Re: [EE]: Synchronising 4 Linear Actuators with 4 x Op Amps and 6 MOSFETS

Hi Justin,

It will work in one direction fine but not so well in the other. When the switches are set so that negative is going directly to the motors and positive is going through the MOSFETs, there won't be enough gate voltage, and you'll be in linear mode. I suppose you could power the LM324 from something greater than the battery voltage.

Using low-threshold MOSFETs might get you into even worse trouble, since they like to blow up if there's too much gate voltage.

How about using an 8PDT switch, lose half the MOSFETs, and put the others on the always-negative side of the switch?

Friendly regards, Bob

________________________________________
From: Piclist <[email protected]> on behalf of Justin Richards
Sent: Saturday, April 19, 2025 7:10 PM
To: Microcontroller discussion list - Public.
Subject: [EE]: Synchronising 4 Linear Actuators with 4 x Op Amps and 6 MOSFETS

resending without attachment too big.

I struggled to find any example circuits that dont use a micro controller
so here is my attempt at an analog design.

Hoping the List could quick glance and critique before I bread board.   The
design seems simple and experience suggests it is never that easy so I am
obviously missing bits.

Problem:

To assist popup section of Echo Chobe camper trailer.

For this task I dont think the 4 Linear Actuators actually need to be in
sync. When the humans do it, it is definitely not synced - unwieldy
perhaps.  As such could simply connect to the toggling polarity output of
the DPDT  to the 4 actuators and the job would be done but ...

Parts:-

4 x Linear Actuators TS-LD-HS with internal 0-5000 ohm pot. 150mm stroke.
 These actuators have built in end stop switches.

1 x LM324N Op Amp (parts bin).

6 x IRF540N MOSFET (parts bin)

1 x DPDT wired to toggle polarity reversal of 13.8v house battery .(LiFeP04
120Ah my video showing how it failed if interested
https://www.youtube.com/watch?v=hDA7KEEZ5p4 )

Theory:

Closed loop control where all the slaves chase the master by attempting to
maintain the same resistance of their internal pots.

A DPDT switch to provide the polarity reversal and eliminate the need for
each slave to be driven with a H-Bridge.

Design:

All internal pots are connected across the polarity toggling supply and the
wipers are all connected to the non inverting inputs of their respective
opamp,

The Master Linear Actuator (the slowest) has its opamp configured as a
buffer,  Its output is fed to all the inverting input of the other
opamps as a ref.  This is the ref the others chase. The supply leads of the
Master are connected directly to the  polarity toggling supply.

The Slave  Linear Actuator main power in are connected to the polarity
toggling supply with two MOSFETS in series. The two Drains are connected.
(This allows the power to be controlled regardless of the polarity of the
supply and relies on the internal body diode.  This seems to be ok practice
but not sure.) The gates are connected to the output of their respective
opamps.

Schematic was attached but too big so a copy is here
https://hackaday.io/project/202902-synchronising-4-x-linear-actuators

Cheers Justin
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