my motor runs at a constant speed

[sumitha]

i have a
PCI 7342 controller.
MID 7654/7652 servo motor drive
motion control software
motion assistant 1.1.
I have setup the hardware connection.
When I try to run the motor using the motion assistant the motor runs at a constant speed irrespective of the velocity that I specify in the motion assistant.My motor velocity is 7 voltage per 1000RPM.The motor keeps running at a constant voltage of 25volts.I donot have a encoder but I have a tachometer.
pls help.

[JesseD]

Hello Sumitha,

You mentioned that you are using a tachometer as feedback. Unfortunately, motion controllers from National Instruments only support position feedback transducers (quadrature encoders or analog position sensors). Servo motor systems with NI controllers require this continuous position feedback in order to close the PID loop.

Is there any way that you could obtain a quadrature encoder for your system? If you can, configure your settings in Measurement and Automation Explorer. You could then use Servo Tune (and this tutorial) to tune your PID parameters.

Best Regards,

Jesse D.
Applications Engineering
National Instruments

[sumitha]

Hi Jesse
Thanx for your reply.Is there any possibilty that I can run my motors just with the tachometer?I tried doing the Auto tunning for servo.
When I did that the Current trajectory data showed the following details:
desired position:-2044
current position:-2044
current velocity :fluctuating from 450 to 0(the motor was runing in opposit direction)
Following error:-3440
I tried seeing the Step response too.When i compared it with the diagram in the tutorial it didnt match..
What am i supposed to do?Can you please tell me why we use servo tune?
Pls help me.I am really struck.
thanx
sumitha

[JesseD]

Hello Sumitha,

In order to use servo motors with National Instruments motion controllers, you must have some form of position feedback. This is required due to the PID control loop implemented on the controller. Without some kind of control loop, there is no way to guarantee that the motor will get to the commanded position following the specified trajectory. Servo Tune is a tool used to adjust the PID parameters, which in turn control the stability of your system.

If you do not have any way of adding position feedback (such as a quadrature encoder), you will not be able to make full use of your motion controller. It is possible to simply read a voltage in and output a specified voltage from the motion controller. However, you lose all hardware timed control and all the benefits of a motion controller. If you are interested in that option, a DAQ board might work better for you. Otherwise, I highly recommend looking into adding a quadrature encoder to your system.

Best Regards,

Jesse D.
Applications Engineering
National Instruments

[sumitha]

hi jesse
thanx a loot for the reply.
Can u please help me with this question?
In the MAX settings for each axis we need to map the primary and secondary feedback and also primary and secondary output.
I read from the tutorial that primary feedback is for position feedback and secondary feedback is for velocity feedback.
The tachometer is connected to the MID 7654/7652 s analog input .So what do i map in primary and secondary feedback?
And how do i find out which DAC channel gives the output?
Please help me.
Thanx
sumitha

[JesseD]

Hello Sumitha,

Let's go ahead and close the discussion forum thread and continue working on this through the e-mail service request you generated.

Regards,

Jesse D.
Applications Engineering
National Instruments

[sumitha]

Hi ,
Has anyone used PCI 7342 card ,Servo motor drive 7652 and motion assistant to drive a servo motor?
I am trying to control the velocity of the motor.
The 1 D interactive is not working properly.
I have a problem which I am not able to solve .
It would be of great help if someone who has done a similar task can help me with this.
PLs help.
thannks in advance .
Sumitha

[Jochen]

Sumitha,

is this a new problem or is it the same issue as you described before? Controlling velocity and position of servo motors is the main purpose of the equipment you have and I have worked with the same devices before without encountering any problems. Do you have an encoder now?
You have sent an email request to NI. Didn't you get a solution, yet?

Best regards,

Jochen Klier
National Instruments Germany

[sumitha]

Hi
I am still facing the same problem that I mentioned in my previous messages.I dont have a encoder.
In 1 D interactive in current trajectory data the motor shows a constant position(count values) irrespective of the
velocity and target position values.The output voltage from the tachometer was 21 volts ,so I added a resistance to it to reduce to 7 volts but still the motor runs at a constant speed.The ADC range is 0 to 10 volts.The hardware connections are done correctly.Pls help.
Sumitha

[Jochen]

Unfortunately there is not much more to say about that issue. You probably need an encoder for your application. If you use analog feedback the voltage is directly interpreted as position feedback. That means +10 V = position 2047, -10V = position -2048. If your analog feedback signal works like that and if the position resolution is ok for you then there shouldn't be a problem. In any other case you will have to use an encoder.

Jochen Klier
National Instruments Germany