汽车控制系统中螺线管电流的测量

Solenoids in Automotive Control Applications
A solenoid is a linear motor with a fixed range of travel. Solenoids may be designed for simple on-off applications, acting much like relays. For example, they are used this way in starters and door locks.

On the other hand, a linear, or proportional, solenoid is one whose position can be controlled in a precise manner. They are used to operate pistons and valves for accurate control of fluid pressure or flow in applications such as transmissions and fuel injection.

Transmissions require accurate and smooth control of pressure on clutches to change gears, and for controlling the locking torque converter. Electronically controlled transmissions may contain more than eight linear solenoids, all of which require smooth, accurate control. Common-rail diesel fuel-injection applications, with pressures in excess of 2000 psi, may require one linear solenoid per cylinder-and one at the fuel pump-to adjust pressure accurately to maintain predictable injector fuel flow.

Example: Electronic Transmission Control
The automatic transmission is one system in which electronic control is largely supplanting mechanical control because of improvements in drive quality and fuel efficiency. Previous improvements in fuel efficiency and acceleration came with the introduction of the locking torque converter. More recently, a combination of software and hardware using electronically controlled solenoids allowed easier adjustment of the shift algorithms, and provided additional benefits in transmission-shift smoothness and quality.

Overall, electronic control of the transmission allows for a simpler, more reliable, and less costly electromechanical system. Electronic transmission control systems improve the control of transmission shift points, with less abrupt gear shifting and improved shift smoothness. In addition, the flexibility of the electronic control allows for better adaptability to changing conditions. Electronic control of shift points with finer resolution allows better acceleration, improved economy, better load control, and reduced emissions, with minimal effort by the driver. In addition, the electronic control allows the transmission to shift more smoothly with varying load and acceleration.

With an electronic control system it is possible to affect the shift-control algorithm by a variety of inputs in addition to shaft speed, vacuum, and driver input. Some of these parameters include spark advance, injector parameters, input speed sensors, shift selection by wire, engine speed, throttle position, torque-converter speed/lock, ATF temperature, engine temperature, wheel-slip sensors, and inertial sensors. Combining these kinds of inputs allows a wide variety of shift optimization points, adapted to the overall operating conditions. To use these inputs most effectively, it is necessary to have a system benefiting by precise and infinitely adjustable electronic control of the shift points and shift speed.

Hydraulic control is still used to change gears in the electronically controlled automatic transmission. In contrast to the mechanical system, electronic control of the hydraulics in the electromechanical system is executed by linear solenoids that vary the hydraulic pressure applied to the actuators attached to the clutch packs. In order for this to work, it is extremely important to have accurate and repeatable control of the solenoid opening-which in turn allows for accurate, repeatable control of the shift points through the application of precise amounts of hydraulic fluid.

Determining Solenoid Position
The linear solenoid’s position is controlled in a feedback loop. For example, a valve’s downstream pressure can be monitored and used as a feedback signal to compare with the setpoint, adjusting the pulse-width modulation (PWM) duty cycle to control the solenoid. However, it may be difficult, impractical, or very costly to measure the downstream pressure.

A practical alternative is to establish the position of the solenoid by measuring the current through the solenoid. This is possible because the force imposed by the mechanical load on a solenoid is directly proportional to the magnetic field, which, in turn, is directly proportional to the current through the coil. Proportional control of the solenoid is achieved by a balance of the forces between the spring-type load and the solenoid’s magnetic field, which can be determined by measuring the current through the solenoid.

PWM Solenoid Control
The solenoid is powered by using a microcontroller-generated pulse-width modulated input signal to rapidly open and close a FET switch in series with the solenoid and a voltage source (the car’s battery). The average voltage is determined by the ratio of the waveform’s on time to the pulse period. Changes in the pulse width and the solenoid’s mechanical load cause the average current flowing through the solenoid to change. The average current is indicative of the amount of solenoid movement, and thus, fluid pressure and flow.

The relationship between solenoid movement and average current for a particular PWM waveform is established through characterization. While it is true that the magnetic force directly relates to the current through the solenoid, the actual mechanical force and movement are not so closely correlated, since they depend on the construction of the solenoid and the nature of the load. So, characterization is required to correlate the average current to the solenoid opening.

For example, the PWM ratio must be increased when the solenoid is first energized to overcome static friction. Once static friction is overcome, a different PWM relationship is used to move it in and out.

Measuring the Current Through the Coil
The current is thus an important indication of the solenoid’s state. The most effective method of measuring the solenoid current is to measure the voltage across a resistive shunt connected in series with the solenoid, the battery, and the switch. There are several different ways to configure this series circuit for switching and voltage measurement.

Low-Side Current Sense with High-Side Drive
The circuit in Figure 1 shows a switch, connected to the high (ungrounded) side of the battery, in series with the solenoid coil and the grounded resistive shunt. A reversed diode is connected across the coil to clamp (i.e., short-circuit) the inductive voltage generated by the coil when the current is turned off. Using a ground reference for the shunt allows an inexpensive op amp-with indifferent common-mode specifications-to be used in the electronic control unit (ECU) to measure the voltage across the shunt.