Proportional hydraulic valves are widely used in mobile machinery to control functions such as boom movement, steering, lifting, hydraulic motors, and attachments.
Unlike simple ON/OFF valves, proportional valves require a variable electrical command to adjust hydraulic flow or pressure.
PWM (Pulse Width Modulation) is commonly used for this purpose. Two common control approaches are:
Duty-cycle PWM control
Current-controlled PWM
Although both use PWM, they control the valve differently. Understanding this difference helps OEM engineers select the appropriate control strategy for hydraulic systems.
A proportional valve typically uses a solenoid coil to move the valve spool.
The controller rapidly switches its output ON and OFF. The percentage of time the output remains ON during each cycle is called the duty cycle.
For example:
25% Duty Cycle → Lower output
50% Duty Cycle → Medium output
75% Duty Cycle → Higher output
Increasing the PWM command generally increases the electrical energy delivered to the coil and changes the valve response.
However, supply voltage, coil resistance, temperature, and wiring conditions can also affect the actual coil current.
This is where duty-cycle and current-controlled PWM begin to differ.
With duty-cycle PWM control, the mobile controller directly commands a PWM percentage.
A simplified control path is:
Joystick
↓
Mobile Controller
↓
PWM Duty Cycle
↓
Proportional Valve
↓
Hydraulic Function
For example, the controller may output 30%, 50%, or 70% duty cycle according to the joystick position.
This approach is relatively simple and can work well for many hydraulic functions.
However, the controller is primarily controlling the duty cycle, rather than directly regulating the actual current through the valve coil.
The electrical resistance of a solenoid coil can change as its temperature changes.
According to Ohm's law:
I = V / R
where:
I = Current
V = Voltage
R = Resistance
As the coil becomes hotter, its resistance typically increases. This can change the resulting current even when the PWM duty cycle remains unchanged.
Supply voltage can also vary in mobile machinery because of:
Battery condition
Alternator operation
Engine starting
Electrical loads
Wiring voltage drops
Therefore:
Same PWM Duty Cycle
↓
Different Voltage / Coil Resistance
↓
Different Coil Current
↓
Potentially Different Valve Response
For some hydraulic functions, this variation is acceptable. For applications requiring more consistent control, current regulation may be useful.
Current-controlled PWM adds feedback to the control process.
Instead of only sending a fixed duty cycle, the controller monitors the actual current flowing through the valve coil.
Target Current
↓
Mobile Controller
↓
PWM Output
↓
Valve Coil
↓
Actual Current
│
└──── Feedback → Controller
The controller compares the target current with the actual current.
If the measured current is too low, it can increase the PWM output. If the current is too high, it can reduce it.
This creates a closed-loop current control system.
The main difference is what the controller regulates.
| Feature | Duty-Cycle PWM | Current-Controlled PWM |
|---|---|---|
| Main command | PWM duty cycle | Target coil current |
| Current feedback | Typically no | Yes |
| Compensation for voltage changes | Limited | Better |
| Compensation for coil resistance changes | Limited | Better |
| Control complexity | Lower | Higher |
| Valve response consistency | Application-dependent | Generally more consistent |
Duty-cycle PWM controls the electrical command applied to the valve, while current-controlled PWM regulates the actual coil current.
The electromagnetic force generated by a solenoid is closely related to the current flowing through its coil.
The basic relationship is:
Controller Command
↓
Coil Current
↓
Electromagnetic Force
↓
Valve Movement
↓
Hydraulic Flow / Pressure
If coil current changes because of temperature or electrical conditions, valve behavior may also change.
For applications requiring smooth and repeatable hydraulic movement, controlling current can therefore provide more consistent performance.
Consider a joystick controlling a hydraulic boom.
With duty-cycle control:
Joystick Position
↓
Controller
↓
50% PWM
↓
Valve
With current-controlled PWM:
Joystick Position
↓
Controller
↓
Target Current: 800 mA
↓
PWM Regulation
↓
Valve
↑
Current Feedback
In the second case, the controller adjusts PWM automatically to maintain the requested coil current as operating conditions change.
PWM control is not determined by duty cycle or current alone.
OEM engineers should also consider:
Minimum and maximum current
Many proportional valves have a defined current range between the point where the valve begins responding and its maximum useful command.
PWM frequency
Different proportional valves may require different PWM frequencies. The valve manufacturer's specifications should be followed.
Output ramps
Gradually increasing or decreasing the command can help create smoother hydraulic acceleration and deceleration.
Electrical diagnostics
Depending on the controller and output design, monitoring can also help detect conditions such as open circuits or short circuits.
These parameters should be configured according to the valve and hydraulic system requirements.
Duty-cycle PWM may be suitable when:
The hydraulic function is relatively simple
Some variation in valve response is acceptable
Precise current regulation is not required
Simpler control implementation is preferred
Current-controlled PWM may be more appropriate when:
More consistent valve response is required
Supply voltage varies during operation
Coil temperature changes significantly
Repeatable hydraulic movement is important
The valve is specified according to coil current
The correct method depends on the proportional valve, controller hardware, hydraulic system, and machine performance requirements.
In modern mobile machinery, hydraulic control is usually integrated with other electronic systems.
For example:
Joystick / Sensors
↓
Mobile Controller
↙ ↘
PWM CAN
↓ ↓
Hydraulic HMI / ECU /
Valves Remote I/O
The controller can process operator commands and sensor signals while controlling proportional valves and communicating with other devices over CAN.
This allows hydraulic functions, machine operating modes, diagnostics, and HMI information to work together within one control architecture.
Duty-cycle PWM and current-controlled PWM can both be used to control proportional hydraulic valves.
The key difference is simple:
Duty-cycle PWM controls the PWM command, while current-controlled PWM regulates the actual current flowing through the valve coil.
Because coil resistance, temperature, supply voltage, and wiring conditions can vary during machine operation, the same duty cycle does not always produce exactly the same coil current.
For simpler hydraulic functions, duty-cycle control may be sufficient. For applications requiring more consistent and repeatable valve behavior, closed-loop current control can offer advantages.
OEM engineers should select the control method according to the valve specifications, hydraulic performance requirements, and capabilities of the mobile controller.
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