While simple on/off-type solenoid valves are very numerous in hydraulic systems, their function is purely to act as a hydraulic ‘switch’, and their rapid operation can sometimes create shock in the system when heavy loads are stopped and started quickly. For this reason they are sometimes referred to as bang-bang valves.
However, by suitably modifying the valve it is also possible to provide it with the capability to control the flow rate as well as the direction of flow. In other words, the amount of spool opening can be controlled electronically, both in terms of the amount of opening and the rate at which it opens and closes. This type of valve is then referred to as a proportional directional valve, because the amount of valve opening (and therefore the valve flow rate) is proportional to the input signal it receives.
As can be seen from Fig. 5.11, modifications can be made to the valve spool by adding notches to the edges of the spool lands to provide a gradual opening and closing of the flow path. Modifications are also made to the solenoid to provide low-friction bushes or bearings (to reduce ‘stick–slip’ effects) and to linearise the magnetic characteristics. As before, the flow direction can be controlled by energising one solenoid or the other, but now the amount of flow can also be controlled by varying the level of solenoid current.
By controlling the rate of change in the solenoid current (i.e. a ramped change), the speed at which the spool opens and closes the flow path can also be controlled.
This has the effect of determining the acceleration and deceleration of an actuator, enabling system shocks to be reduced to acceptable levels. As mentioned, therefore, the comparison between an on/off solenoid valve and a simple proportional valve is similar to that of an on/off light switch and a dimmer switch.
HUMAN–MACHINE INTERFACE (HMI)
INPUTS AND SENSORS
POWER SWITCH
POWER SUPPLY AC
SUPPLY 24 V DC
PLC
Fig. 5.10 PLC control arrangement
POINT OF INTEREST A proportional directional control valve controls both the amount and the direction of flow through it in response to a variable electronic signal.
A proportional flow control valve controls only the amount of flow passing through it (i.e. not the direction of flow).
In theory, a simple variable resistor could be used to vary the current flow through the proportional valve solenoid. However, in practice the problems of heat generation that this would create, together with the variation in the solenoid coil resistance with temperature, means that the degree of control would be very imprecise. In practice, therefore, the coil current, and hence the valve opening, is normally controlled by a simple electronic amplifier arrangement, which not only provides controllability but also additional control features (Fig. 5.12).
ENERGISED DE-ENERGISED
ON/OFF VALVE
PROPORTIONAL VALVE
Fig. 5.11 On/off and proportional solenoid valves
INPUT SIGNAL
AMPLIFIER
POWER SUPPLY
Fig. 5.12 Arrangement for proportional valve control
The input signal to the amplifier can be created by, for example, a simple rotary potentiometer, the output from a PLC or a joystick-type potentiometer. Input signals can be either a variable voltage (often 0–10 V or ±10 V) or a variable current (typically 4–20 mA). Current signals tend to be less susceptible to interference (electrical
‘noise’) and to variations in signal level when transmitted over long signal lines, and so are more suited to some environments. Figure 5.13 shows a block diagram of a typical proportional valve amplifier.
ENABLE
VOLTAGE CURRENT
POWER SUPPLY
OUTPUTS:
SOLENOID A
SOLENOID B INPUTS:
RAMP DEADBAND
PWM DITHER GAIN
Fig. 5.13 Block diagram of a proportional valve amplifier
An adjustable ramp function can be included in the amplifier. This will provide a gradually increasing or decreasing output even though the input may change suddenly from one value to another (step change). This is, therefore, a very useful feature for controlling actuator acceleration and deceleration.
The gain adjustment determines the amplifier output for a given input signal (similar to the volume control on a radio, for example), and the deadband compensation effectively removes the effect of spool overlap (i.e. the portion of the spool movement before the flow path opens).
A dither signal is often included, and this is sometimes adjustable and sometimes factory set. The dither signal is a small high-frequency AC signal superimposed on the DC signal to keep the valve spool vibrating very slightly. This helps to remove the effect of ‘stiction’ in the valve (i.e. the tendency of the spool to stick due to friction or contamination).
Finally, the output signal is converted to a pulse-width modulated signal which, as mentioned earlier, is a series of on/off pulses at high frequency, where the signal level is determined by the duration (width) of each ‘on’ pulse relative to the duration of each ‘off’ pulse. This technique is used to reduce heat generation in the amplifier.
An enable function is normally included in the amplifier circuit to disable the amplifier output unless the signal is present. This is often used as part of the system safety control or connected to the machine emergency stop system.
Simple proportional valves control the spool opening, and therefore valve flow rate, by balancing the solenoid force against the spool spring force. This may be perfectly POINT OF INTEREST
The proportional valve amplifier may be a separate component (mounted in an electrical control cabinet, for example) or be built onto the hydraulic valve itself.
Simple amplifiers may also be mounted in the solenoid plug connectors.
DEFINITION
Deadband refers to the overlap region between a spool and its port. As the spool moves within this region, the port is not opened to flow, hence the term
‘deadband’. The reason for the overlap is often to reduce leakage in the de-energised position. The unwanted effect of the deadband can be reduced significantly by increasing the amplifier gain in this region in order to move the spool rapidly to a position where flow starts to pass through the valve.
adequate for less demanding applications (such as shock reduction). However, when a more precise or faster valve operation is required, a spool position sensor can be added to the valve to provide a feedback signal to the amplifier proportional to the spool position (Fig. 5.14).
The spool position sensor is often a non-contact inductive-type sensor (e.g. a linear variable differential transformer (LVDT)) and provides a feedback signal to the amplifier. The feedback signal is dependent on the spool position either side of the central position. The amplifier then compares the input signal with the feedback signal and acts to correct any error between the two, thus providing a much more precise level of spool control.