Part A – Course 1 Pressure control loop
2. Characteristic curve
First, the pressure relief valve is opened completely. The entire oil flow returns de-pressurised from the pump to the tank. The pressure sensor display shows 0V. Pressure is then gradually increased by closing the pressure relief valve. The pressure levels and the pressure sensor readout are entered in a values table. Once the maximum pump pres- sure has been reached, this series of measurements is repeated with falling pressure.
Note the following when recording the characteristic curve
accurate setting of pressure values
rising or falling direction of measurement.
The characteristic curve of the pressure sensor is represented by plot- ting
the input variable (pressure p in bar) on the x-axis and
the output variable (voltage V in Volts) on the y-axis.
A-8
Exercise 1
3. Characteristics
The most important characteristics of a pressure sensor are:
Measuring range
Connection values
Transfer coefficient
Hysteresis.
These values can be taken from the data sheet. It is, however, often necessary to carry out a check by means of a series of measurements. It is not possible to establish the complete measuring range of the pres- sure sensor with the items of equipment available. Since the pump sup- plies less than 100bar, it is not possible to traverse the entire input pressure range. It is nevertheless possible to calculate the transfer co- efficient in the linear range, which is the most important one for setting a closed control loop. There is no point in calculating hysteresis, since any possible differences are more likely due to the inaccuracy of the pres- sure gauge rather than the features of the pressure sensor.
A-9
Exercise 1
WORKSHEET
Characteristic curve of a pressure sensor 1. Measuring circuit
Familiarise yourself with the required items of equipment. What characteristics describe the pressure sensor?
Input range: _____________________________________________ Output range: ___________________________________________ Supply voltage: __________________________________________
Designate the characteristics of the pressure gauge:
Measuring rang: _________________________________________ Measuring accuracy: ______________________________________
Construct the measuring circuit, starting with the hydraulic and then the electrical part.
A-10
Exercise 1
2. Characteristic curve
Open the pressure relief valve completely.
Switch on the voltage first.
Then switch on the hydraulic pump.
What output signal does the pressure sensor supply?
Slowly close the pressure relief valve. Traverse the measuring range by way of a test.
A-11
Exercise 1
WORKSHEET
Record the characteristic curve of the pressure sensor.
Observe the direction of measurement: rising or falling input variable!
Measured variable and unit
Measured values Direction of
measurement Pressure p in bar 0 10 20 30 40 50 60 70 80 Voltage V in volts rising Voltage V in volts falling
Enter the measured values in the diagram. Identify the axes: x-axis for input variable
y-axis for output variable
Value table
A-12
Exercise 1
3. Characteristics
Establish the following characteristics from the diagram:
Input range: Output range: Measuring rang: Linear range: Transfer coefficient: Hysteresis:
How do you evaluate the use of this pressure sensor within the framework of the circuits given with this equipment set?
State your reasons for this:
A-13
Exercise 2
Closed-loop hydraulics
Forming plastic products
To understand the function of a dynamic directional control valve
To be able to record the pressure/signal characteristic curve
To be able to establish important characteristics from the character- istic curve
Dynamic 4/3-way valve
A dynamic directional control valve is used to set the pressure control loop used in the following. The most important features of this valve are described below.
Hydraulic connections
A and B: Working lines P: Pressure supply T: Return line Switching positions
Flow from P → A and B → T Mid-position closed
Flow from P → B and A → T
Electrical connections
Voltage supply Control voltage (= Input variable) Switching position (= Output variable) 24V +10V 0V -10V P → A and B → T mid-position closed P → B and A → T Subject Title Training aim Technical knowledge
A-14
Exercise 2
Pressure/signal characteristic curve of a 4/3-way valve
The pressure/signal characteristic curve is created by means of record- ing
the control voltage as input signal and
the pressure at the power port as an output signal. The working lines are closed during this.
If the valve spool is moved sufficiently in one direction, then one output is opened and the other closed. This results in maximum pressure at the Fig. A2.1:
Symbols for dynamic 4/3-way valve
A-15
Exercise 2
The pressure/signal characteristic curve consists of two curves, i.e. one each for output A and output B. The following characteristics can be read from this:
Hydraulic zero point
The valve spool covers both outputs equally so that there is zero flow rate. In the diagram, this is the intersection of the two curves.
Electrical zero point
The control voltage is equal to zero. However, the valve spool does not necessarily cover both outputs equally, whereby different pressures may occur at the outputs.
Asymmetry
Asymmetry is the difference between the electrical and hydraulic zero point, which can be compensated by means of an offset added to the control voltage.
Pressure gain
Pressure gain is the ratio of pressure change to voltage change (= out- put/input). It is specified in bar per volt and should be as large as possi- ble so that even a small change in control voltage results in a large pressure change.
Pressure gain often relates to the signal range of the control voltage and is specified in a percentage stating what percentage of the control signal is required in order to reverse the entire pressure. 10% is required for good valves, but only 1% for excellent valves.
Overlap
This can be seen from the pattern of the characteristic curve at the hy- draulic zero point:
With zero overlap, the characteristic curve is almost vertical.
A-16
Exercise 2 Fig. A2.2: Characteristics of a pressure/signal characteristic curveA-17
Exercise 2
Plastic plates are to be precisely formed by means of a hot-forming press. The pressure of the press is to be set automatically by means of a pressure control loop. Pressure is to be controlled via a dynamic 4/3- way valve. Some time after start-up, variations occur in the size of the product. One cause may be that the working pressure is no longer con- stant. This may indicate wear in the directional control valve. The pres- sure/signal characteristic curve must therefore be recorded and an assessment of the operating status made in comparison with the char- acteristic curve of a new valve.
Problem description
A-18
Exercise 2
Pressure/signal characteristic curve of a dynamic control valve
1. Constructing a measuring circuit to plot the characteristic curve 2. Plotting and recording the pressure/signal characteristic curve 3. Establishing the characteristics from a characteristic curve
1. Measuring circuit
The following are measured for the pressure/signal characteristic curve:
the control voltage as input signal and
the pressure at the power port as output signal. The following devices are required:
A generator to set the control voltage between - 10V and + 10V.
A pressure sensor to measure the working pressure.
A second pressure sensor on the other power port facilitates the re- cording of the characteristic curve.
A multimeter for the voltage signal of the pressure sensor, from which the pressure is calculated (see exercise 1).
A voltage supply of 24V for the valve and 15V for the sensor. These are used to construct the hydraulic and electrical circuits.