Optimum servo pid loop rate

Question

Hi,
I want some confirmation on the optimum pid loop rate and the fastest servo motor operation that we can acheive with Pxi-7352 running on Window.
The manual of 7352 says "62 μs PID loop update rate for up to 2 axes". The requirment is to acheive periodic equi sized straight line and rotary motion ( using yaskawa servo motors in torque mode and 7352 to close position loop ) Both motion are independant and not operating simultaneous i.e rotary completes one full rotation in small steps and then linear moves by one small step then rotary completes next rotation in small steps and again linear motor moves this continues till end of test.
you can observe here that both rotary and liner motion are periodic and move equi spaced distance for each step with small stoppage/dwell time of 500 u sec , The catch is in addition to positional accuracy in terms of 50 -100 microns the time for complete each step is also very crucial and very small (many peridoc steps together consitute complete linear and rotary motion). Find expected velocity profile attached for more details on desired timing.
As the motors have simple and periodic straight line and rotary motion i suppose buffered contour moves are not needed which have limitation of 10 msec between two points, then can the time to acheive each step be as low as 2.5 to 5 msec considering best loop rate of 62.5 u sec and highest/worst pid loop rate of 500 usec.
1. Is the expected timing of 2.5 - 5 m sec feasible assuming that the motor is capable of reaching 5000 rpm which is well above desired speed.
2. 735x series being high performance motion controller what is the order of closed loop timing that we can expect for simple and periodic moves (say in order of 1000 usec or 10 msec)
3. Can i interface one stepper motor and another servo motor where both the motors and drives are in position mode on single 7352 and UMI 7772.
4. Are there any limitation if NI motion is not used for code development and NI motion Assistant along with LabVIEW is used instead.
This is precision application and the component is very small ,the weight of the component would be less than 400 grams so i hope we do not have to consider inertial affect on motion significantly.
Appriciate if some one with similar experience can provide more details.
Regards,
Bipin
Solved!
Go to Solution.
Attachments:
Velocity Profile.png ‏21 KB

Answer 1

Hi Bipin,
thank you for your comprehensive post. I had a close look at your timing specs and I strongly doubt, that these specs can be met with a 7352 board. Even though the PID loop rate of 62.5µs might be enough, both trajectory generator and the supervisory control (host communication, move command sequencing,...) are too slow to meet your requirements. Please note, that the trajectory generator always works with a timing interval of 10 ms, regardless of the operation mode (linear move, contouring, blending,...), so there is no way to reach your timing specs with this product. In general contouring is the fastest way to run a quick succession of moves, as it doesn't require host communication (by the way: onboard programs don't execute much faster, than programs that are running on the host).
I don't want to make negative mareketing for our 735x boards, but your timing requirements are just way beyond the requirements of typical motion control applications and it's probably quite hard to find any COTS product in the market, that can meet these specs. 
With this said, I would like to offer you an alternative solution, that is based on NI's FPGA technology. Additionally to a faster trajectory generator a faster control loop is probably also very desireable for your application. Even with the lowest price Virtex-II based R-Series multifunction board (PCI-7830R) you can run PID loops with 5µs cycle time and the trajectory generator at the same rate. You even can use a different type of control algorithm if required. This technology is leveraged very successfull in a lot of applications with very high performance and accuracy requirements (here are some examples).
So these are the pros, but where are the cons? Well, the R-Series boards are general purpose FPGA boards and not ready to run motion control boards, so you need to develop the low-level control application by yourself. The good news here is, that it can all be done graphically in LabVIEW and that there are many ready to run examples for motion control, that can be used as a base for your development. Additionally you may want to have a look at the NI SoftMotion Development Module for LabVIEW, that simplifies the development of motion control applications, but it is not a mandatory tool and the benefits of this module might not be as high for your application, as ithey are for more standard motion control applications.
I hope that helps.
Kind regards,
Jochen Klier
National Instruments