S45 ETL with Angle Sensor (Part 2)

Abstract Previously on iHydrostatics N7: S45 ETL with Angle Sensor (Part 1) has shared the basic information about Electronic Torque Limiting solution presented by Danfoss S45 product. Simply reply N7 in Wechat iHydrostatics to get the history post if you missed it. From the introduction of ETL concept, features and outline, we can get a whole view of this new solution. Any questions, comments or suggestions are warmly welcomed, iHydrostatics encourages you to leave your comments in iHydrostatics QQ Group: 426372753. iHydrostatics will be always by your side.


In this post, iHydrostatics will disclose more secrets of Danfoss S45 ETL in part 2. Hope by introducing of ETL hydraulic circuit, control principle and difference between other similar solutions, you can get the complete picture of this new technology.

1How ETL WorksHydraulic Circuit
ETL solution consists of three main parts: angle sensor which is mounted on the pump housing; normal load sensing/pressure compensated control valve and proportional pressure valve which is integrated into the LS/PC valve as shown below in the hydraulic circuit. Meanwhile, an orifice is also mounted at the X-port.

S45 Electronic Torque Limiting Controls require the use of an LS Signal Line Orifice. The LS Signal Line Orifice is used to enhance system stability, as well as allow the limiting of torque throughout the full range of pump pressure and flow. The standard 1.0mm orifice is best suited for use in a wide variety of applications.
Control Characteristics
In order to figure out how ETL works, the characteristics of PRV and angle sensor should be clarified first.

PRV Characteristics
Proportional Pressure Valve is normally closed and a negative relationship between input current and limiting pressure is built accordingly. This means if no input current to the PRV solenoid valve, the max. limiting pump pressure is set by Pressure Compensated valve; and the pump limiting pressure decreases proportional to an increase in current.

Angle Sensor Characteristics
The angle sensor package incorporates two sensors, allowing two analog signal outputs for improved accuracy and troubleshooting. The sensor is mounted to the right side of the pump, when looking at the pump from the input shaft. This mounting configuration yields a unique voltage versus swash plate angle characteristic curve shown below.

Control Principle

Pump torque consumption is a function of pump outlet pressure, pump displacement, and pump mechanical efficiency. When pump mechanical efficiency is considered constant, the pump torque can be limited when pump displacement is known and pump pressure is controlled. As pump displacement increases, the pump outlet pressure can be limited using the PRV to result in a constant torque limit. Pump outlet pressure is equal to the load sense pressure, which is limited with the PRV, plus the margin pressure setting of the pump.

The Electronic Torque Limiting control allows users to limit pump torque consumption electronically by combining a pressure limiting PRV and angle sensor. This torque limit can be changed with varying engine speeds (as shown in the Electronic Torque Limiting graph below), allowing the use of full engine torque at all engine speeds and increasing machine productivity.

A microcontroller is required to store engine torque vs. speed, receives the pump angle sensor signal, and then calculates and output the pump outlet pressure limit. The basic torque limiting control logic for a single engine speed is shown below.

2ETL Comparison

ETL vs. MTC
As we know, MTC (Mechanic Torque Control) consists of swash plate angle feedback mechanism and a pressure relief valve as well. MTC normally uses two springs (For example A10VO pump) to simulate the ideal torque curve (except the complicated mechanism like A11VO), in this way, the actual torque limiting during the whole operation range does not keep constant; while ETL can perform perfect ideal torque control curve as shown above.

On the other hand, MTC torque limiting acts AFTER the load pressure is beyond the limited setting, it means this kind of control is post-active; while ETL calculates the real-time max. limited pressure based on the swash plate angle feedback, so it reacts more quickly than MTC, this kind of control methodology is proactive.

ETL vs. Rexroth EK Control
Rexroth EK control can also perform ideal torque limiting function based on the EDC. While normally a pressure transducer is integrated into such solution. The pressure transducer sends the pressure signal to ECU, then calculated current is sent to proportional control valve on the pump to adjust the swash plate angle. The main difference between ETL and EK is that : EK senses the pressure first and then change the pump displacement; while ETL senses the pump displacement first and then change the max. pressure limiting setting.

The big advantage of ETL is the proactive action, it does not act after knowing the pressure, and it just calculates the real-time max. permitted pressure. The torque control function response time is just depended on the pump own response capability.