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Momentary Switch to directional DC motor

Bluejets

Oct 5, 2014
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Here is a first pass at the approach I described above.

Notes:


2. There will be a very short (a few milliseconds) current pulse to the motor in the wrong direction before the relay changes state. Most motors cannot start moving in that short a time, so it should not be a problem.


View attachment 46784

Yes but when the motor is already running in the opposite direction, think you'll find this current rather large and sustained, possibly enough to blow anything.
 

AnalogKid

Jun 10, 2015
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The motor runs only when the button is pressed, and cannot change direction once it is up to speed. Direction change happens only during the first milliseconds when the button is pressed and the relay changes state. To change direction, the button must be released, removing all power from the motor, then pressed again. The OP does not give the minimum off time.

As stated, the switch and relay contacts must be rated for at least twice the *startup* current value.

ak
 

AnalogKid

Jun 10, 2015
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What are the voltage and current ratings for the motor? Also, can you post a link to the manufacturer or vendor's data page?

ak
 

Halcyon

Dec 26, 2019
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I’m waiting for the vendor to share that info with me as the model number did not yield much data. I suspect it’s a 12V 10-15A motor. I’ll gladly share as soon as I get it. Here’s the motor for what its worth
Thanks.
upload_2019-12-31_13-5-15.jpeg
 

Minder

Apr 24, 2015
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Apparently from the spec sheet it is a 24vdc 2973rpm 350w 6Nm motor.
peak current is 84amps, 2.5amps no load.
M.
 

Minder

Apr 24, 2015
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A little strange you would obtain a motor for an application without finding the mechanical requirements demanded, and the specs of the motor?
M.
 

AnalogKid

Jun 10, 2015
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Well, I did not see that coming. Would have been nice if the OP had mentioned that the motor was so large.

ak
 

Bluejets

Oct 5, 2014
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The motor runs only when the button is pressed, and cannot change direction once it is up to speed. Direction change happens only during the first milliseconds when the button is pressed and the relay changes state. To change direction, the button must be released, removing all power from the motor, then pressed again. The OP does not give the minimum off time.

As stated, the switch and relay contacts must be rated for at least twice the *startup* current value.

ak

That might be all well and good in theory.
In practice motors have a tendency to keep running in a particular direction when the power is removed, call it run down time if you like.
This will vary with the size of the motor rotor and the load.
So my point is still valid.
 

Halcyon

Dec 26, 2019
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Apparently from the spec sheet it is a 24vdc 2973rpm 350w 6Nm motor.
peak current is 84amps, 2.5amps no load.
M.
It’s on a manual wheelchair, used for tilt forward and reverse. The user’s limitations have complicated the requirements. And limited them to a pushbutton switch only. It was originally designed with a toggle switch, but the user can’t operate it.
 

AnalogKid

Jun 10, 2015
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Given the high currents involved, can you post a schematic or wiring diagram of the current system so we can see the rules of the road?

ak
 

Halcyon

Dec 26, 2019
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Certainly. I will have access to the chair in about a week. From what I recall it was simply a momentary switch that controlled the forward/reverse. And a lead acid battery. I didn’t notice any electronics. I’ll look closer.
Thanks.
 

Halcyon

Dec 26, 2019
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It seems like the connection is straight forward :
2X12V(lead acid 2.9A each) in series.
Momentary toggle switch.
Motor.
No other circuitry.
 

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Bluejets

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Basic manual F/R shown below.
Replace the toggle switch with relay contacts and use 2 push buttons to operate.

Wish I could be as sure of winning the lotto that the op wants to do it with one button.
Think the Op will have to start opening the wallet soon.
 

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Bluejets

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Yes, well, there ya go as they say.
These things take time to design and people offer their help to a certain degree but no one I know will design to your requirements for free.
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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This appears to be a similar problem to one discussed here in an earlier ElectronicsPoint thread. A small dedicated microprocessor would be able to sort your requirements and implement a single push-button switch solution.

There are many push-button switches available for use by the handicapped, some with extremely low force-actuation requirements. I have built some of these from scratch using optical interrupter sensors, which essentially require zero force, with the switch plunger restored to its unactuated position by a very weak compression spring. It all depends on how handicapped the end user is, whether or not they are capable of moving their hand or just a finger and what their range of finger motion is, to determine exactly what push-button switch to select. This must be your first design criterion: push-button switch selection.

It makes little or no difference at all what form the push-button switch contacts are: single pole, single throw, normally-open (type A) contact or single pole, double throw (type C) contact will work fine. Multiple poles are unnecessary. No motor, or actuator, or whatever large current will flow through the push-button switch. The switch is there only to inform the microprocessor of the user's intentions.

For safety, it is advisable to switch in a low-resistance load across the DC motor, actuator, or whatever load you are switching whenever the microprocessor commands the output to be OFF. Relays and contactors are too slow to perform this action. A full-wave bridge rectifier, connected across the load terminals with the bridge output connected to a MOSFET switch, will perform adequately without too much circuit complication. Or you can use a bipolar junction transistor (BJT). Either approach is satisfactory, but either one will require an optical isolator, controlled by the microprocessor, because the load is energized from a bi-polar source (the two 12 V batteries) whose polarity is subject to change.

Ordinary electromagnetic contactors can be used to switch the load current ON and OFF under control of the microprocessor. An intermediate relay or MOSFET switch may be required to actuate the coil of the contactor.

The ONLY function the microprocessor performs is a stateful inspection of the push-button switch to determine what the output should be: ON (forward), ON (reverse), OFF, or OFF with the load "shorted" by a low-value resistor when transitioning from ON to OFF. The load "shorted" condition only lasts for a few milliseconds after the user releases (de-actuates) the push-button switch. It is there only to dissipate whatever mechanical energy was stored in the motor, actuator, or whatever when that device was energized. You can eliminate this "safety" feature if you wish so the microprocessor has only to control two contactors: one for ON/OFF control and the other to select the direction.

I am confused by the images in your post #32. I see no motor there. Instead there is something that appears to be some sort of linear actuator with a small built-in motor rated at 24 V DC and 2.7 A. This is a far cry from your original speculation in post #24 of 12 V 10 - 15 A motor. What is going on here? Have you jumped off into the deep end of the pool trying to help someone who is handicapped and confined to a wheelchair? Are you aware that the linear actuator has only a ten percent duty cycle with TWO minutes ON followed by EIGHTEEN minutes OFF? Does the linear actuator have any safety protection to prevent it from being driven into mechanical limits and perhaps becoming "locked up" and perhaps requiring disassembly to make it functional again?
You can add such safety features to the microprocessor push-button switch control, and I would certainly recommend adding something to prevent exceeding the duty-cycle limitation. Replacing a burned out linear actuator doesn't sound like much fun, but a handicapped person may not realize when they exceed the maximum (two minute) ON time or wait the minimum (18 minute) OFF time before actuating the push-button switch again. Or perhaps this has already happened and is one of the reasons you are involved with this project.

Exactly what are your mechanical, electrical, and electronic experience and training? Are you a newbie at all this? Can you design and build a small microprocessor control for a push-button switch? It's pretty simple IMO but there is a learning curve and a bit of electronics assembly required.
 

Halcyon

Dec 26, 2019
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I much appreciate the detailed run-down. Several interesting points.
The switch that I’m required to use is a SPST push button. The user can hit it with significant force. The issue is aim. That’s why the option of 2 switches is not doable here.
The original Demo chair I saw had no info on the actuator/motor assembly, so it mislead me to think it was a high current based on the toggle switch rating. However, I’ve seen the actual setup now and it’s the picture that shows the LINAK brand actuator. The spec sheet in the other picture is what the mfg provided when I asked for Specs.
And you’re correct, there’s no separate motor- the actuator has the motor built in.
With the regards to the duty cycle but you talk about, how would that have been addressed with the current toggle switch? It seems that if the switch is held for a long time in either position (F/R), or frequently toggled, there is no limitation from it being driven into “mechanical limits”.
I’ve been asked, with option to say No, to try and bring a solution that would make independence that much more possible for this individual. I’m comfortable tackling a project that may not be overly complicated. I have an electrical background (Control Systems) with limited electronics knowledge. Putting in the time is not an issue for me. I would perhaps be able to judge how complicated this is, for me, if I had a look at a circuit. Assembly I’ve done many times. The most recent one was a 2CH TxRx Velleman kit. So building is not an issue for me, but design seems more challenging. Kindly clarify what might be involved.
many thanks!
 

Bluejets

Oct 5, 2014
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Kindly clarify what might be involved.

Hevans1944 covered just about every aspect from a uC control , others ave covered other approaches.

What you do not seem to be able to accept is there are logically 2 states required in the output.
Therefore, which ever approach is used, a 2 state input is required.

If one were to toggle through a flip flop arrangement, it is still a 2 state input regardless.
It may not be beneficial in many instances to have to go say forward before reverse or visa versa.
In fact it would be a confusing arrangement as operation would result in having to remember what the last state was or some indication of same.
If ever powered down then any state would present itself which could be dangerous in some situations.

Basically, I think you do not have a grasp of the logic involved, or have any understanding of what has been presented, even if you have knocked up a Velleman kit.
 

hevans1944

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Jun 21, 2012
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With the regards to the duty cycle but you talk about, how would that have been addressed with the current toggle switch? It seems that if the switch is held for a long time in either position (F/R), or frequently toggled, there is no limitation from it being driven into “mechanical limits”.
Duty cycle limitations are NOT addressed with the current toggle switch. It is a real possibility that an uninformed handicapped person could exceed the ON-time limitation, or not wait long enough for the OFF-time cool-down to occur and therefore exceed the duty-cycle limitations. Depending on how much the maximum ON-time is exceeded, and/or the minimum OFF-time is ignored, the linear actuator internal motor could be burned up before the end-user noticed it had quit working. In all likelihood the end-user will immediately notice the tilt function is no longer working and will call you to "fix it"... again. That's one way to ensure repeat business.

There are Hall-effect switches available as an option for the linear actuator. These can be used to prevent driving the linear actuator into a hard-limit stop. I suggest you contact the manufacturer and talk to one of their application engineers regarding how to order the Hall-effect switches. These devices are also widely available through electronics distributors for after-market add-ons as well as new designs. Conventional limit switches, either electro-mechanical or electro-optical, can also be fitted to the linear actuator. If none of these has been implemented yet, I strongly suggest that you do so now.

I’m comfortable tackling a project that may not be overly complicated. I have an electrical background (Control Systems) with limited electronics knowledge. Putting in the time is not an issue for me.
Good. Because if you decide to use a microprocessor it will take awhile to become familiar with what it can do, and what you must do to learn how to program it. A good place to start is by reading this document, "PICmicro Mid-Range MCU Family Reference Manual." Then you will have a choose a small PIC microprocessor for your project. I suggest choosing something simple, like an 8-bit MCU, preferably in an 8-pin Dual In-line Package (DIP) so you can plug it into a solderless breadboard while you develop the software and circuits to control the linear actuator.

I got involved with PIC microprocessors a few years ago, after retiring from my full-time job as a particle accelerator engineer. Someone here on EP wanted to control a woodworking-shop vacuum system, turning on the vacuum motor whenever either of two power tools was turned on. After both tools were turned off, the vacuum motor would continue to run for several seconds more to allow the sawdust in the ductwork to be cleared. We used a really inexpensive PIC and an Allegro Hall-effect current sensor to sense motor current in a 120 VAC and a 240 VAC circuit, the PIC having an output that controlled a solid-state AC relay that turned the vacuum motor on and off. The Allegro Hall-effect current sensors produced an analog output signal that we digitized with two PIC analog inputs. Pretty fun project, although there was a steep learning curve since I hadn't programmed an embedded microprocessor since the 1990s and the person doing the actual work was a complete novice.

If this sounds like something you would like to do (design, build, and program an embedded PIC microprocessor) I can point you at the thread describing what happened. Very little electronics is involved, just plug-and-pray with off-the-shelf parts.
 
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