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Steve Cheung

Servo MR-J3 Home Position Setting

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Hi All, I was using C++ to develop a customized software to link to the servo amplifier MR-J3 through RS422 port. My software now can communicate well with the MR-J3 servo, control the table move forward and reverse to any position and with vary speed. My application pattern is as: Home position is at twelve o clock, reverse is from 12 o clock to left, and forward is to right side from 12 o clock. I have a problem how to to set the home position data into the servo and the current position of the servo when the power suddently off, so when power on again, servo will remind the current position and can be move to the home position. I think it should not a problem if we using let say FX2N positioning unit. Any idea? Thanks in advance.

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if the drive has option for Absolute Encoder, use it. I was using MR-J2S series drives on higher PLCs (A, QnAS, Q) and all I had to do is install battery and set proper parameters. battery keeps encoder powered even when power is gone and servo keeps track of position until battery is dead (even if positon changed while power is off). normally battery is recharged when unit is powered up. if battery is empty or you don't use is to emulate absolute encoder, you would have to home it every time you power up. then you can go to whatever position you tell it to move. if position doesn't change during power down and accuracy is not critical you can keep writting position to register. on powerup you can write that as encoder position (preset encoder) and turn off OPR request (use data set homing method). or use UPS that will keep the unit powered all the time.

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Keep in mind Mitsubishi states the serial communications on the amplifier is typically only used for configuration and debug, and Mitsubishi does not support using it for control applications.

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The proper way is to use an absolute encoder. Alternatively you can use the index signal of your incremental encoder as a home reference. At startup you only have to rotate the table upto the index point (placed, say, at 12 o'clock) to obtain the correct position. (This assumes that I have correctly understood that the table is a rotating one with no movement limitation at 6 o'clock). The application will still be susceptible to power supply problems (brown-outs etc).

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Hi Friends..... Thanks a lot for all your advices. I have use the Absolute ABS now with battery back up. But, do you guy's know how to reset the ABS to zero and which command I should used? Thanks & best regards, Steve

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When using a Mitsubishi servo, you have always an absolute encoder. Only you need to configure the amplifier to work in absolute mode. This is a parameter. When doing so, you need a battery inside the amplifier, that feeds the encoder during power off. When moving the axis when power off, the current position is stored. You can see this in the rotations after home position. The absolute encoder data is always stored, even if no battery is used, it is an absolute encoder. To reset the ABS encoder data, execute a zero return command, so it will look for the zero pass signal from the encoder. By doing only the rotations after home position will be reset. The ABS encoder data is actually zero. Make a choise in home position return method, like dog method.

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Hi FNMdeJong.... Thanks again for your help. Do you mean if without using a battery, after the power off, we can still have the last position of the encoder? Is it possible to zero the absolute encoder from the Configuration sofware? Otherwise, is there any digital i/o of the servo amplifier that I can use to zero the absolute encoder? Thanks & best regards, Steve

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You have always the exact position of the motor, this is called the ABS encoder. So every position has it own unique signal to the amplifier. When you loose the power the amplifier still knows where the motor is. The problem is that the amplifier does not know how many rotations it has made when there is no power. This is the case to use a battery. To run the motor untill it sees its zero pass signal (once a revolution) is called a home procedure or zero procedure. When you start this procedure it will look for the zero pass signal and when finished, you can see in the monitoring software the ABS encoder data is set to 0. (1 to 5 droop pulses possible). You cannot change the ABS encoder data to another position, because it is fixed by the hardware. But you can change the connection the the reductor, or workpiece it moved.

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Hi FNMdeJong, Can you explain your reply in more detail? If a servo motor is attach in the middle of table to move it left or righ side, I need to assign a home position as a zero to start count where the servo move to left or to right side. This home position can be adjust or calibrate, so need a function to zero the position. Please advice. Thanks & Best regards, Steve

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Take a look at the manual form the MR-J3-A amplifier at the section 14.7.3.

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