Proportional directional valve is a commonly used hydraulic control component used to control the flow and direction of hydraulic oil in hydraulic systems. It adopts the principle of proportional control, which adjusts the opening degree of the valve by controlling the current signal of the solenoid valve, thereby achieving precise control of the flow direction and flow rate of hydraulic oil.
When controlling an electric proportional valve through the PWM port of a PLC, it is often encountered that the proportional valve response is slow, the amplitude of action is too different from the expected amplitude, and the proportional valve stem vibrates frequently.
The main reasons are as follows:
1. The valve core is subject to frictional resistance
2. The valve core is subject to fluid resistance
3. No closed-loop control

During the initial movement of the valve core, it needs to overcome static friction, so the action is slightly delayed. Then, the valve core suddenly moves, causing an increase in flow rate and hydraulic impact, resulting in a larger amplitude of action.
During the power-off process of the coil, the rebound stress of the reset spring is also uneven, which can lead to changes and instability in the flow rate.
The genetic level control difficulty caused by the two-point valve body structure and working mechanism mentioned above is unavoidable.
So how can we solve this problem?
1. Regular current
2. Regular voltage
3. Feedback regulation of electrical signals, continuous calibration of electrical signals, forming closed-loop control
The hysteresis effect refers to the phenomenon where the same control parameter (current) occurs in the electric proportional hydraulic valve from coil electrification control to power-off control, but different hydraulic system flow effects occur
Definition of hysteresis loop in proportional valve:
Hysteresis is an important static indicator of proportional valves, defined as the percentage difference between the widest point of the "flow vs instruction curve" between instruction 0-100% and 100% -0 (refer to the figure below).

The presence of hysteresis will reduce the repeatability accuracy of open-loop control systems.
For closed-loop control systems, excessive hysteresis of proportional valves can cause system vibration and result in the failure of closed-loop control.
Source and avoidance of hysteresis:
The hysteresis mainly comes from the frictional force between the valve core and valve body, as well as static hydraulic force. For direct drive valves driven by proportional electromagnets, the hysteresis and friction hysteresis of proportional electromagnets are also important sources of hysteresis in proportional valves.
The following methods can reduce the hysteresis of proportional valves:
1. Reasonable clearance and shape accuracy of valve core holes.
2. Choose a reasonable working air gap and non working air gap size for the proportional electromagnet, and control the coaxiality between the iron core and the guide sleeve.
3. The use of valve core displacement closed-loop can significantly reduce the hysteresis of proportional valves (<1%).
4. The open-loop control proportional valve superimposes flutter on the coil drive current.  
There are currently three known ways to reduce vibration in force feedback servo valves and proportional valves:
1. Stacked independent flutter: A triangular wave is superimposed on a linear driving current (with low driving efficiency and severe heating), commonly used for servo valve driving with smaller coil currents. Belongs to independent flutter (the frequency and amplitude of flutter can be adjusted separately).
2. Independent PWM jitter: Using high-frequency PWM (equal amplitude pulse width modulation) waves as carriers, with a higher carrier frequency, and superimposing lower frequency triangular waves on the carrier. It also belongs to independent flutter.
3. Parasitic PWM vibration: The vibration source is directly generated by the charging and discharging waveform (approximately triangular wave) driven by PWM waves (50~500Hz) (as shown in the figure below). Belonging to parasitic flutter, the amplitude of flutter is not independently adjustable.

The first two types above belong to independent flutter, which is characterized by the ability to adjust the flutter waveform more finely, and the flutter waveform will not change with the change of control current. Its effect of reducing hysteresis is better. Its disadvantages are high cost and complex on-site adjustment (two parameters of flutter frequency and amplitude need to be adjusted).
The third type of parasitic PWM flutter can meet most engineering applications, and on-site debugging is simple (only adjusting the flutter frequency is required). The disadvantage is that the effect of vibration changes with the change of current value (the above figure is a schematic of the worst-case scenario).  
For example, in order to reduce the response of hysteresis and friction compared to the performance of the control valve, it is usually necessary to superimpose flutter signals, such as sine waves or triangular waves, in the control signal, with a frequency generally ranging from 50Hz to 200Hz and an amplitude of about 0-400. The addition of low amplitude and high-frequency vibration signals to the control current of the solenoid valve is to maintain the movement of the valve core inside the valve sleeve, maintain the thickness of the oil film between the two, and reduce the initial motion resistance. As shown in the figure below, increasing the flutter signal and analyzing it in the time domain, the rise time, adjustment time, and peak time during the process from the initial state to the target state are small, which can effectively improve the transient performance of the control process.