Cool plugs

Cool plugs
Marginal Column

November 2014


The plug-in amplifiers with pulse width modulation, built by Rexroth, have many advantages. They considerably reduce the temperature of directional valves, save massive amounts of energy, and are easy to install. That is of benefit to many industries and here a sawmill serves as an example.


Dry wood and sawdust. In contact with hot surfaces, these can become a source of imminent danger. That is why insurance carriers impose stringent requirements on sawmills. They address surfaces in sawmill lines that exceed the critical value of 100 degrees Celsius and require either encapsulating them or regular cleaning. In the interest of safety, thick layers of sawdust may never build up there. This also applies to the surfaces of hydraulic valves. Operators have to expend a great deal of effort for cleaning. Here, Rexroth’s plug-in amplifiers with pulse width modulation provide a remedy. “We developed these units to reduce power needs and, at the same time, could also lower the temperature of the valves. This is of special benefit to users who have to deal with flammable dusts,” reports Frank Ullrich, product manager at Bosch Rexroth in Lohr am Main, Germany.

Enormous temperature drop

Potential sources of heat are the solenoids in the hydraulic valves. 2-way directional seat valves are often used on hydraulic axes. To prevent any movement should the power fail, these are designed so that “closed” is the default position. This means that power has to be applied continuously during the entire operating cycle. These valves are switched on at the start of the sawmill’s shift and draw power the entire time – power they actually do not need. If the sawmill is in operation all day, then the solenoids heat up and the metallic surfaces quickly rise to more than 100 degrees Celsius – at least with conventional equipment. “The excess current is simply converted into thermal energy,” explains Ullrich. Incorporating pulse width modulation prevents this and reduces the temperature of the surface by up to 80 kelvins. Here the developers made use of the special properties of hydraulics. The valves require full power only for the brief switching cycle. Once the valves have been repositioned, considerably less power is needed to keep them open.

Less current and cool plugs

The integral pulse width modulation makes it possible to quickly alternate between two values for current strength. The switching procedure lasts about as long as the blink of an eye. The plug-in amplifier drops the power back from the peak value after 100 milliseconds. This lower value is achieved by switching on and off rapidly, at frequencies of up to 500 hertz. Since the coil at the hydraulic valve responds fairly sluggishly to changes in the incoming power level, the voltage is sufficient to hold the valve in position during the low-power phase; the mean current strength settles in at a lower value. “The challenge in development was to determine the optimum ratio between the off and on phases,” explains Ullrich. Ultimately, a 60:40 ratio proved to be ideal. This means that 60 percent of the time the plug is switched off; power is delivered during the remaining time. The result is that the surface temperature of the hydraulic valve drops to just about 50 degrees Celsius. This eliminates the need for encapsulation to keep flammable dust from collecting. And regular cleaning as required by insurance carriers is also unnecessary. In this way sawmills not only avoid additional insurance stipulations; they also save a great deal of electricity. Energy consumption can fall by up to 78 percent and that represents an enormous potential for savings at the 100 valves normally in use. The plug-in amplifiers amortize their costs after just a single year due to the energy savings. That has convinced Ullrich: “In the future, plug-in amplifiers with pulse width modulation will play a greater role in the search for savings potentials.”

Simple installation

The installation of these money-saving plugs is as easy as could be. They can simply be plugged in, even in existing systems. The cables are available in three different lengths, doing away with any wiring on the valve. Practice has shown that installers can now make the connection at the control cabinet in ten to 15 minutes instead of the 25 minutes required in the past. There is a further advantage from the system manufacturers’ viewpoint. Previously, the PLC outputs for the valves had to be designed to handle two amperes. With the new plugs by Rexroth, 0.5 ampere is sufficient and a module can, thanks to this, control multiple valves. This reduces the hardware costs and saves on valuable installation space.

Taken together, all these advantages make the plug-in amplifier interesting not only for sawmills. “Essentially, we can use this for all hydraulic on/off valves with 24-volt coils. And they are also suitable for machine tools and plastic processing machines,” emphasizes Ullrich.