TRADITIONAL SERVO-HYDRAULIC CONTROL VS. A SERVO POWER UNIT: WHAT’S THE DIFFERENCE?

The term servo has long been associated with precise motion control. In traditional hydraulic systems, engineers may associate servo control with servo or proportional valves that regulate hydraulic flow to a cylinder. A Servo Power Unit (SPU) uses a different design, integrating a servo motor with a hydraulic gear pump to precisely control the speed (flow) and torque (pressure) delivered to the cylinder.

Both approaches can provide hydraulic motion control, but the way hydraulic power is generated and controlled is fundamentally different. Understanding how each system achieves servo control helps explain the differences between a Servo Power Unit and traditional servo-controlled hydraulics.

How a Traditional Hydraulic System Works

A traditional hydraulic system (Figure 1) typically consists of a motor-driven hydraulic pump, reservoir, filtration, pressure regulation, directional control valves, long hydraulic hose lines, and one or more cylinders. The motor drives the hydraulic pump, which draws fluid from the reservoir and generates the hydraulic flow required to operate the system. Directional control valves then route that flow to the cylinder, controlling whether it extends, retracts, or holds its position.

In many traditional hydraulic systems, the pump operates continuously at maximum RPM and pressure while the machine is running. The motor and pump are typically sized to meet the application’s maximum pressure and flow requirements, keeping the system ready to provide peak hydraulic power at any time, even when that level of demand is only required for a portion of the operating cycle. As a result, energy can be consumed even when the application does not require the system’s full output.

For applications that only require basic extend, retract, and hold functions, on/off directional valves may provide the necessary control. When these valves open or close, maximum hydraulic flow and pressure are applied to or removed from the cylinder abruptly. These violent transitions can lead to inevitable leaks, shock/vibration issues, and stress on connections resulting in unplanned downtime and safety risks.

Adding Servo Control to Traditional Hydraulics

When an application requires more precise control of position, speed, or force, traditional hydraulic systems can incorporate proportional or servo valves. Rather than simply switching hydraulic flow on or off, these valves regulate the amount and direction of flow delivered to the cylinder. This provides smoother, more precise control than a basic on/off directional valve controlling flow or pressure.

Adding a servo or proportional valve does not change how the hydraulic power is generated. The motor still drives the pump to produce hydraulic flow. The servo or proportional valve then precisely regulates that flow before it reaches the cylinder. For closed-loop control, a position sensor mounted to the cylinder or machine mechanism measures the actual position and sends feedback to the controller. The controller compares this feedback to the commanded position and adjusts the servo or proportional valve accordingly, continuously regulating hydraulic flow to achieve and maintain the commanded motion.

Depending on the application, precise position and force control can also require additional control components. For example, force measurement may require an external load cell, adding components, cost, and complexity to the system. In traditional servo-controlled hydraulics, the control loop also relies on a controller to continuously adjust the servo or proportional valve to regulate hydraulic flow or pressure.

Achieving precise control can become more complex in a centralized hydraulic system, where factors such as hose length and expansion, fluid temperature, and changes in viscosity can influence system response. Fluid cleanliness is another important consideration, as contaminants can affect the performance and reliability of servo and proportional valves, requiring proper filtration and ongoing fluid maintenance. Accounting for these factors, along with the additional valves, sensors, and control components required for closed-loop operation, can increase the complexity and cost of achieving accurate position and force control.

How This Approach Changes Hydraulic Motion Control

The difference in how hydraulic flow is controlled can impact energy consumption, heat generation, system complexity, maintenance, and overall machine design.

Power-on-Demand

Because the servo motor directly drives the gear pump, the SPU can respond to the motion requirements of the application. When the cylinder needs to move, the servo motor drives the pump at the speed and direction required for the desired motion. When movement is not required, the system does not need to continuously generate the same hydraulic flow. This power-on-demand approach can reduce unnecessary energy consumption and associated heat generation compared with a traditional hydraulic power unit operating continuously.

Precise Position, Speed, and Force Control

The SPU’s closed-loop control provides precise, programmable control of position, speed, and force without relying on proportional or hydraulic servo valves. By continuously responding to feedback from the position or pressure sensors, the SPU adjusts hydraulic flow and pressure to achieve and maintain the commanded motion or force. This allows the cylinder to follow programmed motion profiles, maintain a commanded position, or apply a specified force based on the requirements of the application.

A More Compact Hydraulic System

The difference also extends beyond how hydraulic flow is controlled. Traditional hydraulic systems require a centralized HPU, large reservoir, external filtration, control valves, and hydraulic hose runs connecting the power unit to cylinders throughout the machine along with a separate, complex controller. The Compact Servo Power Unit integrates the servo motor, gear pump, reservoir, and controls into a compact package that can be located close to the cylinder. This eliminates the need for external filters, large reservoirs, long hydraulic hose runs, and servo valves.

The SPU still uses hydraulic connections between the power unit and cylinder, but locating the SPU close to the actuator allows these connections to be significantly shorter than the hose runs commonly associated with a centralized HPU. Reducing the amount of hydraulic plumbing, fluid, filtration, and external components can also reduce potential leak points and routine hydraulic maintenance.

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