How to control motor speed without tachometer

[nhmh]

Hi everyone,

thank you for dropping by and have a look at my problem 🙂 

I have a PCI7344,UMI7764, DC motor, encoder and motor drive. Everything is working fine. currently, this system enable me to control the motor via encoder feedback (which mean, I need to set my target position first, and the motor will rotate until the certain target position that detected by encoder). 

What I would like to do now is to control the motor speed, however, I do not have  a tachometer feedback since my tachometer only enable me to display the RPM. is there anyway that I can do this (control and maintain the motor speed at certain RPM value) with all above listed equipment I have. Can anyone assist me on this? 

Thank you in advance for all suggestion and recommendation.

Message Edited by nhmh on 07-22-2009 10:56 PM

[Jochen]

With the current equipment you already have what you need. The 7344 does control the motor speed based on the encoder feedback. Even in position mode you can define acceleration and velocity values and the trajectory generator of the 7344 provides dynamic setpoints to the control algorithm to meet these move constraints.

If you don't need a target position, you should run the axis in velocity mode.

You can change the velocity during the move by using either the velocity override functionor by loading a new velocity value and sending the start command.

I hope this helps,

Jochen Klier

National Instruments

[nhmh]

Hi Jochen,

Thank you for your response. I have tried something similar, but using MAX. instead of choosing the mode as Absolute position, I chosed the velocity mode. However, the velocity seems like not accurate, but fluctuate. Saying like, I want to make it stick at 6000, it fluctuate between 5500-6500. The end-user is not happy with this result, do you have any idea why does this happen? However, I have not yet tried to do this using labVIEW. I shall try your solution and come back to you with the result soon. Thanx.

Cheers..

[Jochen · Accepted solution]

Using LabVIEW won't change the motor's behavior. If you see massive oscillations, my first guess is that your PID parameters should be adjusted. Please refer to this document and the other pages linked to it.

Jochen

[nhmh]

Hi Jochen,

Seems like you are my best problem solver advisor, I would like to seek your advice on my current task. 

I have a Geckodrive G340 DC Servodrive, a Clifton Precision Motor attached with Encoder and UMI-7764 with PXI7350. 

Situation : 

Previously, I connected the configuration as found in (Figure 1) where the motor will keep rotating once I power up the geckodrive with 24Vdc. According to geckodrive troubleshooting manual,this happened since I do not connect the encoder to the geckodrive. To avoid this, I reconfigure the connection to (Figure 2). Then only the motor did not immediately rotate once I power up the geckodrive.(however, I can still feel the vibration of the motor)

With this configuration (Figure 2), I proceed  with performing the single axis test (NI Motion Device>Interactive>1-D Interactive). However, I need to ensure that under Axis configuration, i select STEPPER as the motor type (perhaps, the motor is a SERVO motor. however, the encoder unable to do a reading when I select SERVO as the motor type). Below is the setting done for performing the single axis test :

Operation mode : Absolute position

Load velocity in : STEPS

When I enter the target position value, click apply and follow by START, the motor rotate until the encoder meet its target position.

My question is, is there any way I can replace this configuration using PXI-7350 with UMI-7764 with normal Multifunction DAQ? from my understanding, the STEP and DIR signals in the UMI-7764 are used as commands outputs to a stepper driver. Doesn't this mean that this two signals (STEP and DIR) provides a TTL signal. So, is it possible for me to use USB-6009 for this channel on the geckodrive? 

Please advice. 

[Jochen]

Hi,

as stated multiple times in this forum, a USB DAQ device is the worst case scenario for any type of motion control. This is especially true, if you need to run the control loop with this device.

In your situation (figure 2) the control task is a bit easier, as the position loop is closed inside your drive. Still a USB-6009 is not capable of generating a clean trajectory profile.

The reason for that is the fact, that the USB-6009 doesn't support hardware timed output operations for digital output signals, so you won't be able to generate a stable output signal (software jitter!). Additionally the output rate will be too slow (probably less than 100 Hz).

With this said, I really don't recommend to replace a motion control device with a USB-6009. Still with your second scenario you may not need all the features provided by a PXI-7350. If you just want to do simple linear moves, you could replace the 7350 by a 7330 board. These devices are designed forcontrolling stepper motors and obviously your drive provides the same type of command inputs for a servo motor as they are typically used for stepper motors. From a software point of view the 7350 and 7330 are compatible. The same is true for the external connectivity, so you could keep your cabling as it is.

I hope that helps,

Jochen Klier

National Instruments

[nhmh]

Hi Jochen,

thank you for the reply. I understood that, however, user insist to use a multifunction DAQ. so, is there any other multifunction DAQ instead USB6009 that can be used for this case? Please advice.

[Jochen]

There are many more reasons, why you shouldn't use a DAQ device for this task:

1.  With a DAQ device step generation is not smooth. 73xx devices use advanced algorithms for step generation.There is no way to implement these algorithms with a DAQ device. 
2. The API of NI-Motion is optimized for motion control tasks and very easy to use. With a DAQ device the quality of the solution will be much lower and programming will be much more complex. This doesn't sound like a good deal to me.
3. Motion control devices provide real-time capabilities for limit switch monitoring, following error tracking and more. With a DAQ device you will have to run the complete system in a real-time environment to be able to implement these features.

So with this said I really don't recommend abusing a DAQ device for motion control tasks. If you still want to try it, you will need to use a plugin board with hardware timed digital I/Os (at least 10 MHz time base for reasonable velocity resolution), so a PCI-6250 could be an option, but it's definitely not a good option...

Kind regards,

Jochen Klier

National Instruments

[nhmh]

Hi Jochen,

Thank you again on your advice. To share with you, my team and I finally able to control the motor using NI Multifunction DAQ(PXI 6281). Due to some reason, I cannot reveal the solution here.  However, to those who would like to know the solution, they may leave their email address for the solution. However, I could not guarantee that is the best solution. But still, I managed to make the motor rotates when I provide pulses. 🙂

Thanx bro.

Message Edited by nhmh on 08-05-2009 10:40 AM