During my first visit to the U.S., my brother and I drove from New York City to Columbus, Ohio. Although the 650-KM (400-mile) journey was long, our car’s automatic transmission and cruise control made the trip more comfortable. The effectiveness of an automatic transmission and cruise control relies on the precise control of proportional solenoids and motors, which is made possible with accurate current measurement.
Proportional solenoids convert electrical signals into a proportional mechanical force, which allows variance in armature position and force relative to the level of current. Proportional solenoids are used in automotive applications such as automatic transmissions, fuel injection systems, pistons and valves to accurately control position or flow. Controlling proportional solenoids precisely is highly dependent on the accuracy of the position estimation over the automotive temperature range. In general, measurement is used to provide position for proportional solenoids.
As shown in Figure 1, a shunt resistor is inserted in between the switch and the solenoid. The low-ohmic shunt is inserted in such a way to create a low resistance path as well as safely measure and accommodate recirculation current flows. An amplifier in a differential configuration or current sense amplifier is connected to the shunt to measure the current.
Figure 1: Solenoid current measurement
A pulse width modulation (PWM) signal operates the high-side switch. When the PWM is high, the battery connects to the solenoid and current flows. When the PWM is low, the battery disconnects from the solenoid and recirculation current flows through the flyback diode.
The control of the PWM frequency and duty-cycle percentage determines the resulting average current in the solenoid, which in turn controls the force applied to the actuator.
Sensing accuracy Nominal current ranges for solenoids vary from device to device. Because of differing current range needs, current sense amplifier selection for these types of systems is critical as the necessary precise movement of the entire solenoid system depends on the accuracy of the current measurement.
Solenoid current sensors can be designed with external shunts or integrated shunt devices depending on the accuracy requirement. Each has their own advantages. Tables 1 and 2 summarize the differences and benefits of these two architectures.
External shunt devices
Integrated shunt devices
Lower-current solenoids, because it is possible to interface with a higher-value external shunt resistor
Higher-current solenoids, as the integrated shunt is 2 mΩ. Choose based on your accuracy requirement.
Differential configuration register placement
Shunt tolerances independent of device tolerance specifications, with considerable drift over the temperature range
Shunt tolerances included in device tolerance specifications, implying less drift over the temperature range
-4 V to 80 V
Table 1: Differences for external and internal shunt devices.
Enhanced PWM rejection system-level benefit
Inclusion of flyback period in measurements.
Required wide common-mode range to accommodate the solenoid flyback period
Low, in order to reduce voltage swings during state transitions
Relatively lower price (excluding shunt price)
Moderately higher (including shunt price)
Relevant TI reference design
Reference Design for Automotive, Proportional-Solenoid Current Sensor
Automotive Proportional Solenoid Drive with Highly Accurate Current Sensor Reference Design
Table 2: Benefits of external and integrated shunt devices.
Figure 2 and 3 shows the configurations of shunt for external and internal configurations. In Figure 2 shunt is external to the device (INA240-Q1) and in Figure 3 shunt is internal to the device (INA253-Q1).
Figure 2: External shunt configuration with INA240
Figure 3: Internal shunt configuration with INA253
Maintaining solenoid current sensing accuracy
The impedance of a solenoid comprises inductive and series resistive components. The series resistance of the coil (typically copper) has a high-temperature drift of 4,000 ppm/°C. This high drift value may correspond to a 40% accuracy reduction in solenoid motion control over a 100°C range.
Accuracy at lower current ranges is essential over the entire temperature range mainly because lower currents are noise prone and there is a more chance for the solenoid position inaccuracies. Smooth operation of hydraulic systems requires a minimum 2-mA resolution. Variations in this resolution cause solenoid position inaccuracies.
TI has two devices designed for solenoid current sensing applications:
These devices have increased the effectiveness of our powertrain, motor-control circuits and automotive battery systems.
Conclusion Automotive solenoids need higher accuracies to compensate for wider temperature ranges and increased drift as the temperature increases. Our current-shunt monitors enable accurate readings across the entirety of automotive temperature ranges.
All content and materials on this site are provided "as is". TI and its respective suppliers and providers of content make no representations about the suitability of these materials for any purpose and disclaim all warranties and conditions with regard to these materials, including but not limited to all implied warranties and conditions of merchantability, fitness for a particular purpose, title and non-infringement of any third party intellectual property right. No license, either express or implied, by estoppel or otherwise, is granted by TI. Use of the information on this site may require a license from a third party, or a license from TI.
TI is a global semiconductor design and manufacturing company. Innovate with 100,000+ analog ICs andembedded processors, along with software, tools and the industry’s largest sales/support staff.