These days, words like digitalisation and Industry 4.0 are buzzing everywhere. On closer inspection these terms are interpreted vaguely and it quickly becomes clear that there will be many different interpretations of these terms. One of the ways is networking production facilities to ensure trouble-free production is guaranteed for a specific period or a desired quantity before starting production.
Since both hydraulics and lubricating systems are relevant in many production facilities, this article is intended to provide an example of what basic information about liquid level, temperature and filter monitoring look like or may look like in the future.
A look at the common practice to date provides a complex picture:
Both with respect to liquid level and temperature monitoring you can find anything from sight glass and the most primitive bimetallic thermometers to bidirectional continuous communication lines via IO-Link. In filtration simply alerting to the depleted element prevails.
Trying to take a static look at the equipment level with respect to liquid level, for example, shows: Dry-run protection for pump(s) is still the most popular monitoring function. This typical uses an early warning, so two switching contacts. Although the number of continuous monitoring sensors is steadily increasing, it’s still estimated to be in the bottom quarter. In temperature monitoring, on the other hand, there is a decline in simple limit switches in favour of continuous signals.
This clearly shows temperature monitoring is clearly prioritised over monitoring liquid levels, which is no surprise in temperature-sensitive design elements such as hydraulics and particularly lubricant.
Looking further at system equipment, there is a growing percentage of combination devices, but still a large variety of plug versions and parallel wiring.
Even looking at the relevant standards such as DIN EN ISO 4413 doesn’t provide much insight in which path will probably be the most widely accepted.
Lacking this information the following considerations should be put up for discussion:
Going with the slogans which was particularly popular in drive technology at the most recent Hannover Messe trade show, according to which future components will all need to communicate, be connected, etc., liquid level and temperature can consequently only be monitored with continuous signals. In and of itself this is not exactly news, considering all oil-bearing systems have always carried a risk of damage caused by leaks, hence selective monitoring has never been adequate for the risk.
Based on the newly arising background of production networked according to Industry 4.0, however, this risk takes second place and information on the availability of a hydraulic/lubrication system becomes the focus. A networked production can only work if all functional data provided indicate the target production batch can be achieved within the desired amount of time. The pump protection which is currently still being focused on – including under ISO 4413 – is no longer the primary and sole monitoring objective but will rather become a by-product of continuously signalling the level curve.
Intelligent correlation of the level signal not only shows the system is ready for operation, but already detects early deviations and in the sense of predictive availability analysis (predictive service) interpret and if necessary resolve these prior to beginning the production batch.
As a side-effect, in larger production facilities static analysis of the level curves can be used to generate or detect consumption balances and/or wear hotspots, thus meet potential requirements for greater environmental safety.
Apart from continuously signalling there is oil, continuously reporting its current operating temperature is just as mandatory. As a result of the oil’s viscosity changing based on the temperature and the associated relevance to the individual process, this information is vital. However, it provides additional data essential to the availability of the production system.
For example, a subtle temperature change can indicate the cooling system is no longer working efficiently. This can be just as much due to the matrix of the oil/air cooler becoming dirtier, as the coolant starting to become low or inadequate heat exchanger capacity. A relatively rapid temperature increase, in turn, could indicate acute wear. Thorough documentation of the temperature trend can also be used as a parameter to assess the remaining life of the oil.
Unlike the liquid level, where visualising the level normally is not directly relevant to safety, showing the current temperature on site can aid in protecting the operators/maintenance personnel.
Let’s move away from generating information and look at the connectivity of sensors. This is where the benefits of modern communication technology becomes particularly evident. Transmitting the real-time, continuous liquid level information requires less installation hardware than traditional switching signals. This means smaller plug connections, cables with fewer wires, and bottom line less costs. Admittedly, generating continuous signals is slightly more expensive than simple binary switching elements, however the differences in investment are marginal, and looking at the total expense required to providing the information at the connection may even be positive.
This may actually be another core in the attempt to develop a sensible path to the future:
With respect to the desired connectivity, isn’t it important to primarily look at the total costs of the required information, so the signal at the connection, instead of, as is still quite common, the purchase prices of the individual components? Heretically speaking, one could devise that although the purchase may have been a bargain, a lot of money was thrown out on cables and installation. Just as people came to realize a long time ago that mechanics and electronics are an inextricable pair, and this has also been implemented in training, the focus must now be shifted to the benefit generated by this mechatronic component.
Another rationalisation potential which is often overlooked is using installation units with high functional density. In fluid technology this starts with simply combining liquid level and temperature monitoring in one device and comes to an interesting head when combining all key basic functions related to aggregate equipment. The worldwide competitive pressure not only forces us to find innovative functional or product solutions. Far more costs are particularly also hidden in international trade, the so-called supply chain. Even a simple level/temperature sensor combination will cut out one purchase, one incoming goods process, one invoice to audit, one supply process. In addition, it requires one less threaded hole, one less installation, one less cable connection, and one less cable. Taking an honest look at these economic factors a lot more potential can be reaped.
However, there is one more very crucial factor which speaks for using only continuous signals:
Taking a look at historical data, as mentioned earlier, particularly also reflects the infinite individuality of the engineers. One could easily derive from the static distribution of switching points to protect the pump that every oil tank is manufactured to very specific custom dimensions and the switching points would therefore never be the same. But is this in fact so? Of course not. For competitive reasons alone, OEMs are forced to rationalise and standardise internally, and innovative suppliers have already incorporated the requirements of Industry 4.0. But why isn’t the potential to standardise continuous signals to also more widely standardise the liquid level and temperature sensors, at least within the company? After all, with continuous signals it truly does not matter how far in the tank the liquid level sensor is! The relevant signals can be tailored to the final system requirements in the simplest manner. Umpteen versions per company can be condensed into maybe three or four, which in turn would simplify the entire supply chain. With the appropriate setup this could even be reasonably implemented on multifunctional devices and contribute to optimising the costs of signals compatible with Industry 4.0.
So how does this work with filter monitoring?
First a few words about the importance of filtration in modern hydraulic and lubrication systems:
The key function of the filtration, very much in the spirit of Industry 4.0, is to guarantee system availability. This is accomplished by the working filters ensuring the purity class according to ISO 4406 determined by the design. This requires the correct size filters and filter elements with appropriate retention rates. This is clearly defined in DIN EN ISO 4413, however it only requires the service indicator is clearly visible. Therefore the ‘Filter Full’ signal is primarily used, typically as a visual indicator and commonly combined with a simple switching signal. As stated below, this is already outdated now and entirely unsuited to determine availability.
Some basic particulars of filter monitoring:
Pressure changes are detected by the filter element to generate an electric signal. This is caused by particles accumulating on/in the filter element, which slowly reduces the free cross-section and increases the pressure lost through the element. A piston or membrane converts this increase in pressure into a change in displacement, which in turn triggers an electric switching signal at a specific value. This is where the viscosity of the oil based on the temperature comes into play. The detected pressure loss can have two causes, cold, viscous oil or accumulated particles. In addition to this basic problem, when analysing the signal there also is the trend of the pressure increase, which follows a more or less drawn out exponential curve. So over long periods a fresh filter element will show signal changes which are hardly detectable.
The filtration, however, is also a n asset in condition monitoring by responding to acute wear with a shorter filter element life. Therefore in the course of determining availability, continuous filter monitoring is displayed despite the stated shortcomings. However, this should be in a way so signals are only actively processed if the oil is at the desired operating temperature to eliminate viscosity factors as best possible. At the same time the life of the filter element(s) should be logged externally. By comparing the life until the pressure loss curve shows a visible rise and any temperature events already mentioned above, any faults to be expected can be predictive in assessing availability.
The inherent necessities of Industry 4.0 for extensive process networking also requires equipment compatible with digitalisation in fluid technology. This start with tasks as trivial as monitoring the liquid level. After all, if the oil level in the tank is insufficient, the test chain in the availability assessment cannot be ‘thoroughly checked’. The focus shifts to the constantly available information – not the alarm in the event of a fault.
Digitalisation further opens a comprehensive approach to the information required to calculate availability. And the sensor generating the information and transmitting it to the point of use are equal and as a whole constitute the cost of the required information.
Generating the information in installation units with a high functional density holds further, substantial rationalisation potential with respect to hardware.
We should further point out that providing continuous information is the best prerequisite for implementing comprehensive maintenance systems. Smart maintenance under the premise of predictive service thrives on the constant availability of ‘big data’.