Flying referencing positioner of a drive

We have an application where we have to reference on the fly because we cannot determine the exact ratio motor/load.
The drive we are using is Siemens Sinamics S120, in which we use the basic positioner.
The digital input where the referencing sensor is connected is an interupt input with response time of 5µs.
But the homed position depends on the speed the load is passing the reference. The position can differ 5-8mm depending on low or high speed.
The reference sensor is mechanical switch. The belt that activates it moves at a speed of 5-10 m/s.
Should an inductive sensor be more accurate?
What are 'best practices' to perform a good 'reference on the fly', when you cannot reduce speed?
Thanks

Can't you change the way the reference run is made?
I have worked with servo's in the past and a reference run was performed like this:
1) load runs at predetermined speed towards sensor
2) if reference sensor is activated, direction is reversed and speed is reduced ( say 20%)
3) if the reference sensor is not actuated any more, this is your reference point.
we normally would use inductive sensors.

Now your reference run doesn't take very long but is also very accurate.

Are you referencing a timing belt ? Can you add an encoder ? Use the mechanical switch to get the rough location and the encoder Zero Pulse for the fine positioning. It is important that the mechanical switch be ON for only 1/2 to 3/4 of an encoder rotation. The mechanical switch must not be ON for a long enough duration to span 2 Zero Pulses. And the timing must put the Zero Pulse near the center of the ON time / distance.

GlennA said:

Are you referencing a timing belt ? Can you add an encoder ? Use the mechanical switch to get the rough location and the encoder Zero Pulse for the fine positioning. It is important that the mechanical switch be ON for only 1/2 to 3/4 of an encoder rotation. The mechanical switch must not be ON for a long enough duration to span 2 Zero Pulses. And the timing must put the Zero Pulse near the center of the ON time / distance.

Click to expand...

I use the servomotor resolver for position.
I suppose one can never be sure in that case that the zeromark will occur at the same position after the mechanical switch each rotation.

Are you using a timing belt ? Is the load always synchronized to the same rotation angular position of the motor ? At one time ABB robots used Commutation and Synchronization Offsets from the servomotor resolver feedback. Is your resolver feedback processed through an A to D converter that gives you a number / count ? If the Home position is always the same servomotor angle, can you pull a reference count / position from the resolver feedback ?

An inductive sensor may be more repeatable than a mechanical switch. Worth trying. You will still have the issue with signal delays, but it may be negligible.

Let's say that you find a sensor that always turns on at the EXACT same location, and has the EXACT SAME turn-on delay. In other words, repeatable. There is still the fact that the distance traveled during this delay will always be farther when you are moving faster.

The only way around this would be to adjust for this delay based on current speed. When moving at the fastest speed, your reference is at the current location. When moving at a slower speed, your reference is position is a little later (further away). The amount of offset will need to change depending on the speed you are traveling when the sensor turns on.

Some further reading:

http://www.electrocam.com/pdf/tech/SPEDCOMP.PDF



-Eric

Has anyone specific experience on 'homing on the fly' with Sinamics S120,