S7-300 - using SM332 2-output module

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Thread Starter

Lee Rehorn

I am a student in electrical engineering, and I am attempting to learn PLCs from scratch. I have completed the Siemens Level 1 training manual (on my own), and understand what is going on - to a degree. My boss wants me to hook up other devices to our S7-300s to control them, not just the provided conveyor model.

I have read over the SM332 manual, and now I am left wondering: what can possibly be driven by 10V and 20mA, which are the rated outputs of the SM332? What will I need to do to attach, say, a 1.2A, 24V motor to this unit (ie. will it be more difficult than making a voltage/current amplifier to stick onto the output before the motor)? What else will I need to configure on the PLC to get an external device to work? I mean, I guess I'll have to clear out the phyiscal connections currently in the SM332, but is there anything else soft- or hardware wise that must be done?

Thanks very much for any assistance,
Lee Rehorn

 
Z

Zan Von Flue

The S7 module (332) has to be installed in the hardware manager. In the hardware manager you can pick either the 10V or 20mA control signal with the OUTPUT tab (double click on module). This is only a control signal where 0V is 0% and 10V is 10% or 0(4mA) is 0% and 20mA is 100%. For the piece of equipement that needs to be controlled, it needs the correct input, either mA or Volts. If you could be alittle more exact on the type of motors to be controlled I could help you more.
Anyways Zan
 
T

T.P. Mungadze

Hie

To run a motor, you need to use a DIGITAL output card, and not an analogue one. Use the digital output to energise a contactor, which will have power supply to your motor. Hope this helps.

Taj
 
From your message I understand that you need a DC motor speed conntroller which has a 4-20ma analog signal as speed reference signal from PLC.
The controller is powered from a 24VDC.
Is it mandatory that you should use a DC motor. Can you use an AC driven motor?AC speed controller are easily to find in the market.

Kamath
 
L
The motor that I'm thinking of is an old Fracmo 1.2A, 24V, 1/60 HP permanent magnet DC motor and tachogenerator unit.

Another concern I have is about feedback. I'm not sure whether my boss wants position or velocity control, or both. For either situation, would I connect the respective feedback device (tacho, encoder, etc.) to the SM331 Analog Input module?
 
Z

Zan Von Flue

I have a feeling you want a close loop motion control. In this case the SM332 is mostlikely wrong. Yes the analog input card (SM331) would also be needed for the tacho. Or maybe a SM335 Analog in/out. However, in this case only velocity control is possible in connection with a S7 intern PID controller. However, I believe the loop would be about 60 to 100ms slow. Normal velocity loops are at about 0.5ms and position loops are about 2-5ms.
Also possible is a inverter with tacho input, then the S7 gives only the velocity to the invertor. We use a the SCA-PW-70-10 dc inverter (700W) from e-motion.
In any case if positioning and/or velocity is needed a FM354 would be better. The encoder would be connected here for positioning. https://mall.ad.siemens.com/DE/gues...2-781-785-787-788-789-770-773-795-&jumpto=788 is the website with more information.
However, in all cases a inverter is still needed (+-10V input). http://www.e-motion-controls.com/E-Motion/Englisch/produktion/dc/index.htm is our inverter.
Siemens has a controller and inverter package however I can't find the webpage. Try the Siemens website and just look around.
 
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Michael Griffin

0 - 10V and 4 - 20mA are standard signal voltages. If you have a lab power supply with a suitable output range, you may be able to use a 0 - 10V signal to control the output of the power supply. This is the easy way. If you have a 2 or 4 quadrant supply, then a +/- 10V command could drive the motor in either direction.

If you don't have such a supply, you can build one if you feel ambitious enough (I assume that if you are a student, you don't have any
money to buy one). If you are building your own, you may want to try to incorporate some form of isolation in the design so that you don't blow up
the SM322 if anything goes wrong in the rest of your power supply.

I assume that you are a student at Western. If you are putting together a lab demo system, you might want to ask yourself how much safety is required (or rather, you should ask your boss this). This may be determined by whether this is just a test lash-up, or if it is intended to be permanent demo system. We can address such questions later if you wish. Also try to think out how you will do the actual physical wiring.

I'm not familiar with the SM332 in particular, but if you have read the manual on it, and you are familiar with the Step-7 software you should be ready to go. I assume that you are not asking "how does Step-7 work?". You will need to have some sort of program of course, and some way of giving commands or input to the program so it can decide how fast to run the motor.


**********************
Michael Griffin
London, Ont. Canada
[email protected]
**********************
 
J

Johan Bengtsson

Well, the first question is: do you want to run the motor just on/off or do you want to change the speed?

for on/off you do of course just need a relay and a digital output. I think you have figured that out already and need speed control however if I understand your question correctly. In this case you need an analog output, 0-10V 0-20mA or 4-20mA
and - yes it is as easy as making a voltage amplifier, voltage is approximately equal to speed on a DC motor. The amplifier does of course need to be able to handle the power needed by the motor and if you just make a linear amplifier it will be hardest used at low to half voltage
(depending on current drawn).

I don't know what the SM332 is from memory, is it an analog input/output card?

Well besides that you need nothing except the PLC, power amplifier, power source and your PLC-program.


/Johan Bengtsson

----------------------------------------
P&L, Innovation in training
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]
Internet: http://www.pol.se/
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L
> I assume that you are a student at Western. If you are putting together a lab demo system, you might want to ask yourself how much safety is required (or rather, you should ask your boss this). This may be determined by whether this is just a test lash-up, or if it is intended to be permanent demo system. <

Thank you for your response (thanks to everyone, actually). The long-term goal is to replace our PC w/DAC cards and separate controllers with PLCs on all of the lab equipment. However, I must admit that I'm not sure of the compatibility of the PLC with the system I am looking at connecting; it was made in 1967 as a self-contained controls experiment, and there is no documentation on any input/output characteristics of the devices (except the motor). Are you familiar with Feedback Ltd.'s SA150D servo amplifier? I don't know if anyone is anymore! I do not want to connect this system to the S7-300 for the very safety concerns you have voiced. I also question the merit of bothering with connecting this system to the PLC, so I will have to talk to my boss about it.


>I assume that you are not asking "how does Step-7 work?". <

No, I'm familiar with the software.

I will contact you via e-mail as soon as I have access to a computer with e-mail capabilities, and once I make sure I fully understand my boss' wishes.
 
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Michael Griffin

At 16:54 14/06/01 -0400, Lee Rehorn wrote:
&lt;clip>
>The motor that I'm thinking of is an old Fracmo 1.2A, 24V, 1/60 HP
>permanent magnet DC motor and tachogenerator unit.
>
>Another concern I have is about feedback. I'm not sure whether my boss
>wants position or velocity control, or both. For either situation, would I
>connect the respective feedback device (tacho, encoder, etc.) to the SM331
>Analog Input module?
&lt;clip>

The PM DC motor will go at a speed proportional to the applied voltage. Make sure that your drive (or power supply, or whatever you want to call it) can either deliver more than the motor stall current, or that it has true current limiting, or that it can withstand a temporary overload while the motor is starting.
If you have true current limiting to less than stall current, the motor will take slightly longer to reach full speed, as the initial
acceleration will be slower until it is going fast enough to get out of current limiting. You may want to keep this starting characteristic in mind in your overall design.


I'm not sure what the stall current would be for such a small motor. The usual rule of thumb of 10 times full load probably doesn't apply. The motor is probably not very efficient, and so has a high enough resistance to limit it to considerably less than that. 4 or 5 times full load current may be more likely. There are several possible tests you can perform on the
motor which will provide a reasonable estimate of stall current if this is important.


A tach is a velocity feedback device. If you want position, then you should use an encoder. You would need to connect the tach to an analogue input module, but you may need some suitable signal conditioning to ensure that the output voltage of the tach operates within the range of the analogue input. In other words, if the generated voltage of the tach is also 24 volts (I don't know what it actually is), you will need signal conditioning to re-scale that to the range the analogue input can deal with. A robust design would limit the voltage to prevent damage to the input.

You can do closed loop speed control with this set up, but you won't get the same responsiveness as you would if you used a proper servo amplifier. Since you seem to be putting this together on the cheap for educational purposes, that is probably satisfactory. There are regular discussions here about closed loop control algorithms so you can ask about those later.

If this unit is intended to be a permanent demo set-up (as opposed to a one time test), you will probably need to think seriously about circuit protection (fuses), a means of disconnect, and possibly other things as well.


**********************
Michael Griffin
London, Ont. Canada
[email protected]
**********************
 
After some experimentation, it appeared that the SA150D servo amplifier (the one meant to be used with the motor I'm using) has input requirements that closely match the specs of the SM332 module, wihtout any peripheral device. Upon connecting the two, I can get the PLC to control the speed of the motor, via an analog input knob on the PLC's training module board. This makes me pretty happy :) However, now I'm not sure how to implement feedback. I've attached the tacho's output to the second input channel, but I'm not registering any voltage on the oscilloscope. I re-wrote my PLC program to read in from this input and subtract its value from the input knob's value. However, observing the program online as the voltage knob is adjusted reveals that the second input (tacho) constantly has a zero value. I don't think the tacho is broken...

Thanks for any help,
Lee Rehorn
[email protected]
 
J

Johan Bengtsson

Well, how have you connected the tacho? some (if not all???) tacho gives a AC voltage out, direclty
connecting this to a analog input (DC) will give you the average voltage, and that is of course 0 in that case.

If this is your problem you need a converter. The "simplest" form will of course be a rectifyer I really don't think that is good enough.


/Johan Bengtsson

----------------------------------------
P&L, Innovation in training
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]
Internet: http://www.pol.se/
----------------------------------------
 
M

Michael Griffin

A few points for you to consider:
1) Are you sure this is a tach?
2) What terminal connections do you have?
3) Try measuring the voltage open circuit. This will isolate the tach from any other problems in your circuit. When something isn't working correctly, it is best to not introduce too many complications at once.
4) You said you are measuring it with a scope, so whether it is an AC or DC output you should see something. If you are using a voltmeter, try both DC and AC ranges.
5) If you still get nothing, use an ohmeter to check for continuity. You should see a fairly low resistance.
6) You've said you don't think the tach is broken, but you haven't said why you think it is working.
 
Z
>However, now I'm not sure how to implement feedback. I've attached the tacho's output to the second input channel, but I'm not registering any voltage on the oscilloscope. I re-wrote my PLC program to read in from this input and subtract its value from the input knob's value. However, observing the program online as the voltage knob is adjusted reveals that the second input (tacho) constantly has a zero value. I don't think the tacho is broken...>

If on the oscilloscope you are using is not measuring any voltage And the tacho isn't broken, is the Tacho correctly connected? If the the osc. can't find voltage, the PLC can't either.
later
zan
 
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