Problems in moving stepper motor in absolute and velocity mode

Hi Experts,

I am using linear ball screw slide having the stepper motor controlled through S7 1200 PLC but it is giving problems moving it in absolute and velocity mode.

CPU – 1217C DC/DC/DC (6ES7 217-1AG40-0XB0), PTO1 (Pulse & direction, outputs Q0 & Q1 (with double ended to single ended converter chip), technology object – ‘TO_PositioingAxis’ V6.0, Pulses per revolution 6400, load movement per motor revolution 10mm

Stepper motor – Nema 34, 2 phase, 1.8-degree, hybrid stepper motor, Make JSS, China

Microstep driver DM860 – Microstepping 400 to 40000, Input voltage 24-80VDC, Peak current 2.40A to 7.20A, Pulse Input frequency up to 200KHz, TTL compatible and optically isolated, Make China

DIP switch settings – Pulses 6400, Current 4.45A

When I move the motor at velocity 35 in absolute mode, it stops accurately at set distance. But when I increase or decrease the velocity it stops at +30mm/-30mm than that of set distance.

In velocity mode it is working OK (velocity 35) but when I change the velocity to -35 for direction change by removing ‘execute’ command and start it again, it does not change the direction. If I put delay in between stop and start then it is accepting the direction change.

Can you please suggest the solution.

Regards

qwazee

Please consider adding a sample of your project as a ZIP file. You mention numbers without measurement units, for example, which a sample project would not have us guess.

 Hi Qwazee,

Thank you very much for your response and sorry for the delay as I was minimizing the program due to ZIP size issue.

‘Move Velocity’ instruction is used in which I am facing direction change problem.

The sequence I have written in FB2 is –

Auto cycle received

Start sequence input received

Approach move started for 13 seconds (velocity +35)

Delay 2 seconds

1^stWinding move started for 5 seconds (velocity +35)

Delay 2 seconds

2^ndWinding move started for 10 seconds (velocity -35)

Delay 2 seconds

3^rdWinding move started for 5 seconds (velocity +35)

Delay 2 seconds

Home move started till getting I0.7 (velocity -35)

As I am unable get the accurate stopping of slide in absolute mode, I am using velocity mode with counting the sensor pulses to know the travelled distance (HSC).

Regards,

Sant

Hello.

The project sample you sent does not give me an idea of where the absolute positioning is giving a problem. MC_MoveAbsolute is not even mentioned in the program. Also there is no homing routine.

Potentially I see one issue after giving your code a quick look..

The loss of the EN on MC_Power causes the motor to go to a stop. The home position is also cancelled. In turn absolute movement is not possible. Consider how you control MC_Power.. at this point it does not enable the drive.

How is the drive enabled? Premature disable of the drive can also give some issue.

What are your thoughts?

~

qwazee

Hello.

The project sample you sent does not give me an idea of where the absolute positioning is giving a problem. MC_MoveAbsolute is not even mentioned in the program. Also there is no homing routine.

Potentially I see one issue after giving your code a quick look..

The loss of the EN on MC_Power causes the motor to go to a stop. The home position is also cancelled. In turn absolute movement is not possible. Consider how you control MC_Power.. at this point it does not enable the drive.

How is the drive enabled? Premature disable of the drive can also give some issue.

What are your thoughts?

~

 Hi Qwazee,

Thank you for the feedback.

The auto running bit is generated in auto mode when cycle start push button is pressed. It is enabeling the drive. This bit remains ON till I change mode to Manual. So it is ensured that MC_Power is enabled before giving Move commands.

I used Move-Absolute block before but it is not working as slide is not stopping at accurate position. So I tried Move_Velocity by deleting Move_Absolute blocks. But, in Velocity mode when I change the motor direction by giving negative velocity value (say -35), the motor doesn't change the direction. So I used the below logic to change the direction and same is working OK .....

MC_Power enabled

Start Move_Velocity  with velocity +35rpm

Stop Move_Velocity

2 seconds delay

Start Move_Velocity with -35rpm

My question is, why delay is required to change the direction.

Regards

Stepper motors are typically open-loop systems.  You can lose position in a number of ways.  The S7 can't get much wrong, other than dropping MC_Power (as qwazee mentioned) or driving the axis too fast.

How many pulses per second does +/-35mm/s relate to?  And how many steps per second for the stepper motor?

You have three frequency bottlenecks that you need to check:

1. Pulse inputs to the stepper driver.    Does 35mm/sec exceed this?
2. Pulse outputs from the S7-1200.
3. Steps per second of the stepper motor.

If all of these are OK, the stepper might be struggling to accelerate or decelerate the load and so skipping/gaining pulses.

Try to do absolute moves at lower speed.  If this doesn't work, try low speed and also remove the motor from the machine, let it run by itself.  If you are still encountering issues, check the waveforms of the pulse signals (PLC->stepper driver) - they might be malformed or be subject to noise.

cliff.pennalligen

Stepper motors are typically open-loop systems.  You can lose position in a number of ways.  The S7 can't get much wrong, other than dropping MC_Power (as qwazee mentioned) or driving the axis too fast.

How many pulses per second does +/-35mm/s relate to?  And how many steps per second for the stepper motor?

You have three frequency bottlenecks that you need to check:

1. Pulse inputs to the stepper driver.    Does 35mm/sec exceed this?
2. Pulse outputs from the S7-1200.
3. Steps per second of the stepper motor.

If all of these are OK, the stepper might be struggling to accelerate or decelerate the load and so skipping/gaining pulses.

Try to do absolute moves at lower speed.  If this doesn't work, try low speed and also remove the motor from the machine, let it run by itself.  If you are still encountering issues, check the waveforms of the pulse signals (PLC->stepper driver) - they might be malformed or be subject to noise.

 Hi,

Sorry for the late reply. Customer decided to go for servo instead of steppers.

Thanks for the support.