Embedded sensor technology – pressure, speed, position and current measurement directly on the drive
Pressure and weight measurement, electronic load limitation
The highly dynamic movement of heavy loads is the domain of hydraulic drive technology. Loads must be lifted safely, transported efficiently and positioned precisely within a very short time. Examples can be found in moving work machines, e.g. wheel loaders, industrial trucks and municipal vehicles, but also in industrial applications.
Weight determination in vehicles using suitable structures and strain gauges is already state of the art. Hydraulic systems also offer the possibility of determining loads using pressure measurements.
While under purely static conditions it is not difficult to infer the current force applied from the current cylinder pressure via the cylinder surface with the aid of a pressure sensor, this can be considerably more complex under dynamic conditions, where the movement process can be completed within a few seconds. In addition to the pure task of lifting and transporting loads, safety and comfort functions are increasingly being added today. One example is the safe limitation of the maximum mass to be lifted or the determination of the cumulative lifting weight for other purposes. In addition, numerous C-standards in particular require the consideration of functional safety aspects, e.g. in accordance with ISO13849.
Electronic load limitation is illustrated here using the example of a lift system of a refuse collection vehicle for lifting and swiveling refuse garbage cans.
For the theoretical description of the dynamic load determination, the structure of the hydraulic system is required first. In the case of very dynamic lifting processes, it must be taken into account that both hydraulically and mechanically it is a system capable of oscillating.
The task is to estimate the lifted load from the measurement data within the shortest possible time. The pressure values over time are available as measurement data. In addition, the cylinder position can also be evaluated.
The dynamic system behavior is shown opposite. After the waste garbage can is pressed against the lifter, the measurement process is started and the pressure increases rapidly. The oscillations during the lifting process can be clearly seen, but the volume flow control manages to carry out the movement in a controlled and uniform manner. After approx. 1.4 seconds, the lifting process is completed and the cylinder is moved to its end position. The pressure increases abruptly until the DBV opens.
Integration of the pressure transducer
The approach is to integrate pressure transducers directly into the hydraulic control block. Either thin film cells are used, as in this example. Alternatively, thick-film cells can be used or the measuring diaphragm can be placed directly in the control block and evaluated with a strain gauge.
Since the transducers with their full bridges only operate with low voltages in the mV range, the evaluation electronics must be mounted in the immediate vicinity of the cell.
The use of a pressure transducer with evaluation electronics instead of a complete pressure sensor makes sense if several pressure measuring points in a hydraulic block are to be evaluated and if electronics are required anyway to take over further control and measuring tasks.
Possible evaluation approaches
A wide variety of mathematical approaches can be used to infer the weight from the dynamic pressure measurement during the lifting process.
Artificial Neural Network (KNN) approach
In this approach, an artificial neural network, e.g. a so-called feed forward multilayer perceptron, is used to infer the weight from the measured temporal pressure signals and other signals. KNNs provide good results, but they do not meet functional safety requirements.
Another approach used to determine the lifted load is an energy approach. This is based on the fact that there must be a balance between, on the one hand, the hydraulic energy put into the lifting process and, on the other hand, the change in geodetic height and the dynamic situation of the ton. Frictions are neglected in this approach at this point. This approach also provides good results.
Embedded implementation of load determination taking into account functional safety in accordance with ISO 13849
A first important step is the identification of the required safety function. In the system presented, the safety function is “Safe and reliable detection of and safe stop on ton overload”. In other words, it must be ensured that the barrel overload is detected on the basis of the pressure and that this information then leads to a safe stop within a sufficient reaction time in the event of a barrel overload and the further lifting process is thus prevented. The safe reaction time is determined by the maximum safe lifting height at which no serious injuries are to be expected. The safe lifting height was set at 30 cm. This lifting height is reached within 500ms, so the reaction time must be less than this.
Required electronics hardware structure and monitoring measures
Depending on the required performance level PLr and the diagnostic coverage DC, the required hardware structure results. Within the scope of the project presented here, the category 3 controller shown on the right was built with a redundant hardware structure for electronic load detection. In this structure, two microcontrollers monitor each other, and communication between them takes place via a bus. In addition to the mutual monitoring of the microcontrollers, hardware and software measures have been implemented to monitor the function of the power supply and the inputs and outputs and to detect malfunctions. Furthermore, there is a second alternative shutdown path and an emergency stop function that has been implemented.
Black channel communication
In addition to the internal error detection and handling on the controller board, a safe fieldbus transmission to the higher-level controller is required. For this communication, the CAN bus was chosen, which is not secure to begin with. Data loss, data corruption, etc. cannot be reliably detected. Therefore, a so-called black channel must be inserted, a layer through which it can be detected when errors occur. If this safety protocol, implemented in this way, detects errors, an appropriate error response must be made, and the machine must be brought into a safe state.
Position and speed measurement
The motivation to integrate position and speed measurement is
The control of rotary or linear processes
The function monitoring
Hall counters offer a simple possibility for speed measurement and position monitoring. Here, a magnet or an incrementally magnetized ring is applied to the motor shaft. A Hall counter in the vicinity detects each revolution. This solution has the advantage of being very integrated, compact and inexpensive to implement.
The motivation of the current measurement is
The control of the motor current or indirectly of the motor torque
The proportional control of the valve current and thus the valve opening
Monitoring of the motor function or coil monitoring