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Topic: Cross switching 2 current lines (Read 267 times) previous topic - next topic

Jdaniels1730

I have an application where I currently use 2 STDT mechanical ralays to switch to solenoid control wires between the existing controller and the solenoids that control hydraulic pressure.

Basically, the relays take the following circuit

Cntrl 1 ----------------> Sol 1
Cntrl 2 ----------------> Sol 2

And when the relays are switched on the circuit becomes

Cntrl 1 ----------------> Sol 2
Cntrl 2 ----------------> Sol 1

This mechanically binds the device as long as the operator seems necessary via a pushbutton.

This works fine for the current use, but I would like to apply PWM to the relay control so that the device can be "partially" unbound and allowed to turn slowly. To do this would require the relays to pulse on approx 50hz and 25% duty cycle, but that can be adjusted to the individual machine/operator using trim pots if need be. Cannot see it ever needing more than 60hz and 75% duty.

Obviously I first looked into solid state relays. To re-create my circuit with SSRs would require 2 NO, and 2 NC, setup to switch simultaneously. There are plenty that fit my needs with a couple exceptions I'm not clear on. Current leakage and current drop.

In majority of the machine operating time, the relays connecting the circuits would be NC. But there will be normally open relays on standby ready to connect the feeds to opposite circuits. That current leakage I worry about as the solenoids are fed a 500mA to 1.1A current depending on the controllers programming for the conditions. Will current leakage from NO circuits then add to the current on the NC circuits? And vice versa?

Also, in NC condition, will the voltage drop of a SSR relay be enough to significantly alter the supplied current from the controller?

I am not concerned with gate voltage as I can use opto isolated SSR's.

The controllers outputs are not easily altered to compensate for current changes introduced by my circuit, so for machine parts life, the current passed to the solenoids needs to be minimally altered. However with that said, std 12v automotive relays have served me well without PWM.

So basically I need the existing controller to operate as normal until a button is pressed, then the ssr will feed each solenoid with opposite signals. When the operator presses a 2nd button momentarily, he should be able to pulse the relays to allow the machine to creep at slower than normal operating speed by remaining partially bound hydraulically.

This is all kind of like a transmission brake on a race car, but with a pair of current controlled solenoids rather than a single 12v on/off solenoid. (That's where I got the idea for my first implementation and the current idea for pwm the solenoids)  but this is on a transmission driven jet drive for a boat.

I'll add a diagram showing the current mechanical relays as soon as I get in front of a pc and off my phone.

MorganS

but this is on a transmission driven jet drive for a boat.
I'm not picturing what you're trying to do. Maybe when you send the drawing it will be clearer.

Allow me to paraphrase: you have two controllers and two solenoids. Currently you have an extra switch that reverses which controller controls which solenoid. But what do those controllers actually do? Are they forward+reverse controls or just on+off?

What is the voltage and current for each solenoid? AC or DC?

You have discovered that PWM won't work on mechanical relays and it may not work on SSR's either. (If the control current is AC then it's more difficult.)

Assuming PWM control works, I don't see how mixing the two controllers together will help. Let's say we have 25% duty cycle. So 75% of the signal comes from the 'other' controller. But what if they are both set to 'forward'? Then you get 100% forward.
"The problem is in the code you didn't post."

Paul_KD7HB

I think you asking for short lives on you solenoid valves. You may need to study up on proportional solenoid valves and other options. See This description of options

Paul

Jdaniels1730

#3
Jul 11, 2018, 07:23 pm Last Edit: Jul 11, 2018, 07:27 pm by Jdaniels1730
They are promotional valves..  One controller controls both but with separate wires (curcuits). Both solenoids work in the same way whereas 500mA = low hydraulic pressure 1.1A = high hydraulic pressure. The solenoids and controller operate on nominal 12-14vdc. The controller sometimes sees upwards of 18vdc, but I suspect it controls the output voltage to the solenoids to keep them in nominal range

The way this works to bind the transmission is that it turns on solenoid 1 and turns solenoid 2 off when the relays are activated. Inside the transmission, this activates a clutch that wouldnt normally be on, while deactivating another. So when the relays are released (turned off) the hydraulic circuits go back to their controller commanded state and the machine is able to move. When the need arises to pwm the solenoids it is for a very short period of time so I wasnt really concerned with burning up the solenoids. It will be only pwm for 10-20ms at a time.

My computer is giving me crap, so I'm going to use my phone to post a hand drawn version of my existing relay circuit.

MorganS

I hope you mean "proportional".

Yes a hand drawn diagram is perfect.
"The problem is in the code you didn't post."

Jdaniels1730

#5
Jul 11, 2018, 07:50 pm Last Edit: Jul 11, 2018, 07:53 pm by Jdaniels1730
Yes proportional.. auto correct gets me every time


Jdaniels1730

#6
Jul 11, 2018, 08:01 pm Last Edit: Jul 11, 2018, 08:06 pm by Jdaniels1730
Thinking more on what Paul said.. maybe PWM is not the route I should take to achieve this? Looks like I need to look into altering current so I can allow slight amount of clutch slip for that 20ms... hmmm

MorganS

Ok, that diagram helps. I don't really understand what the middle connection is doing for the system. That's another proportional valve and when you activate the relays, it must be off?

So is the existing controller a proportional controller? That means it's already producing a PWM or analog drive suitable for the solenoids? If you still want it involved in the final Arduino-controlled system then you need to find out what it is doing electrically.

You didn't answer the question about how you expect PWM to mix the two controls.
"The problem is in the code you didn't post."

Jdaniels1730

Ok, that diagram helps. I don't really understand what the middle connection is doing for the system. That's another proportional valve and when you activate the relays, it must be off?

So is the existing controller a proportional controller? That means it's already producing a PWM or analog drive suitable for the solenoids? If you still want it involved in the final Arduino-controlled system then you need to find out what it is doing electrically.

You didn't answer the question about how you expect PWM to mix the two controls.
Ignore the middle relay.. it's an old drawing.

Since using the relays to swap signals locks the transmission, I was assuming I could pwm to have them partially lock.. but I was assuming wrong.

The solenoids are current controlled, so I need to intercept the signals and then alter the current fed to them to get my desired result.

MorganS

Can you find a datasheet for the solenoids? It is unlikely that they really are current-controlled.

If they are current-controlled then interrupting the feed wont work. If you are driving your car and something happens which reduces the engine power then you just push harder on the accelerator so the effect of the reduction is eliminated.

I don't understand how the transmission locking helps you. What do you mean "lock"?
"The problem is in the code you didn't post."

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