Using a VFD for Torqe Assist 3

[DaveRosser]

I have an application in which I would like my VFD to drive a load (a carriage on linear rails) for a set speed and a set distance. At this point I want the VFD to merely ASSIST in moving the carriage, which will now be pushed physically by another machine as well.
The carriage is connected by gear belt to a 1000:1 constant torque motor controlled by a Siemens Micromaster 440 drive with encoder feedback from the motor.
We are currently doing this with a Servo drive system, which of course is much more expensive...and therefore is desirable to replace.
If anyone has experience with such an application your help would be greatly appreciated.

Thank you.

 

[DaveRosser]

I should mention that the VFD is controlled by a Siemens S7 PLC as well.

 

[mc5w]

About the only time that you would want to use a VFD and an induction motor as a servo system is the case of an elevator where you need lots of power over a relatively long distance.

If an induction motor is frequently reversed after traveling a short distance such as the distances in computer numerical control machines, the rotor will burn up rather quickly.

What you are better off to do is to use an air motor or hydraulic motor with an open ported 4-way valve for your torque assist. This is a type of 4-way valve that allows the motor to freewheel when the spool is in the neutral position. You could then use the fluid motor for torque assist when starting and put the fluid motor in the freewheel mode when approaching final position. When approaching final position the servo motor would do all of the work. If you are going up and downhill you need a hydraulic motor with a counterbalance valve between the hydraulic pump and the 4-way valve so that the counterbalance valve will provide dynamic braking most of the way.

Actually, for heavy duty applications that go a short distance you could get an electrically controlled hydraulic servo system.

For a short distance you could also use a small electric servo motor and controller to drive a hydraulic power steering unit from a junkyard. An automotive power steering pump uses about 1 horsepower and a unit for a heavy truck would of course use more power.

Depends of whether you want to play around with brand new stuff or play with junk.

 

[jraef]

DaveRosser,
Moving on from that response, you should be able to set up a torque limit in your Vector VFD. In many VFDs, you have dual ramp profiles that you can set up. Use speed control in one profile to do the gross movement that you want. Then use a limit switch or other position sensor to change to the 2nd profile, and set it set up as torque limit so that it doesn't push too hard once the other part of the machine takes over. If you are really good, you could try using the encoder fedback as a position indicator and do the switchover inside the VFD programming. It may take a little trial and error to determine what that amount of torque will be, but I wouldn't think it would be too hard. I'm not familiar enough with the Midimaster to help you with programming it for that, but surely your local Siemens Drives distributor can help you.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[DaveRosser]

I've been in contact with our distributor, but this is apparently above them. They are looking in to getting some more help from siemens. One thing i dislike is haveing to go to distributor level for tech support..they inevitably need to go to a higher level, and sometimes things get lost in translation.
However, my research tells me that what i want should be possible..it is now a matter of HOW to do it. The dual ramp profiles should point me in the right direction, jraef, thank you. As far as postion feedback, we will be able to determine that using a fibre optic photo eye system and some math (this calculation is already used for other parts of the process).

 

[Barry1961]

Would it be possible to change the source of the command signal once the other machine makes contact? If you could have the drive "follow" the other machine it would not fight it after a speed change.

If you already have an analog signal driving the second machine it should be a cheap solution.

The tempoary loss of the command signal during switching should not have a noticeable effect.

Barry1961

 

[jraef]

That's a good idea too. There will be some sort of transition no matter what you do when the 2nd system takes over.

 

[DaveRosser]

great suggestions...i'll have to look into those as options..thank you

 

[ozmosis]

DaveRosser
Where abouts in the world are you?
I work for Siemens and know that what you are trying to do is certainly possible with the MM440. There are several ways of skinning this particular cat and it will depend on your set-up and keeness to try different settings as to the best for you. The MM440 can operate in open/closed loop speed control as well as torque control. The source of the command source can be derived from numerous inputs and even bits within the drive that are monitoring torque or a speed set-point. This is a function called BiCo (Binector Connector) that basically means you have a 'plug-board' internally within the drive that allows certain inputs(getting down to control and status word bits if necessary) to be plugged into certain outputs and even connected via free function blocks to include logic functions (AND, OR, Timers, XOR, flip-flops etc). Basically, you have a simple PLC within the drive, if you want to use it.
The latest MM440 Operating Instruction (part number 6SE6400-5AW00-0BP0)has quite a lot of this detail explained. I would be more than happy to email it but it is 11.6MB. Alternatively, the local Siemens office should help or get one of their application engineers to assist. This is when you tell me you are based in Greenland I suppose

 

[DaveRosser]

I'm located in Southern Ontario, Canada. I'm willing to do whatever it takes to implement this system, as it will have great impact(specifically cost savings...) on our future product manufacturing/developing.
Our local siemens rep. is apparently working on it, but he confesses a lack of experience in this particular area.

Any information that you can send me would be great.

I've tried to get as much info from the siemens web site as possible, but for some reason find it difficult to get what i'm looking for.

Thank you again for your time.

 

[ozmosis]

DaveRosser
I wasn't too far away then with Greenland.
Try this link for manuals:

http://www.automation.siemens.co.uk/

if you look on the right hand side there is a section saying "I want to:" then click on "download a manual". This will whisk you off somewhere hopefully landing you in a site that allows you to search for manuals. Put 6SE6400-5AW00-0BP0 into the search box and there you will find a selection of them. The latest one is at the top. This is a UK website but hey, we're a global company.
I think section 3.23 (open-loop/closed-loop control) is what you might be interested in, with particular reference to torque control.
Let me know if you have problems.
I'll email with details of others who might be able to help.

 

[DaveRosser]

I've obtained the manual, thank you.
i'll try and make sense of it now.
I appreciate your efforts on my behalf.

Cheers,