Above: A large hydraulic cylinder with an integral position feedback transducer mounted in the rear head.
Position sensing cylinders combine the rugged power of hydraulic cylinders with the precision of position sensing electronics in a single actuator. The electronic sensing makes the cylinders "smart". Smart hydraulic cylinders are sometimes also referred to with the following terminology:
Position sensing cylinders include cylinders equipped with the following different methods of sensing:
Position sensing cylinders equipped with position feedback transducers come in two basic styles, Internally and Externally Mounted.
Internally mounted position feedback transducers are usually mounted on the rear cap of a hydraulic cylinder. They usually have a sensing probe that extends from the read head through the piston and into the hollow piston rod. The transducer provides either an anologue or a digital signal as the piston moves through the cylinder. This signal is fed back to a computer that converts the electronic signal to indicate cylinder velocity and position. These feedback systems are called Linear Position Sensors.
One of the most popular of these is manufactured by MTS Sensors and goes by the brand name Temposonics . This system sends an electrical pulse up the cylinder feedback probe. When it reaches the piston it passes through a magnetic ring mounted on the piston. This magnetic ring reacts to the electrical pulse and sends a counter pulse back through the probe. The time for this to happen is measured and gives the electronic controller a clear indication of the distance of the cylinder piston from the rear head. This method is called a magneto-restrictive feedback transducer. Click here to see an Adobe Flash presentation by MTS on Magnetostriction Technology .
Another simpler but less accurate method is to use a linear potentiometer probe. This again is installed in the rear head of the cylinder and passes through the piston and into the hollow piston rod. As the piston moves in the cylinder barrel, a mechanical brush on the piston contacts the linear resistor. This provides a voltage feedback that changes as the piston travels. This method is less expensive and less complicated than the magneto-restrictive sensor, but has a mechanical link that wears out over time. It also has velocity limitations as the brush can surf over the resistor at high speeds.
Externally mounted position feedback sensors come in a wide variety of styles and methods.
One method is a linear potentiometer as described previously except that it is mounted on the outside of cylinder. The unit is fixed to the body of the cylinder with the probe attached to the piston rod or even the cylinder load. The advantages of this method is that it is lower in cost than the internally mounted transducer, it does not add to the overall length of a cylinder, it can be easily repaired or replaced without disassembling the cylinder, and it can be retrofitted to older cylinders in order to modernize control equipment. The disadvantage of this method is that the position sensor is exposed to the elements, is subject to damage from external forces, and it adds to the outer diameter dimension of an actuator.
Another form of external position feedback sensor is an encoder that physically measures the travel of the piston rod as it leaves the cylinder rod gland. One method has a wheel that rolls as the rod moves and counts the rotations of the wheel. Another pulls a cable probe that is measured by an encoder. Yet another method employs an optical eye that counts indicators either embedded in the rod or on something that travels with the rod.
Again these external encoders are subjected to damage, dirt and contamination because they are mounted on the exterior of the hydraulic cylinder. They are, however, easy to repair and replace and are generally less costly than a sensor mounted inside a cylinder.
The hydraulic pressure within a cylinder can sometimes be used as an condition indicator by the electronic controller. For instance, when the cylinder pressure suddenly rises, this is a clear indicator that the cylinder has encountered a load and is working. This information may be used by the machine controller to start another machine function.
When the internal cylinder pressure is low, this may indicate a serious pressure leak, especially if it is combined with a low fluid level signal from the hydraulic fluid reservoir. Thus this combined information can be used to signal an alarm or a machine shutdown.
Pressure switches and pressure transducers can be mounted directly in the front or rear heads of cylinders or in manifold blocks mounted on the cylinder body.
Hydraulic cylinders can be modified to mount electro-mechanical or electronic limit switches. These switches are often installed in the end caps of cylinders to indicate that the piston has reached the end of is travel, either fully stroked out or fully retracted. These switches do not, however, give a progressive indication of the cylinder travel or a mid-stroke position indication. They can be used, though, to give a reset function or "zero position" indicator to a more complex position feedback system.
Above: A hydraulic cylinder with an end of stroke limit sensor. Integral end of stroke limit switches can be activated either mechanically or electronically. The mechanical switch is activated when the piston or cushion boss physically activates a mechanical plunger in the switch mechanism. The electronic switch is activated when it senses the steel mass of the piston or the cushion boss.
Another method that was once very popular used a double rod end cylinder. The second piston rod mounted a fixture that activated externally mounted electro-mechanical switches. This method provided discreet mid-stroke position indications. It was often used to limit the stroke of cylinders when the machine process changed such as the size of the product being manufactured. This method is simple and does not require complex proportional controls or computerization.
Hyco has many years of experience developing Position sensing cylinders for our customers. Which ever of the above varieties and configurations of cylinders you require for your machine application, our experienced engineering team stands ready to assist you.