Closed-loop control of machinery is key to improving productivity and decreasing life cycle costs. In hydraulic applications where exerting a precise pressure is required, automated monitoring of hydraulic pressure is a necessity. Pressure relief valves can play a role in preventing unsafe conditions by ensuring that hydraulic pressure doesn’t rise above a nominal value. However, they make poor pressure or force controllers. A relief valve only limits pressure or force, and on only one side of the piston. Using pressure transducers, on the other hand, enables precise control of pressure or force being applied.
Common applications like presses and injection molding machines have successfully used pressure transducers for many years, but many new fields of application are also coming to fruition. For example, materials and production testing systems are being transformed as awareness and knowledge about how to apply pressure transducers to non-destructive testing has grown.
Beginning with basics
Pressure transducers are commonly applied in pairs for force control applications. Separate sensors are used to monitor the pressure on each side of the piston (see Figure 1). This allows one to calculate the difference in force by using the formula:
Force = PPE APE – POE AOE
PPE is pressure on the powered end of the piston,
APE is the area of the powered end of the piston,
POE is pressure on opposing end of the piston, and
AOE is the area of the opposing end of the piston.
This calculation provides the true net force except for seal friction.
Pressure transducers need to respond quickly to pressure changes. In particular, applications such as presses, where force is applied quickly, can produce extremely fast changes in fluid pressure. Pressure transducers will normally have a rating for how quickly they respond to pressure changes, and these specifications are usually given as a rise time or time to change by a certain percent of full scale. Good controllers can read pressures every one msec, so if the pressure transducer response is slow, the speed of the controller is not being used to full advantage.
The difference in response time ratings sometimes makes comparing specifications a challenge. Transducers that have a rating measured in time constants should know that after three time constants the transducers will be within 5% of the actual pressure after a step change. If the time response is specified as the time it takes for the response to go from 5% to 95% of full scale this is roughly the same. In both cases, the response should be within about 5% after 1 msec if using a 1-msec controller.
The two most common types of feedback from a pressure transducer are voltage and current. Voltage feedback is normally in the range of 0-5 V or 0-10 V, whereas the current feedback is normally 4-20 mA. There are often other voltage and current ranges available. Pressure transducers with voltage feedback are used mainly for laboratory environments were electrical noise and long wire runs are not factors. In industrial environments, the preferred pressure transducer output is a variable current of 4-20 mA. A pressure transducer signal with current output is less likely to pick up electrical noise than a transducer that generates a variable voltage.
Another big advantage of using transducers with current output is in troubleshooting. There is a standard way to determine if the transducer is working properly. If a voltage transducer returns a value of 0 V, it may be a true zero, but it could also mean that a wire has been cut or there is no power to the transducer. In contrast, a 4-20 mA transducer is configured so that a 4 mA output represents a pressure of zero and a 20 mA output represents full rated pressure (which could be 3000 or 5000 psi depending on the transducer selected). If the analog input on the controller sees 0 mA coming from one of these pressure transducers, it knows that either a wire has been cut or the power is off.
Connecting the motion controller to analog pressure sensors requires analog to digital signal conversion. To make the interfacing task as easy as possible, machine builders should seek out a hydraulic motion controller that has the A-D converter built in. With such a controller, the outputs from the transducer can be connected directly to the controller’s analog inputs.
Location, location, location
The locations where pressure transducers are mounted in a system are critical factors for satisfactory operation. It is best to mount the pressure transducers as close as possible to the points of interest on the cylinder, as shown in Figure 1. Here’s why:
|Figure 1. Precise pressure or force control can be performed using an electronic motion controller and pressure transducers mounted on both sides of the piston. |
First, the flow is less turbulent in the larger areas of the cylinder than in the narrow ports of a manifold. Turbulence will result in ‘noisy’ readings and possibly lower pressure readings. To understand this, think about the example of spreading fertilizer on your lawn with a hose and having a venturi system that sucks the fertilizer from a container below the nozzle. The fertilizer is pulled up into the nozzle because of the low pressure above caused by the water flow across the venturi. A pressure transducer mounted in the high flow area would measure a lower pressure in a similar manner. Bernoulli’s equation shows this. However, this may not be of much concern if you are only interested in the pressure when there is no flow.
Second, the distance between the point of interest and the pressure transducer causes delays. Pressure waves travel at the speed of sound in oil — about 4.5 ft/msec. A pressure transducer that is 45 ft away from the point of interest will have a 10-msec delay before it observes a change in hydraulic pressure at the critical location. There is no point in buying a pressure transducer with a 200 μsec response, only to mount it far away from the point of interest.
|Figure 2. Boards of differing thicknesses passing under press rolls can cause hydraulic pressure spikes. |
And third, as distance increases, high-frequency fluid pressure changes tend to become attenuated, so the severity of a pressure spike that occurs at a distance away from the pressure transducer may be go undetected. Pressure transducers often are mounted with an orifice between the point of interest and the transducer. This may protect the transducer in a system that is prone to pressure spikes, but it reduces the fidelity of the control response. It is best to fix the cause of pressure spikes rather than work around them.
Protecting pressure transducers
Pressure transducers can fail when subjected to extreme pressure spikes. Therefore, they should be selected to handle pressure spikes greater than what would be anticipated to occur in the application. But pressure spikes have a way of exceeding the expected. A good way of protecting transducers is to use a small accumulator between the control valve and the cylinder’s piston. However, this is a poor design practice because the accumulator slows down the response of the control system and, therefore, interferes with the force control. But precharging the accumulator to a pressure above the system pressure will prevent it from absorbing oil during normal motion or pressure control, so no degradation in performance will occur. If a pressure spike occurs, it will be absorbed by the accumulator. The accumulator does not need to be very big. Usually, only a few cubic inches of oil need to be absorbed. There are in-line devices for absorbing shock that are really just small bladder accumulators.
Spikes can occur even when a system is working perfectly. For instance, a planer in a sawmill has a press roll that uses a hydraulic cylinder to apply pressure to hold boards down while they are being planed, Figure 2. The press roll normally rolls over each board in turn, but the boards are often not all the same thickness. If the top surface of one board is 18 in. higher than the previous one, then the roll will be pushed up 18 in. This can cause the pressure to spike quickly because the valve cannot respond instantaneously to the movement of the roll. However, a small accumulator would only need to absorb 0.4 in.3 of oil for a 2-in. bore cylinder. This is a small amount of oil, but it can reliably protect the pressure transducer from damage.
By properly selecting, mounting, and protecting a pressure transducer, a designer can make a big difference in the reliability and performance of many hydraulic control applications requiring fast responses to pressure changes.
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