Motor control

[can]

Hi. I'm trying to use a MD888 to give a DAC signal to a motor driver in order to control the speed of the motor. After I've connected dac 1 and 0vdc from the analogue IO port to the motor driver, the motor start turning by itself even though the power to the 888 is not even switched on. The voltage across the 2 pin is 0vdc. Is it a problem with 888 or with the motor driver? How can I make the 2 work together? Thank you

Regards

[support]

Is the motor suppose to turn with 0V signal?

If the PLC is not even powered on and the motor start turning it is a bit hard to say that the PLC is causing problem to the motor driver. The DAC output may have certain impedance and the motor driver doesn't like it? What happen when you turn on power to the PLC? Does the motor turn when the DAC output is set to 0? Note that since the DAC is shared with ADC you must use the SETDAC command to configure the pin to DAC and set the output to zero. Otherwise it will be an ADC and it can be pulled up by neighbouring pin and cause the motor to turn.

You can also use a relay to control the DAC output pin to the motor drive and the relay is under the PLC's control so that the PLC can SETDAC first when first powered up and then turn on the relay to control the motor.

[can]

Hi. When I setdac 1,200, I'm getting 0.2 vdc. However, upon closing a jumper to start the controller, the voltage jumped to 0.6vdc. Any workaround for this problem?

Thank you

[support]

Is the motor analog input pulling it up to higher voltage? Sound a bit strange for an analog input to do it this way. Did you check the spec for the analog input to the motor? Is it a high impedance input?

You can connect a 10K resistor in series to the DAC output from the PLC, thru the resistor and then to the motor driver's analog input. Then SETDAC and measure the voltage at the PLC's DAC pin. It should be around 0.24V. Then start your motor driver and check the voltage before and after the 10K resistor. If the voltage at the motor side is higher than the voltage at the PLC's DAC output then there is some problem with the motor driver's analog input. It is probably pulling the signal high. You will need to connect a buffer to the PLC output to make it a low impedance. A simple IC such as LM324 connected as voltage follower will work to amplify the current from the DAC output.

[can]

Is there any diagram as to how to use the LM324? Sorry it may sound lIke spoon feeding but I'm not too good at electronics. Thank you

[support]

Just have to google (or Bing):

http://terpconnect.umd.edu/~toh/ElectroSim/VoltageFollower.html

http://www.voltagecurrent.info/wp-content/uploads/2009/12/lm324-connection-diagram.jpg

[can]

hi. The resistance of the analogue input of the motor controller is 30KW. Will the use of LM324 be useful to output the correct voltage from the PLC to the controller in this case? What should be connected to pin 4 and pin 11 of the LM324?

Thank you

[support]

Yes, LM324 voltage follower has no problem supplying enough current for a 30 K ohm load.

You can connect +5V or higher (up to the limit specified in the data sheet) to pin 4 and 0V to pin 11.

[can]

Had tried using LM324 but the output is still the same. When I activate the controller and setdac 1,0, there is still a voltage of 0.3vdc between the pin. When I setdac 1,200, the output is 0.5vdc. However, the output drops back to 0.2vdc when I deactivate the controller. Any idea? It is also this problem that is making the excel link to have problem. Will connecting the 0vdc of the controller with 0vdc of the analogue helps?

[support]

Did you measure the DAC output voltage without connecting to the motor controller? I am wondering if you are connecting to the wrong pin. Note that DAC #1 is at Pin 2 of the DB9 connector and DAC #2 is at pin 1. Please refer to the T100MD888+ Installation guide for the location of the ADC and DAC pins.

[can]

Hi. Thanks for the advice. Solved the problem. Turn out that the controller is not grounded. Thought connected to PE is same as grounding. But excel link is still not working due to noise I think. Will try out the diode and capacitor as mentioned in the manual

[support]

The diode and capacitor at the power supply is meant to prevent switching of heavy load from causing a voltage DIP at the PLC power supply and could reset the PLC. But if the communication is affected by the AC motor switching noise, then it will not resolve the issue until you have addressed the switching noise that is coming from the motor controller/motor driver. Also very important to remember is never to bundle the communication cable with other high current wires that carries the motor current.

[can]

Hi. What are the things I need to do to take care of the noise from the controller? Basically, when I take away everything and only connect the earth of the controller and earth of power supply together, excel link still fails when I activate the controller. Why?

[support]

1) what kind of power supply are you using? Is it good quality industrial switching or linear power supply?

2) Are you able to perform online monitoring with the PLC when the power supply and controller is connected to the PLC? If you have no problem doing online monitoring and get smooth data updates (look at the RTC and see if it updates every second), then it could be what you set in the ExcelLink that is giving trouble. E.g. don't specify too big a range of DM to read in a single action.

[can]

Seems like Online monitoring is also not working when the controller is activated. Will putting a noise filter at the incoming ac to the power supply helps?

Happy Weekend