Auto Electronics

Mechatronic Test Making Those Difficult Power Driver Measurements

Mechatronic control systems are the ideal synergy.

Mechatronics is the combination of mechanical, electrical, and software technologies to achieve the performance goals of the system. This combination of technologies forms an ideal synergy, with each contributing key elements to the overall goal of greater performance. The mechanical hardware provides the heavy lifting: force, torque, and mechanical displacement. The software provides the intelligence and finesse, and the electronics binds it all together — efficiently coupling the software domain to the mechanical world.

Mechatronic control systems rely on electronics to provide control to mechanical systems by virtue of power outputs driving inductive loads. Due to the mission critical nature of these power drivers, failure modes that jeopardize the vehicle and passengers must be addressed early in the design, and performance carefully verified during the manufacturing and final assembly process.

These high-power drivers present unique design validation and manufacturing challenges for engineers. This article will discuss these testing challenges, and present measurement methods to help you succeed in your next Mechatronic challenge.

THE ANATOMY OF A MECHATRONIC SYSTEM

Figure 1 shows the block diagram of a mechatronic system. Central to the Mechatronic system is the electronic control unit (ECU), which usually contains a microprocessor for advanced decision management and control for the system. The ECU receives inputs from a variety of sensors. Each sensor has an input buffer designed specifically to interface the external world signals to the electrical world within the ECU.

Once the microprocessor processes the data, mechanical control is achieved via output driver stages. These drivers can take on many forms and are usually constructed from MOSFET or IGBT semiconductor devices. This article will focus on these drivers, how they work, their test challenges and methods to address those challenges.

ON/OFF SOLENOID DRIVER OPERATION, KEY ATTRIBUTES, AND FAILURE MODES

Let's start with the basics — a simple on/off style driver (Fig. 2). Notice the solenoid is driven with a low side driver (drive low side of solenoid to ground). Low side drivers are common, but high side drivers are also used. The rest of this discussion will reference a low side driver, although the concepts apply to low and high side drivers.

Notice the waveform profile of the low side driver (Fig. 2). The resting “off” state is at the battery voltage. When the low side driver is turned on current begins to flow through the solenoid winding.

When the solenoid driver is turned off, the magnetic field must collapse before the solenoid moves. To collapse the field as fast as possible and achieve the fastest switching speed it is common to allow the inductive fly-back voltage to rise to a high voltage level. The time to dissipate the magnetic field is proportional to the fly-back clamp voltage. However, too high of voltage may cause damage to the output device. As such, the voltage is generally clamped to a safe level, generally around 100-150 volts.

Let's discuss the blocks in Figure 2: (1) the output device is generally a MOSFET or IGBT. (2) The fly-back clamp can be active as shown in this diagram, or a simple semiconductor clamp tied to ground. (3) A snubber circuit is used to reduce ringing due to the solenoid inductance and stray capacitance (4) over current detection is commonly incorporated in solenoid drivers.

Failure modes to be aware of include:

  • higher than expected on-voltage of the driver (Vdson) or poor turn-on times;
  • incorrect fly-back clamp voltage (Vpk) due to missing or defective clamp diode;
  • ringing of output due to missing or defective snubber component; and
  • missing or incorrect overcurrent detection and protection.

PWM PROPORTIONAL SOLENOID DRIVER

While the On/Off solenoid provides two discrete settings for the solenoid, there are other applications where a “smoother” continuous control of the solenoid is required. For example, consider an automatic transmission hydraulic control solenoid. A proportional solenoid can be used to “feather” in the hydraulic pressure, proving a smoother shift.

Proportional solenoids are driven with a pulse width modulated (PWM) signal, and the solenoid placement is proportional to the PWM duty cycle.

The proportional solenoid driver is similar to the On/Off solenoid with a few changes. First, a proportional, integral, and differential (PID) control block is commonly used for closed loop control of the proportional solenoid. Second, the clamp diode for the inductive flyback differs from the On/Off solenoid driver version since it clamps the voltage of the fly-back to one diode drop above V+. This subtle, but significant difference results in a gradual decay of the magnetic field. By decaying the field gradually it has an averaging effect and allows proportional setting of the solenoid position by adjusting the duty cycle. This is referred to as a free-wheeling diode.

PWM driver failure modes to be aware of include most of the same failures that can occur with n on/off solenoid driver as well as a couple more:

  • a defective PID control or current sense element results in incorrect average current settings;
  • a missing free wheel diode will cause the solenoid current to decay quickly and results in a choppy current waveform.

MOTOR DRIVERS — UNIQUE DRIVER APPLICATIONS

Electric motors can be driven with low side, high side and H-bridge driver configuration. Also, the H-bridge driver can drive multiple phases. In simple cases the motor drivers can be on-off type drivers and in other cases the drivers may be pulse width modulated.

Stepper motors may use fractional step and micro-step techniques to achieve smoother rotation and consistent torque. This is achieved by pulse width modulating the motor drive, commonly known as chopping controlled current drive. Driving two or more windings of a stepper motor this way allows precise control of the magnetic field (magnitude and direction) so that the rotor placement can be controlled.

TEST METHODS FOR MECHATRONIC DRIVER STAGES

When testing the On/Off solenoid driver it is common to attach a representative load to the output driver and simply toggle the output on and off (Fig. 2). Capture the resulting waveform using a high-voltage digitizer with high dynamic range. Sample rate must be sufficient to capture the fly-back event, which may be as short as 10 uS.

Characteristics of interest include driver on voltage (Von), fly-back peak voltage (Vpk) and pulse width (tpw), driver turn-on time (ton) and ringing characteristics as shown in Figure 2.

To test the output overcurrent detection, place a direct short load from the driver to the V+ source. In Figure 2 this is accomplished by closing Relay A. Turn on the output driver while monitoring the current. This is a fast event and the driver should turn off in less than 100 micoseconds. Also, notice the current shunt is pulled up to V+ and as a result a differential probe or isolated input stage is required. Care must be taken to protect the test system in the event of a defective driver that fails to limit the current as expected. To be sure, power the test using a current limited power supply.

Testing PWM stages present some additional challenges beyond what we've discussed. In the event of a missing or defective free-wheel diode, instead of recirculating the energy of the magnetic field (which results in a slow current decay and averaging effect), the resulting current can be a choppy series of current spikes. In the event the PID control loop or current sense is defective, the proportional driver output will be set to an incorrect level: It may rail high, low, or somewhere in-between. To test a PWM driver, it is valuable to measure the driver voltage as well as the solenoid current. The current shunt may be used to do this.

Testing PWM stepper and servo motors can be challenging. To fully characterize the drivers and motors in operation it is valuable to properly instrument the driver stages to measure motor voltage and currents. This can be difficult when measuring an H-bridge driver, since the drive signals are differential. To measure drive voltage directly across the two sets of windings either isolated or differential instrumentation is required. To measure winding current, a current shunt or a differential current probe needs to be used.

As simple as these measurements may appear, instrumentation with special attributes are required to efficiently and accurately make these measurements. Consider the following:

On/Off solenoid drivers:

  • Fly-back voltages may reach over 200 volts and durations as short as 10 uS. Instrumentation needs to handle these high voltages and fast events.
  • Driver on-voltage may be in the range of several hundred mVs. An instrument with high dynamic range and low input voltage offset is required to capture driver on-voltage and fly-back in a single pass.
  • Driver short circuit current limit is best measured with a current shunt, and proper shut-down typically occurs in 40-100 uS. As such, an isolated, high-speed digitizer stage is best to use for this measurement.

Proportional solenoid drivers:

  • Driver stage current needs to be measured and that means using a current shunt or probe. Current shunts will be at voltages other than ground, so an instrument with isolated or differential inputs may be required.

Motor drivers:

  • Drive signals from H-bridge driver are inherently differential. Instrumentation needs to have differential or isolated input stages.
  • Current shunts or probes may be necessary to verify proper currents in the motor. Since the drive signal may be switching, AC common mode performance is a concern.

MAKING THE MEASUREMENTS: CAPTURING THE DRIVER WAVEFORMS

Figure 3 shows a waveform capture of an on/off solenoid driver stage. This driver is similar to what would be used for a fuel injector. Channel 1 shows the current as measured via the 5 0 mOhm current shunt resistor, and Channel 2 shows the voltage including the 164 V peak fly-back.

In order to make these measurements, you need a digitizer with a high input voltage range and isolate or differential inputs. The waveforms shown in Figure 3 were made using the Agilent Technologies LXI L4534A (4 channel, 20 MS/sec, 16 bit), which is designed for these mechatronic style measurements.

Notice the close up of the on state voltage of the driver stage. In this case, the on voltage is 600 mV. Because of the 16-bit dynamic range and low input offset of the digitizer, the high 164 V fly-back, as well as the 600 mV driver output on-voltage can be measured at the same time, improving test times.

Mechatronic control systems rely on electronics to provide control to mechanical systems by virtue of power outputs driving inductive loads. Failure modes that jeopardize the vehicle and passenger must be addressed early in the design, and performance carefully verified during the manufacturing and final assembly process. Using the measurement concepts outlined in this article will help you get the information you need during your product development and manufacturing test challenges. Instrumentation attributes required for these types of measurements include:

  • high voltage input range (up to 250 Vpk);
  • low input offsets (less than 100 mVs);
  • high resolution (16 bits);
  • sample rates/bandwidths to capture fast flyback events (20 MS/sec); and
  • deep memory to capture long, stepper motor pulse trains (up to 100 MSamples).

Selection of the correct instrumentation is vital to making these measurements. The Agilent L4534A 20MS/sec 16 bit +/- 250V digitizer is ideal for making these mechatronic driver measurements.

ABOUT THE AUTHOR

Al Lesko is an Application Engineer for Agilent Technologies Systems Products Division He joined Agilent Technologies (Hewlett-Packard) in 1980 working as an R&D engineer. Lesko holds a BS degree in Electrical Engineering from the University of Michigan, Ann Arbor, MI, and a BA in Math & Physics from Albion College, Albion, MI.

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