Monitoring the hydraulic power unit of a hydraulic test bench
The hydraulic power unit is the central component of a hydraulic test bench. Pressure sensors are qualified on an impulse pressure hydraulic test bench by means of an “accelerated life” test.
A failure of the facility would increase the development time because tests could not be carried out as planned. Moreover, delivery of certain types of pressure sensors would be restricted during this time as the tests necessary to ensure series quality could not be carried out. This in turn would have a direct impact on sales in this product group and would compromise product availability.
The starting position
The existing hydraulic power unit is already equipped with sensors (temperature, pressure and level sensors). This information is used to control the system and to stop the machine in the event of limit violations. The power unit features two tanks for hydraulic oil cooling.
Another particularity of the power unit are the two redundant pumps which are identical in construction. In existing systems with two redundant pumps, it is crucial to ensure an equal usage of the pumps in order to increase their total operating hours and to avoid damage to one of the pumps (such as seized bearings) due to excessive idle time.
Goal of the project
Comprehensive condition monitoring of the hydraulic power unit
The aim is to monitor the wear in the hydraulic system and the running time of the pumps in order to optimise them. Limit violations are to be detected early by automated alarm management.
Implementation
The signals of the existing sensors in the system are captured via the Y-path/retrofit. Two variants were installed:
Tank
- Parallel tapping of the 0 - 10 V signal of the two temperature sensors via DP1222
- Tapping of the IO-Link signal of the two level sensors via an E43406 IO-Link data splitter
The other sensors were installed and set up according to their relevant operating instructions.
Pump
- Inductive IO-Link sensor with speed monitor
- IO-Link vibration sensors
Return line
- Temperature cable sensor with bolt-on sensor
- IO-Link temperature plug (evaluation electronics for PT100/PT1000 temperature sensors)
moneo RTM is centrally installed on a server. Using the IO-Link master, the sensor values are sent to moneo RTM for visualisation and evaluation.
Success
Comprehensive condition monitoring of a hydraulic power unit with moneo RTM
Through retrofitting (capturing of sensor values from controller) and additional recording of characteristic values of the hydraulic power unit, all requirements of a comprehensive condition monitoring solution could be met.
- Permanent detection of the system pressure
- Permanent monitoring of the level in the tanks
- Permanent monitoring of the temperatures in the tanks
- Vibration monitoring on the pumps
- Calculation of the pumps’ operating hours
- Monitoring of the return temperature
- Calculation of the temperature difference between return line and tanks
- Wear detection in the hydraulic system
- Optimised pump utilisation
System structure
- Bolt-on sensor + temperature plug on return line
- Vibration sensor pump 2
- Inductive sensor + speed monitor pump 2
- Vibration sensor pump 1
- Inductive sensor + speed monitor pump 1
- Temperature tank 2 + converter 0...10 V
- Level tank 2 + IO-Link splitter
- Temperature tank 1 + converter 0...10 V
- Level tank 1 + IO-Link splitter
- IO-Link master
Dashboard
The dashboard provides the user with an overview of the current sensor values in the installation. The following dashboard provides an overview of the process-relevant parameters.
- Return line temperature
- Temperature tank 1
- Temperature difference (return line - tank 1)
- Level tank 1
- System pressure
- Level tank 2
- Temperature tank 2
- Speed pump 1
- Speed pump 2
Separate dashboards provide more information, such as the current characteristic values of the pump:
- Vibration values pump 1 (v-RMS, a-Peak, a-RMS)
- Vibration values pump 2 (v-RMS, a-Peak, a-RMS)
- Speed pump 1
- Operating hours pump 1
- Temperature pump 1
- Speed pump 2
- Operating hours pump 1
- Temperature pump 2
Analysis
The analysis function allows users to access the stored history data of the sensors. For example, this simplifies troubleshooting. Various sensor values can be viewed and analysed over time.
The following analysis clearly shows that the vibration value (v-RMS) and the hydraulic oil temperature correlate. For example, the vibration value can be used as an early indicator of a limit violation of the hydraulic oil temperature.
- Blue: Return line temperature
- White: Tank temperature
- Green: Vibration value v-RMS
Static thresholds
Static thresholds are set for various process values of the hydraulic power unit. If a process value is exceeded or not reached, a ticket is created in moneo, which can be further processed via ticket processing rules.
- Upper alarm threshold
- Delay time for the alarm threshold
For the hydraulic power unit, the following process values are used for limit value monitoring:
- Level tank 1 and 2
- Temperature tank 1
- Return line temperature
- Temperature pump 1 and 2
- Vibration values pump 1 and 2
Ticket processing rules
This function can be used to easily define what should happen after a warning or alarm has been triggered, e.g.:
- Email notification
- SAP integration
Calculated values
Using calculated values and templates, additional information can be generated from measured values.
To record the operating hours, the operating hours counter template was used. As long as the speed of the pump is greater than 50 rpm, the time counter remains active.
- Name of the calculated value
- Trigger for the operating hours counter
- Threshold value for the trigger
- Current time value
The 0...10 V analogue value of the temperature sensor must be scaled to the corresponding temperature value (0 V = 0°C and 10 V = 100°C).
- Temperature as analogue voltage value from sensor (0...10V)
- Analogue end point (100 °C = 10 V)
- Analogue start point (0 °C = 0 V)
- Voltage range analogue value (10 V)
- Calculation of the delta of end point to start point (AEP – ASP = ∆A)
- Calculation of the factor for current to pascal (∆A / 10 V = factor)
- Multiplication of the current value (0...10 V) with the factor
- Addition of the analogue start point as offset
- Result pressure difference in pascal
The level sensor transmits only the measured value without offset via the IO-Link interface. This must be added to the measured value.
- Tank level as raw value
- Offset to level
- Level with offset
