23.3

Control Terminology for Hydrostatic Transmissions

Craig Klocke Sauer-Danfoss (US) Company

ABSTRACT

This paper examines various types of control systems for hydrostatic transmissions as applied in mobile machinery. Because there is a lack of standardized terminology and descriptions for hydrostatic transmissions within the industry, misunderstandings can result which could cause potentially significant problems for the OEM and/or the end user of the machine. The introduction of more complex system solutions increases the need to explain the control of hydrostatic transmissions as simply as possible. The operator of a machine propelled by a hydrostatic transmission is not interested in the functionality of the components or the terminology that describes them.

Simple control types (i.e. single input signal) are reviewed before moving on to those with secondary functions, and then more complex control systems enabled by electronics (specifically software and sensors) are discussed. Explanations and examples of Load Independent and Load Dependent controls will be provided. This paper will also examine and compare how the same function (e.g. displacement control) can be achieved by different methods (i.e. mechanical or electronic means).

The introduction of integrated electronics enables today’s vehicle design engineer to dynamically match the control system to the vehicle function automatically with embedded software or at the touch of a preprogrammed button! Drive control strategies can be optimized for different machine functions and even customized by the end-user. The Tier 4 emissions regulations and the ever increasing demands for lower operating costs are driving machine designers to shift to digital control solutions that dynamically optimize machine performance. An example of this technology is discussed for an “Automotive Control” or “AC” digital control system as applied on a Telehandler or Rough Terrain Forklift machine. Applying integrated electronics to the hydrostatic transmission system offers the machine designer and operator significant benefits in flexibility and performance from a dynamically optimized propel function.

INTRODUCTION

The evolution of control methods for hydrostatic machines is similar to many other paths evolution has taken. As with other subject areas, the pace of the evolution of hydrostatic machines is a balance between technology (what is possible) and value (the ratio

between benefit and cost). Decades ago, technology limited what was possible; whereas today, it is the customer’s cost/benefit ratio decision that has a high influence on the level of technology utilized for a given application.

Figure 1: Closed Circuit Variable Displacement Axial

Piston Pump (Sauer-Danfoss H1P) [1]

Figure 2: Closed Circuit Variable Displacement Bent

Axis Piston Motor (Sauer-Danfoss H1B) [2]

This paper discusses only closed circuit, (sometimes incorrectly referred to as closed loop), hydrostatic transmissions utilizing variable displacement components. The reader should understand the basic design of closed circuit variable displacement pumps and motors. Figure 1 shows a cross-section of a closed circuit variable displacement axial piston pump with swashplate, servo control and integral fixed displacement charge pump. A cross-section of a closed circuit variable displacement bent-axis piston motor is shown in Figure 2. Other types of pumps and motors (e.g. gear pumps and motors) are not discussed as they have fixed rather than variable displacement and as such do not provide proportional control.

Closed circuit systems connect the pump inlet to the motor (or load) outlet; while open circuit systems connect the pump inlet to the reservoir. Open loop and closed loop more accurately describe control terminology – open loop is without feedback and closed loop is with feedback to create an error signal. Hence, a closed circuit system can have either an open loop or a closed loop control system. But, more about that later. . .

Hydrostatic Control Development

Direct Mechanical Electrical

Figure 3: Evolution of Control Types

As shown in Figure 3, control of hydrostatic transmissions started with rudimentary methods (i.e. “direct” control). It then evolved into simple machines (i.e. “levers” and/or positioning cylinders to make a servo control) before moving to more sophisticated devices (i.e. electronics). Today, the evolution has progressed to embedded electronic systems comprised of integrated sensors and micro-processors with CAN interfaces.

First to be discussed is a simple closed circuit hydrostatic system comprised of a variable pump and fixed motor. The speed of the motor shaft is determined by the following three factors:

1. Pump speed

2. Pump displacement

3. Motor displacement

Pump speed is usually a fixed ratio to engine speed. For a system comprised of a fixed displacement motor, variable displacement pump, and fixed engine speed, the motor shaft speed will be controlled by pump displacement. Next, different methods will be examined of how to control pump displacement (i.e. swashplate position).

DIRECT CONTROL

Direct Displacement Control (DDC) provides the means to position the pump swashplate between maximum displacement in one direction and maximum displacement in the opposite direction. This is normally/typically accomplished by attaching a lever directly to the swashplate trunnion. See Figure 4.

Figure 4: Pump Design with Direct Displacement

Control (DDC) [3]

The range of motion required for the Direct Displacement Control lever is exactly the same as the swashplate and is typically ± 15 to ± 18 degrees from a center or neutral position, depending on the design of the pump. For a 15 cc/rev pump, the internal forces may require 30-40 Nm of torque on the displacement lever to hold maximum displacement in all operating conditions. For a 25 cc/rev pump, the torque requirement might be as much as 80 Nm of torque on the displacement lever. In addition, vibration can also be transferred from the pump swashplate to the operator via the displacement lever. Although force and vibration levels can be modified somewhat by changes in porting area of the valve plate, compromises are required. For these reasons, direct displacement is less common for larger pump displacements. In general, Direct Displacement Control is not available on pumps larger than 45-50 cc/rev displacement because of the high forces and power levels required to quickly move the swashplate.

SERVO CONTROL

With larger machines, larger displacement pumps are required. The higher level of control torque and power levels required to use Direct Displacement Control on larger pumps led to the development of servo systems to position the swashplate. (Longer levers were not a practical solution due to the increased distance required to move from full displacement in one direction to full displacement in the other.) Another benefit of the servo system was the reduction in vibration transferred to the operator and corresponding lower operator fatigue.

Servo systems consist of one or more control pistons used to apply a force to the swashplate and a device to regulate the pressure (i.e. force) applied to the control piston(s). Servo pressure is typically regulated by a proportional three-position four-way valve, although other systems may use different designs. Typically, a low pressure source on the pump (i.e. “charge pressure”) is used to supply the servo system. Designs also may use system pressure or other medium or high pressure sources on the pump or machine to feed the servo system. For simplicity, the valve that controls the position of the servo system will be referred to as the “servo valve” for the remainder of this paper.

Figure 5: Pump Design with Mechanical Input

Control (Servo Control) [4]

The means of controlling the servo system has many variations. The first category is “input signal”. The input signal or control method of the servo valve is typically one of the following:

1. Mechanical Input

2. Hydraulic Input

3. Electrical Input

Mechanical Input: Replacing a Direct Displacement Control pump with one having Mechanical Input for the servo valve drastically reduces the forces (i.e. control torque) required by the operator. A servo valve with Mechanical Input may only require 2-5 Nm of control torque to reach maximum pump displacement (vs 40 Nm or even as much as 80 Nm for a Direct Displacement Control pump depending on size). Although the servo system (i.e. control pistons) typically must increase in size with pump displacement, the control torque level is generally constant for a large range of pump sizes (e.g. 25-250 cc/rev). The Mechanical Input generally provides a position input command to the servo valve that generates the required servo pressure to move the swashplate to the commanded position. A typical input signal range for Mechanical Input servo valve is ± 25 degrees with a threshold value of 5-10% of the range. Mechanical Displacement Control or MDC is an example of a common name for a control with Mechanical Input.

Hydraulic Input: A servo valve with hydraulic input can be designed for any signal pressure range. Typically, most hydraulic input servo systems utilize low pressure (e.g.

“charge pressure”) as the signal pressure source and the supply pressure to the servo valve. Hydraulic Input servo systems provide the flexibility of adding additional logic and/or control valves in series or parallel with the operator input. Designs utilizing system pressure to supply the servo system are more common on motors and very large pumps. The advantage of system pressure input is the reduction in control piston size to generate the same force as with a low pressure source. The Hydraulic Input provides a pressure input to the servo valve that generates the required servo pressure to move the swashplate to the commanded position. A typical input signal range for Hydraulic Input servo valves would be ± 10 bar with a threshold value of 1-2 bar. Hydraulic Displacement Control or HDC is one common name for a control with Hydraulic Input.

Electrical Input: A servo system with electrical input can be designed for almost any type of input signal. Historically, these have been either low current DC or high current PWM with 12 or 24 volt supply. The Electrical Input provides current to a solenoid that is converted to a force acting on the pilot valve or spool of the servo valve. This force caused the pilot valve or spool to generate servo pressure to position the swashplate to the desired position. A typical input signal range for Electric Input servo valves would be ± 100 mA for low current DC and 1 to 2 A max current for 12 volt PWM. Designs for 24 volt are also common and generally require half the current of a comparable 12 volt system. Threshold values are generally 10-15% of the input range for low current DC and 30-50% for high current PWM. Electrical Displacement Control or EDC is one common name for a control with Electrical Input. Although less common, CAN-based designs are becoming more popular.

TYPE OF CONTROL

The machine operator is primarily interested in controlling machine function – i.e. for a hydrostatic propel drive this would be the speed and/or torque applied to the drive wheels. Generally, the hydrostatic pump control is commanding swashplate position and consequently output flow of the pump. Pump controls can be divided into two categories – either Load Dependent or Load Independent. Machine function cannot be understood (or explained) until the operation of each of the relevant components is understood. The first step is to understand the function of each component so that the resulting system function can be described.

The second category of controlling a pump servo system is the output (i.e. what is controlled by the input signal). The output controlled by the servo valve is typically either “load dependent” or “load independent”.

Load dependent controls provide a pump swashplate position that, with a fixed input signal, can change with operating conditions (i.e. “load”). These load conditions include input speed, system pressure and temperature.

Load dependent controls are typically described by one of the following names:

1. Non-Feedback or Non-Feedback Proportional

2. Proportional

3. “Soft” control

The dependency on system pressure and input speed can be changed by design parameters. See Figure 6 for a typical example of system pressure load dependency for a Non-Feedback Proportional Control with electrical input (i.e. NFPE).

Figure 6: Load Dependent Control – NFPE [5]

Load Independent controls provide a constant pump swashplate position (i.e. displacement control) that, with

a fixed input signal, does not change with operating conditions (i.e. “load”). Displacement control is achieved by providing feedback of swashplate or servo piston position back to the servo valve via force or position. The servo valve will adjust the spool position, which changes servo pressure, to move the swashplate to the commanded position. It is important to note that

Figure 7: Load Independent Control – EDC [6]

even for a constant pump swashplate position, the output flow will not be “exactly” constant for all load conditions due to volumetric efficiency. In a “perfect” component (i.e. no losses) the output flow would be the product of speed and pump displacement, but in an actual machine, the losses that change with operating conditions or load (e.g. input speed, system pressure and temperature) will change the flow. Figure 7 shows the load independent characteristics for an electrical input servo control (e.g. EDC – Electrical Displacement Control). In addition to the basic or primary control function (i.e. load dependent or load independent), secondary functions can be added. These secondary functions typically provide an “over-ride” or “limiting” feature. These functions may either be integrated into the control or incorporated into the hydrostatic transmission circuit. Examples of these secondary functions include system pressure limiting and control over-ride. A control over-ride, when activated, will over-ride the normal input command with a command for the swashplate to move to the neutral position (i.e. zero displacement).

CHOICE OF CONTROL TYPE

What is the best type of control? It depends not only on the application, but also on the preference/experience of the operator or machine designer. If the machine operator desires a smooth start-up under various load conditions, then a Load Dependent Control is a good choice. Applications that require precise vehicle velocity under various load conditions should use a Load Independent control. In the past, this choice was made by the machine designer and it was fixed until the next design iteration. The operator was forced to use the control type chosen by the machine designer, regardless of the machine function, operating conditions or operator preference. However, digital control systems now allow the operator and machine designer to have either type of control at their fingertips.

As described earlier, in “Direct Control” the operator’s input signal (i.e. lever position) determines the displacement of the pump – hence the term “Direct Displacement Control”. Servo pressure control is achieved by providing feedback of the servo pressure to the input spool.

DIGITAL CONTROL

Digital Control or Electronics is an enabler for more sophisticated control systems. With the use of electronics, control types can be changed at the touch of a button or automatically without operator input. A basic control type can be “over-ridden” or merged with another using sensors and electronics. The obvious benefit is increased machine performance (even with un-skilled operators). Sensors can detect changes in operating conditions providing information to the electronic control system which results in optimized performance and function of the system, more quickly and more accurately than the best-skilled operator could control the machine.

One example of this “merger” of control types is demonstrated by adding a swashplate angle sensor to a load dependent control. This allows the pump to operate

Figure 8- Hysteresis & Linearity of Load Dependent

Control

Figure 9- Hysteresis & Linearity of Load Dependent

Control with Closed Loop Swashplate Position

Figure 10- Hysteresis & Linearity of Load Independent

Control

as either a load dependent (basic function) or load independent (via the sensor and closed loop digital control system) control type. Hysteresis data at 0 bar system pressure is shown in Figure 8 for a basic load dependent control with electrical input (e. g. NFPE). A system comprised of the NFPE control, swashplate angle sensor, and a digital controller was designed and built. PLUS+1

TM electronic hardware and software was

used to provide a digital closed loop control of swashplate angle. Sample test results of this system are

shown in Figure 9. It is obvious that a large reduction in hysteresis was achieved with the digital control. A load independent control with electrical input was tested under the same conditions and the results are shown in Figure 10 for comparison. With this digital control system, the machine designer does not have to decide which control type to apply. But rather the software decides (based upon operating conditions) which control type to utilize. These results demonstrate the ability to optimize machine performance and function for all operating conditions!

Figure 11- Closed Circuit Axial Piston Pump with

Integrated Sensors & Embedded Digital

Controller (Sauer-Danfoss H1P) [7]

The next step in control evolution was taken by integrating the digital controller and swashplate angle sensor into the pump (see Figure 11). This system contains a Load Dependent control with speed, swashplate angle, and pressure sensors. This flexible system with an embedded digital controller can be configured to provide Load Dependent or Load Independent control as well as secondary functions. The pressure sensors provide the information to perform electronic pressure limiting function with a fixed setpoint or a variable setpoint based upon operating conditions (e.g. temperature, engine speed, etc.). It can also provide Automotive Control function (Load Dependent Control with input signal proportional to pump speed) or non-Automotive Control (Load Dependent Control with input signal directly from the operator via Human Machine Interface or HMI).

MOTOR CONTROL

Motor controls have the same general descriptions and classifications as pump controls. In general, variable displacement motors may have Load Dependent or Load Independent Controls with mechanical, hydraulic or electrical input signals. An additional variant for motors is the option of two-position control (sometimes incorrectly labeled “two-speed”). Motors generally have

Direct Displacement Control available only for smaller displacements (i.e. <50 cc/rev). Motors generally do not have Mechanical Input. Consequently, the majority of variable motors have servo valves with Hydraulic or Electric Input. As with pumps, electric input signals for motors are typically either low current DC or high current PWM with 12 or 24 V supply.

LOAD DEPENDENT: Motors with Load Dependent control are generally pressure dependent and commonly labeled “pressure compensated” or “PC”. Fixed and variable setpoints versions are available. Neither version is directly controlled by the machine operator. They react automatically to the system pressure in the hydrostatic circuit and adjust motor displacement accordingly. When the system pressure is above the setpoint, motor displacement is increased, thus resulting in slower speed and greater torque. When the system pressure is below the setpoint, motor displacement is reduced resulting in faster speed and lower torque. Hence, the default or “startup” position for a motor with PC control is minimum displacement. Internal system pressure provides the signal (and typically also the supply pressure) for the servo valve. Both versions have the option of a defeat valve or “over-ride” (e.g. Pressure Compensated with Over-Ride = PCOR). With the Over-Ride activated, the motor is commanded to maximum displacement regardless of system pressure.

LOAD INDEPENDENT These controls are generally proportional or two-position and commonly labeled “displacement control”. Load Independent controls for motors are very similar in function to those for Pumps. Secondary functions such as Pressure Compensator “over-ride” are available in fixed or proportional versions. Electrical Displacement Control and Electric Proportional Control are two common names for a Load Independent control with Electrical Input. Load Independent controls are well suited for applications that require precise speed control such as dual-path machines like crawlers.

THE COMPLETE PUMP/MOTOR SYSTEM

In general, all combinations of Load Dependent and Load Independent control types for pumps and motors are possible. Dual path systems and circuits that use one pump with multiple motors generally do not mix control types among the individual motors.

If the machine has an engine speed dependent control strategy, then a Load Dependent pump control is usually required. The motor control may be either Load Dependent or Load Independent. The Load Dependent motor control will automatically adjust displacement to match engine power to load. The Load Independent motor control can function similar to a gearbox or as an “automatic” ratio system with a Pressure Compensator over-ride function.

Machines that use an engine speed independent control generally do not use a Load Dependent pump control. Although the Load Independent pump control provides

precise speed control of the machine, a horsepower management or “anti-stall” system may be needed to prevent the engine from being overloaded. The Load Independent pump may be used with either Load Dependent or Load Independent motor control. The Load Independent motor control can function similar to a gearbox or be used in a “phased” system providing smooth speed control through the entire operating range. It can also provide an “automatic” ratio system with a Pressure Compensator over-ride function.

A Telehandler is a machine that operates in different work modes. Rapid reversing and quick speed changes are required for some work modes and others demand smooth starting and stopping. A digital control system provides the flexibility of operating the electronic pump and motor controls as Load Independent or Load Dependent allowing the operator, or the control system, to select the best control type based on the conditions. A subset of Load Dependent control is an “Automotive Control” or AC. The AC is dependent upon both engine speed and load. The HMI for AC typically includes a foot pedal controlling engine speed and an FNR controlling direction of machine travel.

To illustrate why an operator would choose a particular control type, the behavior of a Telehandler with AC accelerating from zero speed to maximum travel speed is given below. The operator first selects “forward” from an F-N-R lever controlling travel direction and depresses a foot pedal controlling engine speed. Depressing the pedal proportionally increases the engine speed from low idle towards high idle. When the AC senses that engine speed has crossed a predefined threshold, it triggers a pump current command per the Engine Speed Drive map (i.e. Engine speed vs Pump current). The Automotive Control knows which direction the pump swashplate should be actuated (if at all) by the position of the F-N-R lever. The pump current is converted to servo pressure in the pump servo valve. The servo pressure applies a force to the control piston causing the pump swashplate to move towards maximum displacement. The actual pump swashplate position is a result of a balance of the forces acting upon it. For example, if the machine encounters a high load that increases system pressure, pump displacement is reduced as a result of the force balance.

The output flow of the pump is converted to wheel rotation by the motor. A Load Dependent (i.e. Pressure Compensated) motor control starts at minimum displacement and automatically increases motor displacement when the system pressure exceeds the setpoint value and then reduces displacement when pressure drops below the setpoint. For aggressive machine acceleration (or when heavily loaded), the system pressure will probably exceed the Pressure Compensator setpoint and motor displacement will be adjusted during machine acceleration.

SUMMARY & CONCLUSIONS

This paper examined the various types of control systems for hydrostatic transmissions as applied in mobile machinery. Standard terminology for control types (i.e. Load Dependent and Load Independent) was introduced and examples of typical performance data provided. Controls were further defined by classifying their Input Signal (i.e. Mechanical, Hydraulic or Electrical). Secondary control functions such as pressure limiter and control over-ride are utilized in many machines where hydrostatic transmissions fulfill the role of propel.

The introduction of electronics (i.e. sensors and digital control) for hydrostatic transmissions provides the opportunity to design more complex system solutions. These “new” systems are actually built up from the basic control types and/or functions introduced and described at the beginning of this paper. In many cases, mechanical devices and control functions are being replaced by electronic solutions. Test data was provided comparing hysteresis for a “mechanical” Load Independent Control with that of an “electronic” solution. The “electronic” Load Independent Control system is comprised of a Load Dependent Control, swashplate angle sensor, and digital controller. The “electronic” solution provides improved hysteresis as well as the flexibility of either Load Independent or Load Dependent functionality. In fact, hydrostatic transmission components are available with integrated sensors and an embedded digital controller that provide a very high degree of flexibility in control function. These electronic systems with sensors and digital controller are enabling today’s machine designer (and the operator) to adjust control function and machine performance “on-the-fly” to optimum levels. This is needed more than ever due to the tightening Tier 4 regulations and demand for lower operating costs with increasing fuel prices.

REFERENCES

1) Sauer-Danfoss Literature, 520L0958, Rev BA,

October 2010, page 8.

2) Sauer-Danfoss Literature, 11037153, Rev CE,

Sep 2010, page 6.

3) Sauer-Danfoss Literature, 520L0635, Rev EJ,

October 2010, page 41, 58.

4) Sauer-Danfoss Literature, 520L0603, Rev FF,

August 2010, page 29.

5) Sauer-Danfoss Literature 11063344, Rev CA,

Jul 2010, page 14.

6) Sauer-Danfoss Literature 11063344, Rev CA,

Jul 2010, page 10.

7) Sauer-Danfoss Literature 11071849, Rev BB,

Oct 2010, cover page.

CONTACT

Craig Klocke is currently Leader of the Global Propel Systems Technology Team for Sauer-Danfoss and

based at the Ames, Iowa location. He has held various technical & leadership roles during his 30 year career at Sauer-Danfoss. His professional career started at John Deere Ottumwa Works. Klocke holds two patents and graduated from Iowa State University with a degree in Agricultural Engineering. Klocke may be contacted at cklocke@sauer-danfoss.com.

DEFINITIONS, ACRONYMS, ABBREVIATIONS

closed circuit - hydrostatic systems where the pump inlet is connected to the motor (or load) outlet

closed loop - a control system with feedback (used to create an error signal)

DC – Direct Current

DDC – Direct Displacement Control

EDC – Electrical Displacement Control

FNR – Forward, Neutral, Reverse

HDC – Hydraulic Displacement Control

HMI - human machine interface

hydrostatic system – transmission system consisting of hydraulic pump(s), hydraulic motor(s) and ancillary components (e.g. valves, filter, hose, etc.) using hydraulic fluid to transfer energy from a power source (i.e. engine) to drive a load (i.e. propel a machine)

MDC – Mechanical Displacement Control

Neutral – swashplate position for pump displacement of zero

NFPE – Non Feedback Proportional Electric

open circuit - hydrostatic systems where the pump inlet is connected to the reservoir

open loop – a control system without feedback (no error signal)

PLUS+1 – product family of digital controllers and HMI devices

PWM – Pulse Width Modulated

servo valve - a valve that controls the position of the servo system (i.e. swashplate) proportional to an input signal