Position control without servo

We have an very old welding machine for stainless steel metal sheets where the welding torch is fixed on bracket and is moved by a gearbox motor which is controlled by an ac drive.
The operator sets the system in motion and it's start welding until a mechanical limit switch stops the ac drive.
The speed and the position of the limit switch are set manually by the operator depending on metal sheet thickness and and length.

We wan't make the process a little bit more automatic.
The obvious solution is to use a servo system but I think it would be overkill for the particular application since the required position precision is around 2mm.

I guess it's possible to install an encoder on the motor and then use an ac drive (vfd) in closed loop for position control.
I'm not very experienced in motion control applications so I need some help with this.

Yes, that is possible. The trick is finding the positioning servo drive that can run an induction motor. They are out there. ABB ACSM1 could to it, but I believe that is obsolete. KEB S6 and F6K can do it. I am sure there are others.

Siemens has several ways
Least expensive would be
G120 system
-CU250S control unit (This has epos positioner functionality)
-PM240-2 power module

ou can use an encoder & inverter I have done this and managed to get repeatability to about 1-2 mm, however, a worn gearbox may have an impact on the accuracy, the inverter needs braking etc. Servo is the best solution without doubt. In saying that welding is quite a slow process so accuracy should be quite easy to achieve with an inverter. Some suggestions if using an inverter & encoder, have a home position sensor this will serve as a home position reset of the encoder. You mention closed loop on inverter, not quite sure what you mean by that but you will need some way to calibrate it, input speed & distance variables assume you will be using a PLC & HMI.

If you aren't requiring high amount of accuracy, I'd install a highspeed counter in your PLC and add an encoder somewhere. Install a home prox, program a homing routine, and replace the limit switches with actual_position >= setpoint_position intructions and you should be done. I would think it would be as accurate as the limit switch, depending on where you get your feedback.

JRW said:

Siemens has several ways
Least expensive would be
G120 system
-CU250S control unit (This has epos positioner functionality)
-PM240-2 power module

Click to expand...

Plus one on the CU250S and Epos with profinet encoder. Used one a few years ago for a positioning system and it was a really clean system to install and commission. Plus it has simple safety stock and more advanced safety with a license.

Slick system

Thank you for your suggestions.
We don't use Siemens products so I would prefer something from ABB, Schneider or Yaskawa.
I've been asked to install an HMI so the operator can enter recipe sets with speed, torch power in amps and the length of the welding line. This is the easy part.
As PLC I would prefer to use Schneider's M241 because it's Codesys based, has high speed counter inputs for the encoder and we already using a lot of them.

I guess breaking is needed because the motor will have to stop immediately as soon as the desired end position is reached. Right?
I will also install a homing proximity switch for encoder reset.

The only problem I see is how to install the encoder on the motor.

Another solution for position feedback could be a magnetostrictive sensor but I will only use it if I will not be able to install the encoder on the motor.

You may not need it, keep the VFD ramp up & down times short and as it is probably running quite slow it should stop pretty quickly, we have used them for placing trays ready for arms to pick up & no brake was needed, this was running at a reasonable rate (as fast as we could without making the trays slip) it sounds like you only need a single phase encoder and the resolution will probably not need to be very high.

kallileo said:

Another solution for position feedback could be a magnetostrictive sensor but I will only use it if I will not be able to install the encoder on the motor.

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NO! that would be too expensive!

Incremental encoders are cheap. Slide resistors are cheap and absolute.
It depends on which feedback is easiest to interface to your motor's drive or PLC.

All you want to do is add a simple closed loop control and to make the PLC do what the person does now.

If it's just a back and forth motion, look at Kinetix 300 Indexing servo, cheaper than full blown and suitable for more simple projects.

Janner
That’s not on his manufacturers list
Post 7

JRW said:

Janner
That’s not on his manufacturers list
Post 7

Click to expand...

His loss then.

So I will everything as is and I will just mount a rotary encoder on the motor and wire to the PLC. Then after counting the appropiate quanity of pulses the PLC will stop the ac drive or just the disable the welding torch and make the inverter stop using a ramp.

Unfortunately I don't have access to the gearbox so Ι have no information about it.
As far as I understand the only way to find the distance/pulses is use a measuring tape.

I guess a 500 RPM rotary encoder be ok for this application.

I have actually built a few of those machines
I used Yaskawa Flux vector drives
In my case the there was lot more to the machine it was a Sub-arch welder
we has 2 400 AMP or 600 AMP I don’t remember the exact size
the bridge span 25 ft the travel was originally 200 ft but it depended on the power cable feeding it.
The bridge could travel vertically up to 10 ft
I had an AB 1400 PLC with remote point IO.
We were running a welding bead travel speed was below 1hz on the drives.
The weld could be run the long axis or the bridge. The vertical travel of the bridge was automaticly controlled by sensors following the work.
The weld bead was better than you could run manually but I don’t know anybody that can run a continuous bead for over 150 ft
The main thing you need to control is the travel speed, you need to determine the travel speed you need to form the bead I our case they ran about 17 in/min.
The high speed travel to go between the jobs. I ran the drives at up to 120 hz
If you need to control the travel length then you will need an encoder and high speed counter for
feedback. The vector drive encoder can be used for position feedback a small PLC can be used to control and don’t forget you will need to add a small HMI for the operator.
In our case we controlled both welders from the plc as well as travel.