be split with a simple voltage divider as a control reference (stability more important than precision) I'd estimate the whole thing at US$ 900-1000 As for the lab supply, I don't think a dead stable bus is that critical. You would be adding a chopper for regen. More important is that variation/ripple be << slower than your current loops in the PMAC. For very high precision, it may matter more than I realize. For the price, the unregulated supply you found with a chopper is worth a try. You can always add a little capacitance if needed. You'd be
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Haven't needed to operate anything at 60v myself, and in looking I see that the choices are indeed limited. First off - as someone alluded to already - are you sure the Brick LV does not have a regen facility ???? Big omission in my mind, but given the range of potential bus voltages.... Here's what I would consider: 0) I share your concern regarding a switch mode supply. I wouldn't, unless I could add capacitance, a 'start-up' resistor circuit, and an isolating diode. Even then not my 1st choice. 1) Three phase unregulated supply with enough capacitance to absorb xx% energy with a +yy vdc rise (you have to define the xx% & yy bus voltage for your app). I would say the ripple reduction of a 3 phase supply is worthwhile, but at low voltage & low power, capacitance is pretty cheap. Slew rate of the bus should be many orders of magnitude slower than your servo loops, even with single phase. 2) Since the power supply is unregulated, feeding it from a Constant Voltage Transformer (1% voltage reg) will help. These can be expensive enough that you may want to consider 1 phase supply instead of 3 and add some capacitance. 3) Here's where it gets difficult. Looking for a freestanding regen bus module didn't come up with as much as I would have found 10 years ago. It was more common then. Even more limited for a 60 v bus. I did find this at Automation Direct. Regulates based on differential between supply & bus, across an isolating zener diode. Might be ok with a switcher (??) Provides some bus capacitance. https://www.automationdirect.com/adc/shopping/catalog/motion_control/stepper_systems/power_supplies_and_regen/stp-drva-rc-050?gclid=Cj0KCQjw9JzoBRDjARIsAGcdIDVZTdub9oTACM_fCXuh3UQRTS3kAcxth77_QjBt3Qg8TGBQh8qfSLgaAgKnEALw_wcB Cheap, rated 80v, 50W (100W with external resistor, 800W peak) https://cdn.automationdirect.com/static/manuals/surestepmanual/surestepclamp_datasheet.pdf https://cdn.automationdirect.com/static/manuals/surestepmanual/suresteppower_datasheet.pdf Automation Direct + Delta Tau Is that an oxymoron??? KB Electronics has a DC-DC drive (KBBC) that could be configured in a voltage feedback mode as a bus regulator. Likely would need a small inductance + power resistor so it looks like a motor rather than a resistive short. (dI/dt fault). Only rated at 48v, but with a voltage configuration setting of up to 1.25 rating allows 60v. But you'll need 65 or 70 to allow some capacitive energy storage before dumping. http://acim.nidec.com/drives/kbelectronics/products/variable-speed-dc-drives/battery-dc-to-dc If you have any volume on this KB can be pretty flexible. Relatively cheap, you might even consider one of their AC-DC drives as a regulated supply to avoid the CVT in #2) above. Kinda' a science fair project at this point.....
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You have explained your difficulty precisely. If not familiar with the 'Nyquist criteria' check the attached link: https://en.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_sampling_theorem This is a basic concept of digital signal processing that is universally accepted in applications ranging from A-D conversion to serial communications, including quad encoder interfaces. In general, to digitally sample a signal, the absolute minimum is 2x the fundamental frequency of the sampled signal. I generally prefer 4x for reliability. You are losing your signal integrity at ~fundamental=60% of the sample freq., which roughly agrees with Mr Nyquist's predictions not to exceed 50%. I would say you should not depend on being successful above 50mm/s, and that would be sketchy. Classic clash of speed vs resolution. [Clarification: your system is near the Nyquist limit, my comments are biased by my preference for a safety margin by using 4x margin] At 20mhz you'll need to be sure you have proper termination, very good cable, minimized length, & proper impedance matching to maximize performance.
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Thank You for the follow up. Closure is helpful.
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I tried to put together an example that is based on your application. It is bare-bones & likely has some mistakes. Hopefully it is good enough to help explain the concept of using the Kinematic approach. Perhaps this will help you understand the explanation and example in the User Manual and between the two it will make sense. Being only one axis, your application is a great way to learn this method. Best regards and hope your application is a great success. Kinematics How To.pdf