The programmable logic controller (PLC) can be an intimidating piece of equipment if you only login to interact with it on occasion.

It is not untypical for a maintenance tradesman to be asked to adjust a timer in a machine’s control program. Perhaps a solenoid that closes an unloading valve a few seconds after a hydraulic pump start-up should now be triggered a little later to minimize newly discovered electrical current spikes.

In this edition of Newsletters that Teach we will refresh the basic parameters of the timer (Timer On Delay) function as used in ladder logic programs. Our examples will be drawn from Rockwell Automation's RSLogix 5000 and Schneider Electric's Unity Pro XL.

Mobile Equipment Note: On many mobile machines a PLC is often referred to as an ECM (Electronic Control Module). The manufacturer may have locked out many if not all programming changes in order to protect safety critical functions such as brakes and steering. If program updates are offered, they are usually "flashed" into the controller as a batch of files that cannot be edited.

We will assume basic knowledge of ladder programs with bit level (Bool) instructions that process inputs or energize outputs where the value can only be true or false (1 or 0). You may also wish to refer to our previous editions of Newsletters that Teach; PLC Primer and Function Block Diagrams.

The timer is an instruction (an entire function block really) that lives on the output (right hand) side of a program rung. A timer is typically enabled by a bit level input instruction that becomes true when some logical condition (example, a start switch is pressed) dictates that timing should begin.

Once enabled, a timer with a delay-on function starts a clock that actively times all the way up to a preset value. The preset time value may be in seconds or some other smaller unit of time.

Once the clocked timing is complete, a "done" status is triggered and used as an output to turn on a piece of equipment or trigger another stage of Program logic.

Once done, the timer will remain in that state until the enabling instruction on the left (input) side of the rung is cycled to a false state.

In this edition of Newsletters that Teach we will examine some of the operating principles of what is likely the most popular hydraulic pump design in use in both plant and mobile machines today. In the case of many mobile machines, pumps often have more controls than just a pressure compensator. For those machines, we will look only at the pressure compensation (a.k.a. pressure override or pressure cutoff) feature in this tutorial.

Let us start with a quick look at pumps that do not have any displacement or pressure controls.

A gear pump like the one in use on a simple hydraulic machine displaces the same amount of fluid for every rotation of the pump’s shaft. The gear pump shown also has no pressure controls built in. The hydraulic system must contain a relief valve to limit the maximum system pressure. The system of cylinders and hydraulic motors may not always need the full displacement of the pump. To vary the amount of flow to one part of the circuit with a flow control, the pump’s flow will have to be divided. The system relief sometimes serves as the divider. If the relief valve is cracked open to any degree, then the system is obviously near maximum pressure.

Examine the graph to see that the system overall is at full displacement near the maximum pressure. The combination of these two maximums also means that the power requirement (the diagonal vector on the graph) from the prime mover (diesel engine or electric motor) is at it’s maximum as well. A prime mover at maximum power is consuming maximum energy (fuel or electricity). Much of this energy is being used for nothing other than a conversion to heat over the system’s relief valve.

Now let us look at how a system with a pressure compensated pump saves energy. For this tutorial we assume that the reader has basic knowledge of pumping action of the rotating group (pistons and cylinder barrel) in an axial piston pump. The pump is equipped with a pressure activated valve (compensator) that stays closed under spring pressure when the hydraulic system is below the maximum pressure. The maximum system pressure is set by adjusting the compensator. Clockwise rotation of the adjustment screw typically increases the maximum system pressure.

When the pump’s flow is sufficiently restricted or blocked in the system, causing the pressure to rise to maximum levels, the compensator opens directing high pressure fluid into an internal control piston. As this control piston extends, the pump’s displacement is reduced to near zero (off-stroke).

The pump now only comes on-stroke to displace enough fluid to maintain the maximum system pressure. This reduction in displacement conserves input energy and prevents the build up of excess heat (as caused by flow through a spring loaded relief valve). If the flow control setting remains the same the actuator's load has not changed, the pump will go off-stroke again.

Overheating in a system that features a pressure compensated pump may be due to maladjustment. If the system has run at correct temperatures in the past, it is possible that someone may have adjusted the system relief valve. Most systems with a pressure compensated pump still feature a relief valve to control sudden pressure spikes. This relief is typically set a few hundred PSI higher than the pump’s compensator.

If the relief valve is adjusted down to a level below the pump’s compensator setting (or the compensator is set higher than the relief valve), then the energy saving feature of a pump that can reduce it’s own displacement has been removed from the system. At this point, the pressure compensated pump is offering no design benefits beyond that of a fixed displacement pump.

In future editions of Newsletters that Teach we’ll look more closely at the behaviors and the adjustment of a pressure compensated pump.

Plant Pressure System

In this plant machine example, a cylinder is being used to move a machine component into position (e.g. head of a large filter press). Once in position, the cylinder must then continue to exert a force on the machine to keep it positioned against all counter-forces that may occur, and take up any slight machine changes that require the cylinder to extend a bit further.

System pressure spikes quickly once the cylinder has extended as far as possible. This pressure spike triggers the unloading valve. The flow from high volume pump no. 2 is routed back to tank at low pressure. With the check valve in place, low volume, pressure compensated pump no. 1 continues to supply a very small flow to make up for internal system leakage and hold the needed pressure value in the cylinder.

With pump no. 1 unloaded for a cycle of the main manufacturing machine, electric energy is saved in the driving of an unloaded pump, and needless heating of the hydraulic fluid is avoided.

Front End (Wheel) Loader

A loader will typically have several hydraulic pumps. In order to avoid stalling the diesel engine when an extremely heavy bucket load is being lifted, an unloading valve will respond.

The pressure in the example circuit is determined by how much resistance there is to the two loader boom cylinders as they extend. In other words, the heavier the bucket load, the greater the pressure.

If the pressure rises to the level of the spring setting (e.g. 1500 PSI) on the unloading valve pilot section, then flow will occur through this section causing the main unloading poppet to open. Once this occurs, pump no. 2 will be routed back to the tank at low pressure.

Only pump no. 1 will continue to supply flow to the boom cylinders. The boom cylinders will still continue to extend, but at a slower rate due to the use of the single pump.

The result of this unloading function is that the demand for power from the engine will be reduced.

In some cases the purpose behind this horsepower limiting function is to allow the loader to travel forward while lifting the bucket. In this case, the electronic control module in the loader will sense forward motion and energize the travel unload pilot valve (solenoid ‘a’) automatically. Both travel and boom lift require power for which the engine may not have enough if the bucket is extremely heavy.

In this edition of Newsletters that Teach we will examine the unloading valve and find out how it minimizes energy waste.

Mechanical Engineering Symbol

This symbol indicates a mechanical point of mount or friction. It is used in the schematic below to indicate that the spring is mounted to a fixed point.

The unloading valve is a device that is typically used in a high-low circuit. The circuit is sometimes referred to as a fast approach, slow-feed circuit. The valve is put to good use in a system where a large flow volume is needed at a lower pressure, and then later, a very low flow volume is required with a higher pressure.

Typically, two pumps, plumbed in parallel, are used in such a circuit. One fixed displacement pump provides volume for the lower pressure, high flow mode, while a small pump provides the low flow at the higher pressure. Both pumps provide a flow volume in the low pressure, high-flow mode. The purpose of using the two pumps is to save energy draw at the prime movers during the long periods when the only function of the hydraulic system is to maintain an even maximum pressure with very little or no flow.

When the system cylinders are extending quickly at low pressure (any pressure below the setting of the unloading valve), both pumps send their full volume into the cylinder.

When the cylinders encounter enough mechanical resistance to bring the system pressure up to the setting of the unloading valve, the high flow volume pump is directed to tank through the piloted unloading valve. Unlike a relief valve, the restriction of the operated unloading valve on the flow passing through it is minimal. Therefore, the unloading valve does not pose a heating problem. The volume of the smaller pump cannot escape to tank through the unloading valve due the check valve in the system. The presence of the check valve makes the schematic symbol for the unloading valve distinct from a typical relief valve.

As the cylinder continues to move against resistance (system pressure) that is higher than the unloading valve setting, the motion is slower as only the volume from the small pump is available. In many cases the cylinders come to a halt as the machine is actively clamping an assembly or machine apparatus of some sort. At this point, the main relief valve will open and limit the maximum system (or clamping) pressure. If a relief valve is not used to limit maximum system pressure, then the small pump will be of the variable displacement/pressure compensated variety.

In many systems, the large pump is referred to as the volume pump. The small pump is often referred to as the pressure pump since it is often just used to support a system pressurizing function when the cylinders are used in a clamp mode. The small pump will only supply enough volume to the cylinders to make up for any internal leakages in the system.

In The Unloading Valve - Part II, we'll look at a plant application and a mobile machine circuit where unloading valves are utilized.

Jack cylinders are common to most large diameter press rams. They are used for the initial low pressure stage and are often critical for retracting the ram. During the main pressing stage, the jack cylinders will likely be used to contribute additional force. If the press ram is oriented vertically it may also have a prefill valve. This is especially true if the ram has a free fall stage. During free fall the prefill valve allows the natural partial vacuum that occurs as the ram drops to draw in the initial volume of fluid.

Jack cylinders (circled in green) are used to retract the large ram.

During free fall, a partial vacuum is created because of the huge volume of the ram cylinder. The prefill valve (magnified at the right of the image) allows fluid into the blind end of the cylinder to reduce the partial vacuum that is being created. If fluid were not allowed into the cylinder, the vacuum created would overpower the combined forces of gravity and the jack cylinders, and the ram would not drop.

While the jack cylinders are extending the ram, or while the ram is dropping under the force of gravity, fluid must be allowed to flow into the ram on the blind end port. A simple check valve arrangement is typical here to allow the ram to draw in fluid directly from the reservoir. These large diameter check valves are specifically made for a press prefill function and will also feature a pilot port to force the valve open later during the return stroke stage. In our example the prefill valve also has a built-in decompression poppet.

On some press rams that move very slowly in a horizontal orientation, the large and specialized prefill valve is replaced by a smaller, basic hydraulic directional valve. Decompression, however, must still be handled very carefully through a control valve of some sort at the end of ram stroke.

A basic directional valve handles the cylinder prefill for a slow horizontal press application.

In our main vertical press example, the ram moves through free fall first. Then, as pressure builds in the jack rams, the sequence valve opens and the pump's flow is directed into the main ram. The prefill valve, which is essentially a check valve, will now be closed.

When the press cycle is complete and the directional valve is reversed, the ram is not immediately set into a retract motion. The ram cannot retract even with the jack cylinders powered into their retract mode. The reason for this delay in retract motion is the fact that the prefill valve is not open. If the prefill valve could be opened at full pressure, the explosive force of the compressed fluid would cause serious damage. Decompression of the main ram must occur before opening a large flow valve. In our example, a small decompression poppet is slowly opened via a small flow control orifice. The ram pressure is bled down safely through the small poppet. Only when the ram is sufficiently depressurized does the pilot piston develop enough force to open the large diameter prefill valve. At this point main ram retraction can occur.

The decompression poppet opens...

...and the jack cylinders can retract the ram safely after decompression is complete.

Going back to our horizontal press schematic we see that the main ram is extended and retracted by a standard solenoid controlled, pilot operated valve. The decompression function is handled by a separate solenoid operated valve with a flow control acting as a bleed down control orifice. Can you imagine what negative effects will occur if that bleed down orifice is maladjusted to an opening that is too large? The circuit also requires that the press sequence controller (a PLC of some sort) continue to read pressures correctly from the pressure transducer, and to operate valve solenoids in the correct order.

In our slow horizontal press application, decompression is occurring via the valve circled in green.

Fluid Science Feature

The bulk modulus (K) of a substance measures the substance's resistance to uniform compression. (source: Wikipedia, Bulk Modulus)

Of course in a hydraulic press we're actually dealing with inverse bulk modulus and the fact that the fluid can be compressed as opposed to stretched.

]]> http://www.carldyke.com/compression-ii/feed 0 http://www.carldyke.com/compression http://www.carldyke.com/compression#comments Tue, 12 Jan 2010 18:39:09 +0000 Carl Dyke http://www.carldyke.com/?p=1768 Can hydraulic fluid be compressed? In this edition of Newsletters that Teach, we're going to have a look at unique issues around large volumes of fluid under pressure.

Can hydraulic fluid be compressed? In this edition of Newsletters that Teach, we're going to have a look at unique issues around large volumes of fluid under pressure. Hydraulic circuits are often used in applications with a press function. These circuits typically feature a large hydraulic ram that compacts or reshapes material in some way, or a number of large cylinders in parallel that provide a clamping function.

Some examples include:

• Pulp mill bale or slab press
• Plywood press
• Forage feed compactor
• Residue filter press
• Aluminum profile extrusion press
• Cardboard baler
• Garbage compactor

Decompression

At the end of a press cycle, with the pressed material pushing back against the ram, the fluid in the ram is still under great pressure. Correctly controlling the rate of decompression before retracting the ram is a serious issue that is common to most hydraulic presses. If you have always heard and believed that hydraulic oil is not compressible, you’ll have to open your mind to some new teaching. It's true that hydraulic oil is not very compressible when we compare it to a gas such as air (used in factory air compressors), but that's not the end of the story.

Without compression, the available 400 gal volume is filled with 400 gal of hydraulic fluid.

Most petroleum-based hydraulic oils compress by approximately ½ of one percent for every 1000 PSI of hydraulic pressure. This means that at 1000 PSI you can fit 502.5 gallons into a 500 gallon ram. At 5000 PSI you can fit 512.5 gallons into that same 500 gallon ram.

Under compression, 512.5 gal have been crammed into the 500 gal space.

A typical industrial press function such as an aluminum extrusion press or a pulp bale press features a very large cylinder (or ram) that may have several gallons (perhaps 10 litres) or more of fully compressed fluid at maximum pressure when fully extended. This is an enormous amount of potential energy.

The trapped and pressurized (5000 PSI) 512.5 gallon volume of fluid inside a 500 gallon press ram will expand very rapidly when a valve port opens to expose the fluid to atmospheric pressure.

The compression of the hydraulic fluid was carried out in a gradual and controlled fashion as the ram was moved against the load (the pressing function). The release of that compressed fluid must also be very carefully controlled. The decompression is usually handled by opening a very small orifice valve. Once the pressure has dropped to atmospheric level, a larger valve is opened for a quick and efficient retract cycle.

Decompression is initiated by opening the decompression poppet, allowing a controlled release of the extra hydraulic fluid.

If the fluid is released at an uncontrolled rate, as would happen through a large valve port that has immediately moved to its maximum position, the fluid will move at a very high velocity. This sudden burst of energy against the walls of hoses and tubing that are essentially undersized during this moment can cause a pressure build-up that may rupture the line. A flexible return hose may bulge diametrically during a sudden ram decompression, and in doing so may contract in length and break free from a fitting, or tear out a portion of the reservoir wall surrounding the return line connection.

Smaller diameter press cylinders such as those used in grocery store garbage compactors may not feature a decompression control. These cylinders with their smaller volumes and lower pressures may only have a small amount of compressed fluid inside at the end of the stroke. The inexpensive hydraulic components typically only include a bang-bang style of directional valve that fully releases the fluid from the cylinder’s blind end port at the end of stroke. The effects of uncontrolled decompression, however, are still often heard and felt as the shock wave is amplified by the machine’s entire metal enclosure. Even these small volume shocks can have a destructive effect over time.

In the next edition of Newsletters That Teach, we'll look at some of the specific valving at work for decompression control on vertical and horizontal press machines.

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Function block diagrams (FBDs) in Programmable Logic Controllers (PLCs) are an alternative to ladder logic programming. When they are well designed, a FBD may be more readable than ladder logic, especially when dealing with large and complex programs. Using a FBD may also allow you to skip intermediate coils and registers that could be necessary work-arounds to ladder’s limited format.

Function block programming is not inherently better or worse than using ladder logic; a lot depends on your own preference and experience. However, both styles of programming are in use in most modern PLC environments, often within the same company, so a basic understanding of both is valuable.

Don't be afraid!

The ladder rungs that you knew so well from the days of relay programming have not disappeared; FBD is just a new graphical interpretation of the same old logic. Many PLC programming environments let you use ladder and function block routines within the same controller project. The two different types of routines can communicate and share data with each other without issue.

A few FBD basics

• Function Blocks are always read from left to right.
• Inputs are always on the left side of the block, and outputs are on the right.
• Unfortunately, a comprehensive visual standard has not yet been adopted, so most programs have some unique elements. It pays to review the basics of the program you are working with to avoid confusion.

Let’s look at a simple start/stop diagram with seal-in functionality.

Believe it or not, these two diagrams create the exact same routine. When the start button is pressed (TRUE), AND the stop button is NOT pressed (FALSE), System_Run is energized (TRUE). The OR functionality creates a seal-in, and System_Run remains energized (or TRUE) until the stop button is pressed (System_Run = FALSE).

Lets have a look at the components of a FBD, and how they translate from Ladder format.

Tags (Variables)

It is a good idea to make your tag names as informative as possible, to save yourself and others time if you need to review that routine down the road. In our example, this tag is named Start_SW_B. It's not much of a stretch to realize that this refers to the state of the Start Switch for routine B. You may already have your own vocabulary and abbreviations for naming tags; remember that remaining consistent will save trouble.

In our example, red = FALSE, so the red tag name indicates that the Start Switch is not powered at the moment.

Boolean (BOOL)
(A.K.A Bit Level Instructions)

If you’ve ever written a true or false test, you are already familiar with booleans. Boolean operations mean that there can only be two states, which could be called true/false, on/off, open/closed etc. For example, a classic light switch that you can only flick on or off has just two states. But if you were to swap that lightswitch with a dimmer, it is no longer an example of boolean operation.

BOOL across the top of these Function Blocks reminds you that there is no "halfway" state; the operation must be true, or false.

Function Block

A function block simply takes input and provides output as determined by the operation specified at the top of the block. The input(s) are always what is evaluated by the function, and the output always depends on that evaluation.

In our example, we have an OR_BOOL. Think of it as saying," If IN1 (Input 1) OR IN2 is active, make OUT (Output) active." And that's all there is to this very basic function block.

Some simple boolean functions include:

OR

• If any or all inputs are true, the output will be true. Only when all inputs are false does the output become false. This is an "inclusive OR."

XOR

• This is an "exclusive OR". It follows the same rules as the regular (inclusive) OR, except that the output will be false if more than one input is true.

AND

• AND requires all inputs to be true in order for the output to be true.

NOT

• NOT simply flips the input; true becomes false and false becomes true. Like a normally closed ladder instruction, NOT only accepts 1 input and has only one output.

NAND

• NAND is like combining an AND function with NOT, or a pin inversion. If your inputs are all true, then your output would be true, except that the NOT part of the function inverts the output. So, all true inputs = false output. Mixed true and false input = true output. And all false input = true output.

In Plain English, Please

Now that we have a basic understanding of some boolean function blocks, lets talk through the diagram that we saw earlier.

Pre-existing knowledge: This program uses RED to indicate FALSE, and GREEN to indicate TRUE.

And the same diagram, at the moment you press the Start button (not shown) to turn Start_SW_B true.

The arrow points out that that System_Run_B is the same variable as is used in the OR condition check. This variable has now been turned true (green) by pressing the start switch; it will be true (green) in both locations from the moment the start switch is pressed.

So after the start button has been pressed and then released (above), the seal-in condition looks like this.

The seal-in condition is broken when the stop button is pressed.

Once again, it's important to remember that the output variable that we have just turned false is the same variable that we use in the OR function at the beginning of the routine. This breaks the seal-in condition, and we return to the original state of the circuit.

Don't forget that you can login and interact with the dynamic simulations at www.carldyke.com/sample. Username and password are included in each emailed newsletter, so if you're not already a subscriber, click here to sign up for free!

In this edition of Newsletters that Teach, we will finish up the series on parallel flow paths by looking at load sense circuits.

The basic assumption of a load sense design is that the system pressure, (measured at the pump outlet before any restriction or actuator load,) should never be more than a few hundred PSI higher than the heaviest parallel load. This saves input energy while still providing enough hydraulic potential energy (system pressure) to meet system needs.

Pressure Compensated Flow Control

But first let's back up a little. A pressure compensated flow control is a basic device that features a needle valve for restriction. However, the rate of flow through any fixed amount of orifice opening will change if the pressure changes on one side of that orifice or the other. What changes the pressure?

If you increase the load on the cylinder, then the load pressure will rise on the downstream side of the needle valve (orifice). On the other hand if the pump is shared with other applications where pressure is rising and falling, then the pressure is changing on the upstream side of the needle valve. In either case, as the difference (ΔP) in pressure changes from the upstream to downstream side across the needle valve, the flow rate will change.

If you add a pressure compensator to the flow control with the bias spring chamber plumbed to the cylinder load, you now have a flow control that adjusts automatically for pressure fluctuations. The compensator spool will open up when either supply pressure drops or when cylinder load pressure increases. Conversely it will close down when either supply pressure increases or when cylinder load pressure decreases. In this way the pressure differential across the needle valve remains steady (determined by the strength of the spring) which ensures a consistent flow rate (steady cylinder stroke speed). The pressure compensator spool becomes an automatic variable restriction in series with the main needle valve.

If you can grasp the basic functioning of this flow control, (login and try out the interactive simulation model!) then you're ready to think about a load sense valve bank with parallel functions.

Load Sensing Hydraulic Circuits

The operator of an excavator may wish to move both the boom and the stick cylinders simultaneously. If the two functions represent different load pressures, the maximum system pressure will be set slightly higher than the greater of the two loads. This adjustment is done automatically with a sensing line from the valve bank that instructs the pump to increase its displacement slightly.

With this adjustment made we would expect that the lighter load would speed up. This would be undesirable. If the operator has opened the proportional, directional valve to move a load at a certain speed, it is then hard to operate the machine well if that load speeds up when another parallel function causes an increase in system pressure. We are now ready to answer for the mystery components from the previous two lessons. If you guessed that the symbols represent a pressure compensator, then you are correct.

Just like the pressure compensated flow control we spoke of earlier, each section of the valve bank has its own pressure compensator. As the overall system pressure automatically increases in a load sense system to support the heaviest load, the pressure compensator on each section of the valve bank with a lighter load pinches down a little to keep that section from speeding up.

Troubleshooting Note

The pressure compensator in most valve bank designs is a small spool that is located below the main proportional/directional spool. If the pressure compensator jams due to contaminants in the system, then the lighter loads will speed up when another parallel application takes on a heavier load during multi-functioning (more than one parallel valve is open for flow at one time).

Feel free to login and try out the interactive models that accompany this lesson. In the next edition of Newsletters that Teach we'll head back into the plant environment.