Motor Control

[draynor]

On an actuator i have an RVDT that provides a voltage of 0V at minimum stroke and 10V at maximum stroke. How do i use this voltage to shut off the 28V dc motor at either end of the stroke?? What should the circuit contain?

Please help! it'd be greatly appreciated.

 

[AFab]

A PLC with Analog input

 

[buzzp]

Most of the time these can come with limit switches of sorts to stop travel at either end. They can be external or internal. If yours dont and you don't want to add limit switches, then your stuck using the 0-10V output to indicate position. If your just using this to stop the actuator at either end, it might be easier to find a good spot for limit switches. You could come up with some analog circuitry to stop the actuator but the accuracy might not be good enough. Another option is to actually read the position using an AD converter in a microcontroller or PLC. Let me know how accurate this needs to be and I can give you some details.

 

[draynor]

Thanks buzzp.

Well, i'm considering using LVDT & RVDT's to replace current unreliable micro-switches.

I have an aerospace unit that currently has an RVDT attached onto it. I also have a signal conditioning chip that i am currently linking up on some veri-board. I can scale the output voltage how i want, which will probably end up as 5V at min stroke & 15V at max stroke. It is using these output DC voltages to cut the motor that is giving me grief.

 

[nbucska]

What about replacing the uSwitch with opto-sensor?

<nbucska@pcperipherals.com>

 

[draynor]

You mean an optical encoder of some sort?

If you do, then i have considered it and found they cannot operate at the altitudes & temperatures required.

 

[nbucska]

optical interrupter. You haven't specify altitude and
temp. Is there any other limitation or requirement
we need to know to answer your question?


<nbucska@pcperipherals.com>

 

[draynor]

Can't say i've heard of one of those! I'll check it out, thanks.

It's on a Hawk aircraft so altitude 25,000ft, temp from -60 to 125 degrees C.

 

[nbucska]

Try <

http://www.panasonic.com>,

<

http://www.optek.com>

, <

http://www.sharp-world.com>

or <

http://www.agilent.com>.

Someone must make mil.spec part...

<nbucska@pcperipherals.com>

 

[nbucska]

Try <

http://www.panasonic.com>,

<

http://www.optek.com>

, <

http://www.sharp-world.com>

or <

http://www.agilent.com>.

Someone must make mil.spec part...

You may try HALL-sensor, too. Optical one may get dirty...

<nbucska@pcperipherals.com>

 

[draynor]

Yeah, i've had a look at Hall sensors. Stuggling to get the range i need. + & - 49.50mm.

I have already settled on an RVDT and need a circuit that will take a voltage, compare it against a reference and disconnect the motor.

 

[nbucska]

send motor current, RVDT data and specify the switching
sensitivity or positional accuracy.

<nbucska@pcperipherals.com>

 

[draynor]

Thanks a lot for your help.

Can you quickly explain what a dual power dc supply is and what advantages it has over a single power supply?

Many thanks.

 

[buzzp]

The temp and altitude make it a little more difficult for sure. As Nbucska said, the accuracy is important to know so we know where to send you for a solution. I think you definately picked the right technology for determining end and start of stroke for such a short range.
Comparators could work depending on the accuracy. You can get a DC rated relay/contactor to handle the motor load (need more info on the 28VDC motor). This relay would be energized by the comparator circuit. There is lots of ways to do this that is just one way. Ill wait for more info.

 

[nbucska]

draynor:
The dual supply is just that: one supply with two or more outputs usually with different voltages.


<nbucska@pcperipherals.com>

 

[draynor]

Thanks for your advice evryone.

The accuracy isn't gretaly important at the moment, just a generic circuit to compare an input voltage with the position voltage and disconnect motor.

 

[draynor]

Consider a black box with one input that ranges from 5 - 15 V dc. At the output are the negative & positive motor inputs.

I need to know what to put in the black box to cut the motor at the 5 & 15V.

 

[buzzp]

The first circuit that comes to mind is a comparator circuit. It essentially compares the input voltage to a reference voltage (reference voltage provided by you or built into the chip). If the input is above the reference voltage then the output goes high or low depending on how you set it up. You would need two comparators, one for comparing to 5V and one for comparing to 15V. Some integrated circuit IC's have built in transistors that may be able to drive the mechanism that is turning off and on the motor. I assume there is a relay or similar device hooked to the actuator supply power. So that when the relay is energized the actuator moves. If this is not in your system already then you need one to make life simpler. The relay will allow you to turn off/on the motor without the comparator having to switch the actuator load directly.
The high or low output of the comparator would energize or de-energize the relay that is carrying the electrical load of the actuator.
Some links below (randomly selected-search google for comparator circuits) will help, some are very basic some are detailed.

http://www.maxim-ic.com/appnotes.cfm/appnote_number/886

http://www.allaboutcircuits.com/vol_6/chpt_6/2.html

http://home.cogeco.ca/~rpaisley4/Comparators.html

Pay close attention to the data sheet for operating temperature and altitude. A commercial grade part will not be adequate. Military spec parts are definately in order.

There are other methods that might work better such as a PLC with a analog to digital card or a microcontroller with the same AD card. These would take more time to develop (possibly) depending on if you have worked with either before.

One other note, the temp range your dealing with will cause the reference voltage to drift, how much depends on how much you want to spend. If the reference voltage drifts, say +-10% over the operating range (cheaper voltage reference) then this will directly affect where the actuator motor is stopped.

 

[ScottyUK]

What about using a pair inductive proximity sensors in place of the microswitches. They have a number of advantages:

- a digital output which will be easier to interface to than an RVDT

- simple power requirements

- no moving parts other than whatever they are detecting the position of

- are physically small and light

- capable of service in some pretty hostile environments.

They're popular in the chemical process and power generation industries because they are tough and reliable, plus they are simple - which suits us just fine! I think you may be making this unnecessarily complicated involving RVDTs.



-----------------------------------

Start each new day with a smile.

Get it over with.

 

[draynor]

Many thanks for your help guys. Very interesting indeed.

As for the proximity suggestion, i'm looking to replace the discreet micro-switches with a continuous system. This way, any later systems e.g auto pilot can interface with the output voltage.