Lenze Servo positioning ECS, speed question

[userxyz]

Hi,

When we build machines, then we make a settingsscreen on the HMI device for the drive settings. Speed and reference positions are the settings to be made.

Question that I have:

When I set the speed at 50%, then the ramp acts shorter, so the drive is accelarating faster at lower speeds.

Because:
Ramptime = Speed / acceleration

Example:

We have 600mm/s Speed and 750mm/s² acceleration.
then ramptime = 600/750 = 0,8s

When we set the speed on 10%
then ramptime = 600/750 = 0,4s.


Is it better to add the ramptimes to the settings on the HMI, or is changing the speed without the acceleration no problem ? At lower speeds, the ramptimes are pretty short, and it isn't as smooth motions.

regards
G

 

[Peter Nachtwey]

You probably need to take the jerk into account.

Your 10% numbers don't make sense.

Nothing can instantly accelerate or decelerate. Doesn't the drive have a jerk limit? I would imagine it would be about 75000mm/sec^3. It may be lower. 75000mm/sec^3 would result in ramping up to the acceleration rate in 10 ms which is reasonable. Since the jerk must ramp up and down during the acceleration there is 20 ms out of the acceleration where the acceleration is not at the full acceleration rate. Your simplified calculations do not result in a significant error when accelerating to full speed but it is is a significant error when accelerating to 10% speed. You probably need to take the jerk limit into account when computing your ramp times. This has bit me too in the past. One must pay attention to detail, always.

 

[userxyz]

hmm

10% = 60/750 = 0,08 ramptime,

made a mistake in my explanation

Hi,

When we build machines, then we make a settingsscreen on the HMI device for the drive settings. Speed and reference positions are the settings to be made.

Question that I have:

When I set the speed at 50%, then the ramp acts shorter, so the drive is accelarating faster at lower speeds.

Because:
Ramptime = Speed / acceleration

Example:

We have 600mm/s Speed and 750mm/s² acceleration.
then ramptime = 600/750 = 0,8s

When we set the speed on 10%
then ramptime = 600/750 = 0,4s.


Is it better to add the ramptimes to the settings on the HMI, or is changing the speed without the acceleration no problem ? At lower speeds, the ramptimes are pretty short, and it isn't as smooth motions.

regards
G

 

[Peter Nachtwey]

But what about the jerk parameter in drive?

If you want to avoid this problem you can alway ramp up in a constant time but sometimes that is too conservative.

A more accurate method ramps up to the desired velocity in 3 motion segments but you must know the jerk limit or acceleration rate.
t0 to t1 acceleration is ramping up until the desired acceleration is reached.
t1 to t2 the motor accelerates at the desired acceleration.
t2 to t3 the motor decelerates to 0 mm/sec^2 so it can run at the the desired velocity.
Now you need to calculate the total time from t0 to t3. If the allowable jerk or acceleration ramp rate is very long the motor may never reach the maximum acceleration before requiring to decelerate to 0 mm/sec^2. This is the case that causes the problems with short moves or acceleration to small velocities.