April 27, 2006



Hardware Control Strategies for the New Family of Electronically Controlled Automatic Transmissions

Joseph M. Funyak, K. Shawn Williams - Infineon

One of the major considerations for automotive design innovation over the past several years is the quest for improved fuel economy. The recent surge in the price of oil accentuates this motivation. Acceleration of the adoption of 5, 6, and 7-speed automatic transmissions over the typical 3 and 4-speeds that dominate today’s market is part of the equation to improve fuel economy. Along with adding more gears, additional, more complex electronics are required to ensure smooth operation and ultimately realization of the expected improvement in fuel economy. This paper reviews the changes in the market, as well as the control electronics for automatic transmissions. Also presented are possible solutions for tighter control and smoother shifting.

Introduction

The recent surge in oil prices and the subsequent increase in the price of gasoline have initiated new interest in better fuel economy from the consumer standpoint. This in turn motivates OEMs to implement new technology to improve fuel economy of their vehicles. Converting 3 and 4 speed automatic transmission to 5, 6, and even as many as 8-speed transmissions is a way the OEMs are improving the fuel economy of their product offerings. An added bonus is the improved driving experience for the customer experiencing a smoother shifting transmission.

Market Overview

Light vehicle production volume for 2005 (in millions) was approximately 16.1 in North America, 15.7 in Western Europe, 10.0 in Japan, 4.0 in China, 3.6 in Eastern Europe, 2.5 in South America, and 9.2 for the rest of the world. The rankings are projected to remain the same for 2011 with the volumes estimated to be 17.6 for North America, 16.6 for Europe, 10.2 for Japan, 6.9 for China, 5.3 for Eastern Europe, 3.1 for South America, and 10.9 for the rest of the world.

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Fig. 1: Global Light Vehicle Production

North America, Western Europe, and Japan are the largest producers of light vehicles. They also account for 85% (approximately 26 million) of the total automatic transmissions globally. In 2005, the percentage of vehicles produced in North America, Western Europe, and Japan with automatic transmissions as compared to their total production was 90%, 20% and 84% respectively. In 2011, the percentage for all three regions is projected to increase to the 92%, 24%, and 88%.

Globally, the 4-speed transmission was the dominant transmission in 2005 making up approximately 60% of all automatic transmissions. Five (5) speed transmissions were the next transmission of choice securing roughly 23% of the global automatic transmission market and the 6-speed transmission made up the third largest market share at 12% (see figure 2).

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Fig. 2: Global Automatic Transmission Gear Breakout

The North American market reflected a similar percentage breakout with 4, 5, and 6-speed transmissions making up approximately 65%, 29%, and 6% of the automatic transmission regional market respectively (see figure 3).

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Fig. 3: North America Automatic Transmission Summary

The Asian market was geared more heavily toward 4-speed transmissions with roughly 76% of the automatic transmission market for the region (Figure 4.). In contrast, the European market had a higher percentage of 5, 6 and 7-speed transmissions for all automatic transmissions in the region (combined 72%) with 4-speed transmissions making up only 28% of the market (Figure 5.).

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Fig. 4: Asian Automatic Transmission Summary

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Fig. 5: Western European Automatic Transmission Summary

Future Trends

The trend in North America is a move from 4-speed to 6-speed transmissions. The 6-speed transmission is expected to improve fuel economy by 3% to 7% depending on the type of vehicle [1]. Ford alone has 8 different vehicles among their Ford, Mercury and Lincoln lines that offer a 6-speed transmission on Model Year 2006 vehicles [2]. Ford has also announced a new 6-speed automatic transaxle that will mate up to a new 3.5-litre V6 on the Model Year 2007 Ford Edge and Lincoln MKX. This transmission was designed to work with engines producing up to 300 horsepower and 280 pound-feet of torque with the capability to shifting speeds up to 7,000 rpm [3].

General Motors introduced the 6L80 in 2005 (used on Cadillac XLR-V, STS-V, and Chevrolet Corvette). Also, GM recently announced it is investing $125 million to add capacity at the Ypsilanti, Michigan Transmission Plant to produce the new 6L50 6-speed Hydra-Matic automatic transmission [4]. GM plans to introduce 8 other 6-speed transmissions globally by 2010. They are projecting they will equip as many as 3 million vehicles with 6-speed transmissions in 2010.

DaimlerChrysler will enter the 6-speed transmission market in Model 2007 with the 62TE on the Sebring. This joins the 5 and 7-speed transmissions in the Mercedes-Benz lineup.

Lexus is taking the number of gears in an automatic transmission one more step further. Lexus will replace the LS430 with the LS460 in Model Year 2007. This vehicle will mate a new 4.6 liter V8 (estimated horsepower at 380 and torque at 370 pound-feet) with a new 8-speed automatic transmission [5].

Transmission Controller Architecture

Figure 7 is a block diagram of a typical 4-speed automatic transmission electronic controller. These functions are either part of an all encompassing Powertrain Control Module (PCM), or a separate Transmission Control Unit (TCU). The major functions illustrated include the micro-controller (typically 16 bit for a TCU or 32 bit for a PCM), sensor input with signal conditioning, communication circuitry (CAN or LIN), voltage regulation, and power circuitry to drive the various types of solenoids.

The types of solenoids typically found in these applications are on/off, Pulse Width Modulated (PWM), peak and hold, and Variable Force Solenoids (VFS). The on/off solenoids are controlled by the supply voltage being switched fully on or off. The PWM solenoids are controlled by a signal varying the time the solenoid receives full supply voltage over a set period of time. In other words, the on-time or pulse is varied or modulated within a particular frequency. Figure 6 depicts a typical peak and hold solenoid wave form. The initial current value, I1, is typically the current needed to move the solenoid armature and I2 is the sustaining current value.

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Fig. 6: Typical wave form of peak and hold solenoid control

Peak-and-Hold Parameters:

I1 = Peak current

I2 = Hold current

T1 = Peak time

T2 = Hold time

P = PWM period

The VFS is controlled by supplying a constant current to the solenoid. Thus this solenoid is sometimes referred to as a constant current solenoid. This control technique is used in order to control the position of the pin of the solenoid thus regulating the flow of hydraulic fluid through the path causing a specific pressure required for a

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Fig. 7: Typical US OEM 4 Speed Transmission Controller Architecture

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Fig. 8: Valve Assembly for GM Hydra-Matic 6L80 6-speed Automatic Transmission [6]

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Fig. 9: Typical US OEM 5/6 Speed Transmission Controller Architecture

particular function within the transmission (e.g. engagement of clutches).

The solenoid driver circuitry for US and Korean manufactured vehicles typically incorporates a low-side technique for switching power on and off for the solenoid control. This means a power switch, typically a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is connected in the electrical path between the solenoid and ground, and the other end of the solenoid, the high side, connected to the power supply. Automatic transmission controllers for Japanese vehicles typically incorporate the opposite type of solenoid driving technique, or high-side driver circuitry. The MOSFET is located in the electrical path between the power supply and the solenoid with the other side of the solenoid, the low side, connected to ground.

Another difference in the solenoid driver circuitry encountered is whether the MOSFET driver control is located within the micro-controller (software control) or located with in a separate Integrated Circuit (IC) specifically designed for solenoid control (hardware control). The software required to control the on/off and PWM solenoids is relatively small compared to the VFS. Each VFS can consume approximately 2% of the 16-bit micro-controller used in a TCU. For the typical 4-speed TCU, this does not present an issue; however, for the new 6-speed and higher gear transmissions, this can take up a significant portion of the computational power of the micro-controller.

Figure 8 above is an illustration of valve assembly for General Motor’s (GM) Hydra-Matic 6L80 6-speed Automatic Transmission. This transmission incorporates 6 constant current type solenoids [6]. Figure 9 above provides a block diagram of the how the TCU could be configured to control this valve assembly. This would equate to approximately 12% of the total micro-controller computational power if the solenoid control strategy were software based. A hardware solution or an IC dedicated to the control of these types of solenoids frees up the micro-controller for other computational functions within the controller. Just for GM alone, this means an approximate increase of 15 million constant current solenoids from the conversion of 4-speed to 6-speed transmissions by the year 2010.

VFS Driver Circuitry

Automatic transmission manufacturers are moving to Variable Force Solenoids in order to improve engagement control of the clutches thus improve sifting and ultimately drivability of the vehicle. This improved shifting is accomplished by precisely regulating the hydraulic oil pressure to the various clutches. The precise control of the hydraulic pressure is achieved through precise control of the VFS which requires precise current control from the driver circuitry. Figures 10 and 11 illustrate a typical open-loop, low-side configuration of the VFS application.

[pic] Fig. 10: Block diagram of an open loop pressure control with hysteretic current regulation of the VFS

[pic]Fig. 11: Block diagram of an open loop pressure control with fixed frequency current regulation of the VFS

Block description

1. Load: inductive load e.g. variable force solenoid

2. Shunt for providing a load current proportional voltage signal

3. Free wheeling element e.g. silicon diode, shottky diode or switched MOSFET

4. Electronic power switch e.g. MOSFET

5. Driver including protection and diagnostic circuitry for the power switch and a charge pump if a high-side switch topology is used

6. PWM generation used e.g. for a fixed frequency current regulation

7. Signal conditioning for the floating input signal voltage including level shifting, filtering and amplification of the signal

8. A/D converter for digital control topologies

9. Error correction of the input signal e.g. offset compensation

10. Current control block e.g. digital PI control or analog hysteretic control [7]

11. Block handling additional functions like superimposed dither current or symmetrical current clipping

12. Look up table or algorithm converting the required hydraulic pressure to an average current value

13. D/A converter for analog current control topologies

Figure 12 is a block diagram of the TLE7241E, a dual-channel, hysteretic, constant current control solenoid driver with integrated DMOS power transistors showing an external sense resistor and recirculation diode driving a VFS via low-side configuration.

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Fig. 12: Application circuit including block diagram of TLE7241E

The average load current of the TLE7241E can be programmed to a value in the range of 0 mA to 1000 mA (with a 1 Ω external sense resistor) with 10 bits of resolution. Load current is controlled using a hysteretic control scheme with a programmable hysteresis value. A triangular “dither” waveform can be superimposed on the switching current waveform in order to improve the transfer function of the solenoid. The amplitude and frequency of the dither waveform are programmable by the SPI interface. The device is protected from damage due to over-current, over-voltage and over-temperature conditions, and is able to diagnose and report open loads, shorted loads, and loads shorted to ground.

One of the major advantages of this part is dither control. This function is primarily used to minimize or eliminate the static friction of an armature of the solenoid at complete rest. This function is very difficult to reproduce via software in the microprocessor thus making it ideal for a hardware control IC.

Another major advantage is the accuracy of the device. Figure 13 is example data for voltage measured across an ideal 1 ohm resister at one of the output channels of the TLE7241E.

Fig. 13: TLE7241E Example data of voltage measured across an ideal 1 ohm resistor from -40oC to 120oC, center value 1,000 mV, major unit along the y-axis is 10 mV

Figure 14 represents a block diagram of the TLE7242G, a quad-channel, fixed-frequency, closed loop, constant current solenoid pre-driver in conjunction with external sense resistor, MOSFET, and recirculation diode driving a VFS via low-side configuration.

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Fig. 14: Application circuit including block diagram of TLE7242G

The average load current can be programmed from 0 mA to approximately 1.2 A with the TLE7242G in steps of 0.78 mA when a 0.2 Ohm external sense resistor is used. The four channels are independently controlled via a SPI interface. The PWM frequency, dither frequency, and dither amplitude are all programmable via SPI over a wide range. This high level of programmability enables the user to optimize the control of each solenoid design.

The average current is controlled using direct measurement of the load current as feedback to a PI controller. The output of the controller is a fixed frequency / variable dutycycle gate drive signal to the external MOSFET. The coefficients of the PI controller are programmable via SPI. This flexibility allows the user to set the desired step response characteristics according to the requirements of the system.

The TLE7242G also includes a full set of diagnostic and protection features. Short circuits to ground and to battery at the load are detected by the device, and communicated via SPI to the microcontroller. Open load conditions can be detected both in the “off” state and in the current regulation state.

Comparison of Control Methodologies

Figure 15 illustrates the effect of the inductance of the solenoid on the solenoid current waveform when hysteretic current control is used. Note that the outer envelope of the waveform is independent of the solenoid characteristics, whereas the switching frequency is not.

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Fig. 15 : Hysteretic Current Control

Figure 16 illustrates the effect of the inductance of the solenoid on the solenoid waveform using the PI controller method. In this case the switching frequency is independent of the load but the envelope of the waveform is not. Also note the slight “rounding off” of the triangular dither waveform due to the dynamic characteristics of the PI controller. Table 1 lists the relative advantages and disadvantages of the control methods.

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Fig. 16: PI controller with fixed frequency PWM

Comparison of MOSFET integration vs external MOSFET

There are both advantages and disadvantages with integrating the power MOSFET transistor with the control circuitry. Table 2 lists the relative advantages and disadvantages of these two approaches.

|Characteristic |Hysteretic |PI control |

| | |Fixed Freq. |

|Ease of Use |+ |- |

|Adaptability |+ |+ |

|Response Time |+ |+* |

|Accuracy of |- |+ |

|Current Control | | |

|Accuracy of |+ |- |

|Dither Amplitude | | |

|Accuracy of |+ |+ |

|Dither Frequency | | |

|Control of |- |+ |

|PWM Frequency | | |

Table 1: Comparison of Control Methods.

* when transient mode is enabled.

Note: + means advantage and not greater than.

|Power MOSFET |External |Internal. |

|Ease of Use |- |+ |

|PCB Area |- |+ |

|Slew Rate Control |- |+ |

|Scalability |+ |- |

|Unused channel cost |+ |- |

|Diagnostics |- |+ |

|Cost (Silicon BOM)* |+ |- |

|Power Dissipation |+ |- |

Table 2: Comparison of Power MOSFET Integration.

* not considering PCB area and manufacturing costs

Note: + means advantage and not greater than.

Conclusion

This paper started by discussing the current global market noting approximately 85% of the automatic transmissions are produced in North America, Europe, and Japan. Further, the trend for these regions is for this percentage to increase slightly by 2011. The other important trend identified for this time period is the migration from a 4-speed gear set to a 6 and higher-speed gear set transmissions. This is primarily due to the improvement in drivability (smoothness of shifting) and fuel economy (4-speed vs. 6 and higher-speeds). The impact to the overall system, in addition to the higher gear set, is the types and number of solenoids needed to operate these new transmissions. The new 6 (and higher)-speed transmissions incorporate multiple Variable Force Solenoids (VFS) instead of the typical On/Off or Peak-and-Hold type solenoids dominant in the 4-speed automatic transmission. This is a significant increase of VFS for the future; a potential of 15 million VFS for GM alone.

In order to control these solenoids to the accuracy required to achieve the improved shifting qualities, new integrated circuits (IC) were designed and developed. Two different control methodologies, hysteretic and fixed frequency, were discussed. Both of the ICs discussed have the control strategy built-in thus off loading the microprocessor with this burden of control (hardware vs. software). Also included was a table of advantages/disadvantages of the integration of various features (such as MOSFET drivers) in the new solenoid driver ICs.

Overall, the feature set and different control strategies available in the new ICs provide greater flexibility for the automatic transmission system designer. Realization of the full potential of these ICs will become more apparent as the new 6-speed transmissions are introduced in the marketplace.

References:

[1] J. B. White, N. Shirouzu, Gear Wars: The Race for an Eight-Speed Car, The Wall Street Journal online, August 18, 2005.

[2] , June 6, 2006.

[3] J. Fossen, Ford Hybrids Get V6 Engine While Crossovers Get New Six-Speed Transmission, Ford Communications Network,, November 3, 2005.

[4] B. Hampton, GM to Launch New Six-Speed Automatic at Michigan Plant, Autotech Daily, May 31, 2006.

[5] Motor Trend Magazine on-line,, June 15, 2006.

[6] GM Service Document ID#1701750 for CadillacSTS,, September 2, 2005.

[7] Kelling, N., Katzmaier, E., Zarcone, A., Cost efficient partitioning for new generation of automatic transmission gearbox controllers, SAE-2006-01-0403

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Pulse Width Modulated Driver

With Protection and Diagnosis

On / Off (X 2-3)

PWM

Constant Current

Temperature Sensor

Pressure Transducers

Pressure Switches

PRNDL switches

Speed Sensors (VR or Hall)

On/Off Driver

With Protection and Diagnosis

Constant Current Driver

With Protection and Diagnosis

CAN / Transceiver

Signal Conditioning

Voltagel)

On/Off Driver

With Protection and Diagnosis

Constant Current Driver

With Protection and Diagnosis

CAN / Transceiver

Signal Conditioning

Voltage Regulator

μController

High Side Switch

With Protection and Diagnosis

Constant Current Driver

With Protection and Diagnosis

On/Off Driver

With Protection and Diagnosis

With Protection and Diagnosis

On/Off

(X 2)

Constant Current

5 to 7

Temperature Sensor

Pressure Transducers

Pressure Switches

PRNDL switches

CAN / Transceiver

Signal Conditioning

Voltage Regulator

μController

Speed Sensors (VR or Hall)

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