OT - Dual Pot

cjd1965 · Sep 3, 2009

Hi
I have a requirment for a 10K potentiometer which can be used to provide fine and coarse control of a drive. A selector switch to go from fine to coarse is OK.

I dont know if the drive has 2 analogue inputs (dont know the make)

I wa considering 2 pots but realised the speed could have a large 'step' when fine to coarse etc if is selected if the pots are somewhere close so I started googling dual pots

An alternative would be for me to use 2 pots in series, say a 1 turn and a 10 turn

Any ideas?

dYoast · Sep 3, 2009

Just a thought

Wire the pots to analog inputs for the PLC.
Use the coarse as a 0-100 and the fine as 0-10.
Manipulate the two values to get a value to send to the drive.

Skip the pots and recommend an HMI.

cjd1965 · Sep 3, 2009

Hi thanks for the reply.
I dont know if there is a PLC

I am looking for an old fashioned electrical fix, although a cheap PLC/HMI may be a solution

Cheers

Edit: Anyone know of a cheap and cheerful PLC/HMI 0-10V DC analog output. Max 2 digitals in, no digitals out.
Someone is going to site to take some photos of the panels for me so I may know more then. The end user thinks a drive is where he parks his car

Brian123 · Sep 3, 2009

You could wire two pots of different value in series. One pot could be a 9K pot, and the other could be 1K. Together, they would add up to 10K. The 9K pot would be your coarse adjust and the 1K would be the fine. As there is no switching between them, there will be no "stepping" to the new value.

Edit: Also, most drives will accept a range of input resistances, so you wouldn't necessarily have to have the pots add up to exactly 10K. You would want to maintain a fair ratio between them though. A 10K pot and a 1K pot would likely work fine and be easier to source.

Brian

keithkyll · Sep 3, 2009

If these are fixed speeds, the drive should have 3 or 4 digital inputs that you can program for fixed speeds.
For the HMI, look for a digital process controller similar to a temperature controller. Loop the analog output to the analog input to make it act as a pot.

moggie · Sep 3, 2009

Have a look at the programmable relays from Siemens (Logo) or Moeller (Easy).
These give you digital I/o as well as analog and with the Moeller Easy a screen for messages etc.

cjd1965 · Sep 3, 2009

Hi thanks for replies

I thought of 2 pots in series but the only downfall would be fine tuning 'down' once it hits the lowest end of the pot but that may be workable.

Pre set speeds wont work, needs to be 'infinetly variable between 0 and 100% with coarse and fine speeds

I will check out the moeller stuff, I am thinking a PLC with 2 AI and 1 AO would suffice

theDave2 · Sep 3, 2009

Why don't use a multi-turn pot w/multi-turn dial ?

http://www.p3america.com/precision_potentiometers.htm

cjd1965 · Sep 3, 2009

Hi
It is basically a line speed pot. I am told that in most cases the existing single turn (10K I am guessing) pot is good enough but for a handful of products they need finer control than they currently have.

I need to speak to them in more detail to determine the exact requirments but it is looking like a micro plc to sum 2 pots or to configure the drive to use 2 pots if it supports it (speed and trim)

I thought I saw a pot years ago with an inner and outer dial for this but maybe my memory is fugged

I may try the series pots if they can accept maybe having to set the fine to midway then get the coarse somewhere handy then fine tune the speed

cjd1965 · Sep 3, 2009

Found it... Dual Concentric pot

gonna ring them tomorrow and see if it fits the bill

http://www.farnell.com/datasheets/14851.pdf

milldrone · Sep 3, 2009

cjd1965 said:
Hi thanks for replies

I thought of 2 pots in series but the only downfall would be fine tuning 'down' once it hits the lowest end of the pot

Wouldn't this be a problem with any "pot" that has stops on the end ?

cjd1965 said:
It is basically a line speed pot. I am told that in most cases the existing single turn (10K I am guessing) pot is good enough but for a handful of products they need finer control than they currently have.

I once came across an application that used an encoder mounted on a panel door with a knob on it for operator control. The knob was for speed control of some VFDs. The site had absolutely no analog modules of any kind. The VFDs were controlled by rs485 and one digital signal for enable (I'm presuming so that if the comms failed the drives could be stopped via the enable input).

For your application perhaps a coarse speed encoder knob and a fine encoder knob to give an continuously variable fine tune. You would have no stops at the end of travel on either knob. Obviously everything would have to be "massaged" in the PLC to give you an analog out.

cjd1965 · Sep 6, 2009

Hi Thanks for the replies

The customer was OK with the series pot idea so I am going tomorrow to set it up for him. I explained that they may need to start with the fine pot in mid position and he is fine with that.

Cheers.

shooter · Sep 8, 2009

i would use two buttons one for up and one for down speed.
if there is a plc in between you can make anything within, otherwise a potmeter from a amplifier is also very good to work with.

erdemsvri · Sep 8, 2009

Also you can try digital potentiometers this way you can see the output voltage or current scaled and also some models have a fine tuning input but with the digital pot I think you wont need another fine tuning because you can exactly see the value of the voltage or current...

OT - Dual Pot

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