SVX Transmission Line Pressure Control.

[Trevor]

Unlike another who posts prominently, it is not my practice to record what amounts to an unsolicited lecture. However statements have been made on the alternative forum which may mislead members to their disadvantage. Therefore it is desirable that correct information is published here.

SVX Transmission Line Pressure Control.

Line pressure is initially controlled via pulse width modulated, (PWM) electrical, normally closed, solenoid valve “A”. When open, this valve bleeds off pressure, rather than interrupts pressure, as a means of control. It provides precise control, but has only limited capacity. Therefore amplification is necessary, in order to achieve final control of the overall operative line pressure.

The adjusted pressure from solenoid “A” is applied as pilot control pressure, to a fully hydraulic control valve, the pressure modifier valve. As the name suggests, even this valve constitutes only a control device and does not provide final control. The pressure modifier valve, in turn controls the main pressure regulator valve. The result is a system of amplification, in two stages.

The use of a normally closed valve “A”, for electronic control of line pressure, renders the system fail safe. However maximum pressure must be limited and not allowed to runaway at high pump speeds. This is achieved by including the pilot valve, which sets the pressure, and the pressure modifier valve, within a feed back loop.

Solenoid valve “A” is controlled by means of a PWM signal, delivered by the transmission control unit, (TCU) via a direct circuit. This electrical signal comprises a series pulses, delivered at a fixed frequency of nominally 50 cycles per second. The length of the pulses, rather their frequency, controls fluid output from the valve.

The output pressure is therefore delivered in the form of waves at 50 CPS. However, due to the low volume delivered, the high frequency and the reciprocating mass involved in the pressure modifier valve, these waves have little effect. Furthermore, final control pressure from the pressure modifier valve, is smoothed by the pressure modifier accumulator, a device incorporated for this purpose, as is mentioned in section 3 within the Subaru manuals.

The dropping resistor circuit.

It will be immediately apparent that the sudden on off pulse width modulated duty, to which normally closed solenoid valve “A” is subject, tends to cause what could be called a hammering of the valve seat, even though this is largely reduced/damped by the flow of the controlled fluid.

The dropping resistor introduces a second series of current pulses, applied in parallel with the control signal. These pulses are applied across the off cycles, so as to check the travel of the armature as it moves, thus reducing both shock and noise. These secondary parallel signals mean that in effect, during the closing/closed period, the voltage does not fall completely to zero.

This second series of pulses must be at a lesser level than the control signal, hence the dropping resistor. A resistor with a high current rating is required, which can not be mounted within the TCU enclosure.

Full voltage from the direct circuit operates the solenoid and quickly opens the valve. The low voltage dropping resistor circuit, holds in the solenoid and thus controls the point at which the valve is allowed to close. Therefore controlling the length of the low voltage pulse, sets the overall pulse length.

The full voltage direct circuit signal, comprises a very short fixed length pulse. This is immediately followed by an independent low voltage pulse, from the resistor circuit. The sum of the two provides the total pulse length delivered during each cycle.

It will be appreciated that increasing the resistance in the circuit, or opening the circuit by omitting the dropping resistor, will upset the normal pulse length, thus increasing the line pressure and making shifts more abrupt. Secondly, as an undesirable issue, shock loads applied to solenoid valve “A” are increased.

The resistor should measure between 9 and 15 ohms to be within specifications and is usually close to 12 ohms.


Pressure Control via Solenoid Valves.

Control via a Simple Solenoid Valve.

In the interests of simplicity and economy, a simple on/off bistable solenoid valve is often used for the pressure/flow control of fluids. It will be appreciated that this form of valve has limited armature travel. When used in this form, the component is sometimes referred to as a pulsoid.

It is absolutely impossible to electrically hold an unsupported object stationery within a varying environment without some form of feedback, arranged to detect any movement requiring a necessary correcting force to restore equilibrium. In the instance of solenoid valves, this is achieved in the design of special proportioning spool valves, by detecting change in applied current, or otherwise using feedback from a second coil winding.

As a means of variable fluid control, the simple valve is in effect turned rapidly on and off at an adjustable rate. The output when a normally open valve is in operation, amounts to the sum of the valve on/open time, as opposed to the total off/closed time. Often the ratio is expressed as percentage.

The on/off ratio is established by energising the solenoid operating the valve, by means of an on/off signal and the procedure is known as pulse width modulation. (PWM). The frequency of the pulses is held constant, while varying the percentage of on/off time. A frequency of around fifty cycles per second is often used.

It will be appreciated that the down stream flow, will be in the form of a high frequency undulating stream. In many instances, devices down stream will not have there operation affected, due to inherent reciprocating weight/mass. Furthermore, smoothing components are often incorporated.

Where the application is more critical, a more complicated approach is necessary and a linear solenoid can be used to drive a valve. This provides the necessary degree of travel to enable intermediate positions to be established and held in order to provide a variable valve opening. Thus flow/pressure can be regulated in a linear fashion. But it must be appreciated that continual variables must be continually offset as an ongoing contingency.

The answer to this difficult problem utilised within process control equipment, is to provide a feedback loop, established via a feedback signal, whereby control can be balanced/adjusted, even though variables exist. In its simplest form this can be a signal from a down stream sensor, but when this can not be arranged, sensing can be achieved directly from the controlling device. The further down stream the sensor, the more accurate is overall control, but control becomes less constant.

Spool Type Pressure Reducing Valves

In order to operate as an effective pressure reducing device, the valve must incorporate a feedback loop. e.g. This is arranged within the SVX pilot valve, by means of a restricted interconnection between the outlet and a feedback chamber. When the valve is open there is feedback pressure via the chamber against the included coil spring. Immediately equilibrium is established the valve closes, with this process of establishing equilibrium being continuous. In this way outlet pressure depends on the chosen spring pressure.

The Linear Solenoid.

In respect of a linear solenoid, the operating current is monitored, as a means of securing a direct feedback signal. It will be appreciated that solenoid supply current will vary, as result of the mechanical load, which is directly proportional to the magnetic field and the current through the coil. The circuitry involved naturally becomes complex, and in addition the mechanical construction of the valve must be taken into account, when establishing set points and actual flow.

Proportional Solenoid Valves.

These purpose designed components, incorporate specific means towards directly providing a feedback signal. Often an auxiliary coil winding is incorporated. The most important feature of note, is extended armature travel so as to provide an increased ratio of movement and therefore degree of control.

Please note that there can be confusion in what should, or should not, be termed a proportional/proportioning solenoid valve, as a linear solenoid valve can perform this function.

Feed Back Loop

A feed back loop operates, by feeding back any change which may have altered a desired set point. e.g. When the output of a valve falls, a feed back signal via the loop, results in the valve opening being increased. If the valve is opened excessively and flow is raised above the set point, the reverse takes place. The action proceeds until equilibrium is established.

The process is normally continuous and can occur at very high speed, i.e. at many cycles per second depending on the dynamics involved. The rate of repetition, and therefore accuracy, depends on the time constant affecting the return signal. In applications involving fluid, this is often adjusted by means of a restriction inserted within the loop.

Copyright, (C) 23/02/2010. T. R. Sheffield.

[ShiuludeSVX]

This is a very informative writeup. Thanks for the info.

I do appreciate the effort, and encourage you to add to the database if you see fit.

[oab_au]

I was directed here for “Definitive information” on Trevor’s A solenoid / Dropping resistor Theory, but I find that this “Definitive information” is different to his “Definitive information” that he has posted for the last 6 years.

I won't post all of it, but this is a link to his 2004 posted “Definitive information” statement. http://www.subarusvx.net/forum/showt...t=35942&page=2

The source of the problem is your description of how the A solenoid is driven by the PWM signal from the TCU, you wrote;

This solenoid valve is opened and closed repeatedly, in a rhythmical manner by a control current which is turned on and off by the transmission control unit (TCU) at a very fast rate. The valve is a normally closed device, and remains closed in the event of the loss off a control current. After passing through this modulated solenoid valve, the continually interrupted pressure is in the form of a pulsed flow. When the peaks level off with the troughs, there is a resulting overall steady reduced pressure. The level of this pressure is adjusted by varying the on/off intervals.

and also;

The resulting adjusted output pressure is therefore delivered as a rapidly fluctuating stream.

When I read this in 2004, it was clear that the Author had no idea of how the hydraulic system works in the valve body of an auto transmission. It was obvious that it cannot work like that. The solenoid valve controls a Pilot pressure that is a low regulated pressure, derived from the line pressure. The line pressure cannot be used for any control purposes, as it fluctuates too much from servos and clutch operations, and rpm, so a smoothly regulated pressure is used to control the spool valves that operate the box.

Trevor’s theory has this Pilot pressure chopped into 50cps pulses. So that when a 50% PWM duty cycle powers the Solenoid, it is open for 10millsec, closed for 10millsecs. He then says that, these pressure pulses even out so that it does not matter. This is a completely ridiculous statement to make about a hydraulic system. When the ABS pulses the brakes, you feel every pulse on the pedal, that is the way a hydraulic system works. The pressure acts in all directions equally.

I now see that his new “Definitive information” has filtered out all the “opened and closed repeatedly”, “ the continually interrupted pressure is in the form of a pulsed flow.” And “When the peaks level off with the troughs”,
and that

“The resulting adjusted output pressure is therefore delivered as a rapidly fluctuating stream.

We are now told that it is an undulating stream.

He has thought that, due to his solenoid action the valve on it might get a bit of a battering. So he has devised another theory to accommodate it.

THE DROPPING RESISTOR CIRCUIT It will be immediately apparent that a sudden on off cycle tends to cause what could be called a hammering of the valve seat, even though this is largely checked by the viscosity of controlled fluid flow. The dropping resistor introduces a second series of current pulses applied in parallel with the control signal. These shorter pulses are applied during the off cycles and timed to check the travel of the armature as it reaches the closed position, thus reducing both shock and noise. These secondary parallel signals in effect, “round off” the closing period and reduce the closing shock.

This whole theory is to prevent the “hammering of the valve seat”. He states that the A solenoid is a normally closed valve. I don’t know if he has ever looked at the mechanics of the normally open/normally closed valves, but the normally closed valve does not have a valve seat, it uses a ported sleeve valve, no seat to hammer. Only the normally open valve has a ball and seat. So I see all this 'Dropping resistor theory, as superfluous.


This is the problem; he now has two computer outputs working the one valve. The C solenoid works the same as the A solenoid, works all the time, regardless for the resistance or operating voltage, or if N/O or N/C, and requires none of the dropping resistor, or two computer outputs to control it, that Trevor says the A solenoid needs

So if he now longer supports this, why does he still maintain his Dropping resistor theory that needs two computer outputs to work the valve, when the C. solenoid does the same work in controlling operational spool valves, and does not need a dropping resister, or two computer outputs to control it.

I just find the whole Theory unbelievable.

Harvey.

[Trevor]

I will be pleased to discuss the information I have put forward, with anyone posting intelligently, but not a person who claims that ---

There is a similarity between a 50 cps PWM signal and an ABS brake system.

A normally closed valve does not have a valve seat.

Solenoid “A” --- involves a ported sleeve valve.

Only the normally open valve has a ball and seat.

The C solenoid works the same as the A solenoid,

--- What is more in addition to the above, has personally published this theory. ---

“If we apply a DC current to the solenoid, the magnetic force will pull the armature up to compress the spring and close the valve. If we keep fitting a stronger return spring, we will eventually get to the position, when the magnetic force will not be able to move the armature, and it will remain in the open position. If we fit a spring that is half way between the weak and strong spring, the magnetic force will be able to pull the armature half way up, to compress the spring, and just stay there with the Pilot flow reduced.

So we can vary the armature’s travel, by varying the return spring’s force. The more conventional way is to leave the same spring force, and just vary the magnetic force with the PWM signal. A 5% PW signal will hardly move the armature at all, but as we progressively increase the PWM percentage, more current flows to increase the magnetic force, and the armature will be pulled further along its travel, till the magnetic force equals the spring and Pilot oil pressures, and stay at the point, to smoothly control the pressure’s flow.“


N.B. I have already quoted scientific fact. ---

It is absolutely impossible to electrically hold an unsupported object stationery within a varying environment without some form of feedback, arranged to detect any movement requiring a necessary correcting force to restore equilibrium.

N.B. In respect of solenoid valve "A" the quite miniscule length of travel and port cross section presents, an interesting additional factor.


Four years ago, I proved all of this same stupid nonsense from Harvey absolutely wrong. The thread became a ridiculous fiasco, so that for some light relief I added some humour. For the original Fairy Story click here. ---

http://www.subaru-svx.net/forum/show...t=35942&page=5

The current statement is quite beyond reason and furthermore, is rather sad.

[kwren]

Originally Posted by Trevor

A normally closed valve does not have a valve seat.

(

Trevor, a normally closed valve could have a valve seat, and many do. It depends on the design and purpose of the valve.

You actually own more than one that does, right in your home!!!

Keith

[Trevor]

Originally Posted by kwren

Trevor, a normally closed valve could have a valve seat, and many do. It depends on the design and purpose of the valve.

You actually own more than one that does, right in your home!!!

Keith

Keith,

Please readdress your comments.

As is usual, you have not correctly read the post. It was Harvey who made this muddled and completely stupid statement. ---

"I don’t know if he has ever looked at the mechanics of the normally open/normally closed valves, but the normally closed valve does not have a valve seat, it uses a ported sleeve valve, no seat to hammer."

Whatever, thank you for drawing special attention to this stupidity.

[kwren]

Trevor, your charitable "thank you" was appreciated... though slightly diminished by your comment... "as usual, you have not read the post correctly".

However small, your attempt at kindness was noted!

Keith

[SoCal LS-L]

Originally Posted by kwren

"as usual, you have not read the post correctly".


Keith

You guys are both wrong..... the main ingredient in a blueberry pie IS blueberries.

[kwren]

Originally Posted by SoCal LS-L

You guys are both wrong..... the main ingredient in a blueberry pie IS blueberries.

Makes my mouth water to think about those great blueberry pies!

Wife makes the ultimate pie crust... every time!!

Keith

[Freeman]

In ingredient is normally blueberries.. Unless you buy it at walmart. Then it's something that just looks like blueberries.

[Trevor]

Interesting information has been posted confirming that solenoid valve “C” is audible, due to the difference in applied PWM signal. Solenoid “C” is not provided with the soft closing secondary signal, as is solenoid “A”.

The easiest way to test a sol C. is by my own personal (ok It was taught to me but i'm taking credit) duty C test, and it requires no tools. Park the car over some concrete or some sound reflecting surface in a quiet area. Then, turn the ignition to ON with the shift in P or N. (radio&climate OFF)but do not start the engine. In the quiet area, slide the shifter to any moving position. this action will cause a very distinct buzzing noise that you can hear if you lean your head out the open door near the undercarriage. It should sound like a healthy bee flying (loudly) in the tail shaft of the transmission. This noise should continue as long as the shifter is in a moving range. If there is no noise, if there is a single click, or if there is a bzzzz...zzzz..zzzzz....................zz......... .(nothing) then you've got a bad C solenoid.

Refer here, page, 2. post 17. --- http://www.subaru-svx.net/forum/show...906#post638906

[Trevor]

I have added further detail as follows ---

The use of a normally closed valve “A”, for electronic control of line pressure, renders the system fail safe. However maximum pressure must be limited and not allowed to runaway at high pump speeds. This is achieved by including the pilot valve, which sets the pressure, and the pressure modifier valve, within a feed back loop.

[Trevor]

In view of incorrect information published within the other SVX forum, the following has also been added. ---

The output pressure is therefore delivered in the form of waves at 50 CPS. However, due to the low volume delivered, the high frequency and the reciprocating mass involved in the pressure modifier valve, these waves have little effect. Furthermore, final control pressure from the pressure modifier valve, is smoothed by the pressure modifier accumulator, a device incorporated for this purpose, as is mentioned in section 3 within the Subaru manuals, under the heading Line Pressure Control.

[oab_au]

Originally Posted by Trevor

In view of incorrect information published within the other SVX forum, the following has also been added. ---
.

You forgot to supply the link.
http://www.svxworldforums.com/forum5/2144.html

Harvey.

[Trevor]

Originally Posted by oab_au

You forgot to supply the link.
http://www.svxworldforums.com/forum5/2144.html

Harvey.

To have done so might have incurred the wroth of the powers that be.