795F AC Off-Highway Truck: Sensor Inputs, Switch Inputs & PWM Sensor Inputs

Input components that the Drivetrain ECM will use for system control

Note: This section of the manual will cover the components that provide an input circuit to the Drivetrain ECM. These circuits and components are located outside of the Inverter Cabinet. The components inside the Inverter Cabinet that provide input signals primarily to each motor ECM are covered in the Systems Operation, "Electric Drive System" section of this manual.

The Drivetrain ECM will utilize various types of devices to provide status information for the systems that are under control. The ECM will use this information to meet the operational requirements of the electric drive system based on programmed memory and software parameters.

All of the system input components in this section that supply inputs to the ECM fall into one of the following groups: sensor type inputs and switch type inputs.

The ECM will monitor most of the circuits of the input components for diagnostics. If the ECM determines that an abnormal condition exists in one of the circuits, the ECM will log a diagnostic code or an event code. Sensors or switches that are not monitored for diagnostics will be noted.

Sensor Inputs

Sensors provide an electrical signal to the ECM that constantly changes. The sensor input to an ECM can be one of the several different types of electrical signals. The types of sensor signals are:

Pule Width Modulated (PWM) Sensors

When powered up, PWM sensors continuously send a pulse width modulated square wave signal to the ECM. The voltage of the signal ranges between 0 VDC and 5 VDC. The ECM monitors the voltage and the duty cycle of the signal.

The PWM duty cycle is the percentage of time that the signal is high as compared to the time interval of one complete square wave cycle (hertz). The voltage of the signal corresponds to the duty cycle of the signal. A higher duty cycle results in a higher signal voltage.

The operating frequency of the signal for most PWM position sensors is approximately 500 hz. However, some PWM sensors operate at a frequency of approximately 5000 hz.

An ECM will monitor the duty cycle, the voltage, and the frequency of the PWM signal. The voltage of the signal is used by the ECM as a status indicator for the sensor signal. The measurement of the signal duty cycle using a multimeter is used by the technician to determine the signal status.

Position sensors are the most commonly used type of PWM sensor. Any movement on the axis that is attached to the sensor will change the duty cycle and the voltage of the sensor output signal. The signal frequency can also change slightly, but the frequency change should not be great. The duty cycle of the PWM sensor signal changes according to the direction and amount of movement on the axis.

When an axis is moved, the ECM will interpret a specific duty cycle or voltage as a specific axis position. The ECM will determine the position based on the detected travel limits of the axis and the PWM duty cycle for those limits. The ECM determines the axis limits either by a manual calibration procedure that is performed by an operator or by an automatic calibration procedure that is performed by the ECM usually at machine startup.

The duty cycle signal from a typical position sensor that the ECM will recognize as valid is 10 ± 5 percent to 90 ± 5 percent at the extreme ends of the axis movement. A joystick, pedal, or actuator that is in the center or neutral position would result in a duty cycle signal of approximately 50 ± 5 percent.

An internal pull up voltage is present at all PWM input circuits in the ECM. If the voltage signal is interrupted due to an open circuit, poor connections or the loss of supply power, the signal circuit will be pulled high. The ECM will activate a "voltage above normal" diagnostic code.

Voltage Input Sensors

Voltage input (active analog) type sensors provide a voltage input signals to the ECM that generally range between 0 VDC to 5.0 VDC. Active analog sensors are normally used for measuring pressure. The ECM will associate a specific voltage to a specific value for the medium that is being measured.

Most active analog sensors are powered by the ECM 5.0 VDC power supply and return circuits.

An internal pull up voltage is present at all analog input circuits in the ECM. If the voltage signal is interrupted due to an open circuit, poor connections or the loss of supply power, the signal circuit will be pulled high. The ECM will activate a "voltage above normal" diagnostic code.

Resistive Input Sensors

Passive analog type sensors provide a resistive input signal to the ECM. the ECM also monitors the voltage of the circuit.

This type of sensor is normally used for temperature sensors. The ECM will associate a specific circuit resistance to a specific temperature value for the medium that is being measured

Most passive analog sensors are powered by the ECM 5.0 VDC power supply and return circuits.

An internal pull up voltage is present at all resistive input circuits in the ECM. If the voltage signal is interrupted due to an open circuit, poor connections or the loss of supply power, the signal circuit will be pulled high. The ECM will activate a "voltage above normal" diagnostic code.

Frequency Inputs

Speed sensors provide a frequency input signal to the ECM. Most of the speed sensors that are used on late model machines are "fixed mount" type speed sensors. No adjustment of the sensor is required once the sensor is installed.

A magnetic coil in the sensor creates a square wave voltage signal when a ferrous metal object, generally a gear tooth, is passed under the sender tip. The ECM will determine the number of square wave signals (frequency) of the sensor circuit in order to determine the speed of the gear that is being monitored.

Switch Inputs

Switches provide input signals to the ECM. When a switch is moved to a position that will cause the switch contacts to close, an open signal, a grounded signal, or a voltage signal will be detected on the ECM input circuit.

Switch to Ground / Voltage Inputs

Switches will provide one of the following types of input signals to the ECM:

An open signal
A ground signal
A voltage signal

The contacts of a switch have two contact states. The switch contacts are open or the switch contacts are closed.

When switch contacts are open, no signal is provided to the corresponding input of the ECM. The "no signal" condition is also referred to as floating.
When switch contacts are closed, either a ground signal or voltage signal is passed through the switch contacts to the corresponding input of the ECM.

Switch to ground type input circuits have an internal ECM "pull up voltage" that is present at the ECM contacts. An above normal voltage is internally connected to the ECM input circuit through a resister. This pull up voltage allows the ECM to detect a problem in the switch circuit. During normal operation, the switch signal will hold the circuit low. However, circuit conditions such as a disconnection or an open circuit will allow the circuit to be pulled high by the ECM pull up voltage. The result will be an above normal voltage condition at the ECM contact. When the ECM is expecting the circuit to be low, the ECM will activate a diagnostic code for the circuit.

Switch to battery type input circuits have an internal ECM "pull down voltage" that is present at the ECM contacts. A below normal voltage is internally connected to the ECM input circuit through a resister. The pull down voltage allows the ECM to detect a problem in the switch circuit. The circuit will be at the system voltage level when the switch contacts are closed. If the circuit is open or has a bad connection, the pull down voltage will pull the circuit low. When the ECM is expecting the circuit to be high, the ECM will activate a diagnostic code for the circuit.

Many switches often provide two inputs to the ECM. Generally, the two inputs are switch to ground type inputs. In each switch position, one of the inputs will be grounded and the other input will be floating. If the ECM determines that both of the inputs are grounded or are floating at the same time, the ECM will activate a diagnostic code for the switch.

PWM Sensor Inputs

Excitation Field Regulator - Current Feedback Circuit, Diagnostic Feedback Circuit


Excitation Field Regulator (EFR) diagnostic feedback circuit connections (bold) and current feedback circuit connections (heavy bold).


Excitation Field Regulator harness connectors (view: under the Generator looking from the left side of the cooling air duct). (1) 14 contact control circuit harness connector(2) 4 contact exciter circuit harness connector

Note: During machine operation, the Excitation Field Regulator (EFR) supplies 144.0 DC volts to the Generator exciter windings through current drive circuits that are connected between the EFR and the Generator low voltage enclosure. These circuits are connected in the four contact exciter circuit harness connectors at the EFR and at the Generator auxiliary enclosure. After machine shutdown, these circuits can contain stored capacitive hazardous voltage levels for approximately 5 minutes. After Engine shutdown, wait for at least 5 minutes before disconnecting the four contact exciter circuit connectors at the EFR and at the enclosure or before entering the Generator auxiliary enclosure. Always use a multimeter to verify that there is 50.0 VDC or less at any exposed exciter circuit contacts before any other action is taken.

The Excitation Field Regulator (EFR) is used by the Drivetrain ECM to control the three phase voltage output of the Generator.

Based on commands from the Drivetrain ECM, the EFR will send a 0 amp to 20.0 amps electrical current to the generator exciter winding. The exciter current will result in a controlled variable voltage output from the Generator. For more information on how the Drivetrain ECM controls the EFR, refer to the Systems Operation, "Electric Power Generation" section of this manual.

The EFR will self-monitor operation and detect abnormal conditions. The EFR will use the following output circuits to provide feedback to the ECM that will indicate the status of the EFR system.

EFR Diagnostic Feedback Circuit

The EFR will use a PWM diagnostic feedback signal circuit and a return circuit to indicate operational status to the Drivetrain ECM. The Drivetrain ECM will use the 5.0 VDC sensor power supply to provide the power for the feedback circuit.

The EFR will adjust the duty cycle of the PWM signals on this circuit to indicate the operational status to the Drivetrain ECM.

The following table lists the PWM duty cycles that the EFR will use for diagnostic feedback and the indication to the Drivetrain ECM.

Table 1
EFR Diagnostic Feedback PWM Signals
PWM Duty Cycle Percentage	Indication to the Drivetrain ECM
Less than 5%	Feedback line short to ground.
10%	EFR not enabled, power, and PWM are received - waiting for "EFR Enable" command.
20%	EFR in Standby - Internal boost voltage is high with output command disabled.
30%	Input voltage out of range
40%	Output short circuit - Output shorted high or low
50 %	Normal operation - No faults present.
60%	Output open circuit.
70%	Output current too high
80%	PWM command from Drivetrain ECM out of the normal range (3% to 97%)
90%	Undefined internal EFR fault.
100%	Feedback circuit shorted to another voltage source.

The following points should be considered if a problem is detected with the EFR diagnostic feedback circuit or with the operation of the EFR:

If the PWM duty cycle of the diagnostic feedback circuit is 5 percent or less, the Drivetrain ECM will activate a CID 2800, FMI 04 for a diagnostic feedback circuit short to ground.
If the PWM duty cycle of the diagnostic feedback circuit is 100 percent, the Drivetrain ECM will activate a CID 2800, FMI 03 for a diagnostic feedback circuit short to a higher voltage.
A loss of the Drivetrain ECM 5.0 VDC sensor power supply to the EFR will cause the diagnostic feedback circuit and the current feedback circuit to go to a high voltage condition (FMI 03). The ECM will activate a CID 2008, FMI 03 diagnostic code for the diagnostic feedback circuit AND a CID 3500, FMI 03 diagnostic code for the current feedback circuit. If both of these codes are active at the same time, verify that the Drivetrain ECM 5 VDC power supply is present at the harness connector for the EFR before any other action is taken.
During normal operation with the key start switch in the ON position and the Engine not operating, the feedback PWM duty cycle is expected to be 10 percent. When the Engine is operating, the PWM duty cycle is expected to be 50 percent. Any other PWM duty cycle on the diagnostic feedback circuit indicates that the EFR has detected a problem.

EFR Current Feedback Circuit

The EFR continuously monitors the 0 amp to 20.0 amps electrical current output circuit that is being sent to the Generator exciter winding.

The EFR will use a current feedback circuit and a return circuit to send a PWM duty cycle signal to the Drivetrain ECM. the signal will indicate the actual measured excitation current output that is being sent to the Generator exciter windings

The Drivetrain ECM will use the same 5.0 VDC sensor power supply that is used for the diagnostic feedback circuit to provide the power for the current feedback circuit.

The acceptable duty cycle range of the current feedback signal is 3 percent to 97 percent. The duty cycle of the signal will correspond to the 0 amp to 20.0 amp EFR current output.

The following points should be considered if a problem is detected with the EFR current feedback circuit or with the operation of the EFR:

If the current feedback signal is lost due to an open circuit, a pull up voltage at the ECM contact will result in a high voltage condition on the circuit. The Drivetrain will activate a CID 3500, FMI 03 for the current feedback circuit.
A loss of the Drivetrain ECM 5.0 VDC sensor power supply to the EFR will cause the current feedback circuit and the diagnostic feedback circuit to go to a high voltage condition (FMI 03). The ECM will activate a CID 3500, FMI 03 diagnostic code for the current feedback circuit AND a CID 2008, FMI 03 diagnostic code for the diagnostic feedback circuit. If both of these codes are active at the same time, verify that the Drivetrain ECM 5 VDC power supply is present at the harness connector for the EFR before any other action is taken.
During normal operation with the key start switch is in the ON position and the Engine is not operating, the EFR feedback PWM duty cycle is expected to be 10 percent. When the Engine is operating the PWM duty cycle is expected to be 50 percent. Any other PWM duty cycle on the diagnostic feedback circuit indicates that the EFR has detected a problem.

Throttle Pedal Position Sensor

Location of the Throttle Pedal Position Sensor (arrow)


Throttle Pedal Position Sensor connections

The Throttle Pedal Position Sensor provides a 500 hz PWM signal to the Drivetrain ECM. The sensor is powered by the Drivetrain ECM 8.0 VDC power supply.

The sensor PWM input signal allows the Drivetrain ECM to determine the position of the throttle pedal. The ECM will recognize a valid duty cycle range of 5 percent (0.25 VDC) when the pedal is not depressed to 95 percent (4.75 VDC) when the pedal is fully depressed.

The Drivetrain ECM will use the CAN A Data Link circuits to send the throttle pedal position status to the Engine ECM, the Chassis ECM, and the Brake ECM.

The Drivetrain ECM will use the requested gear input information from the Chassis ECM and the position of the throttle pedal to determine the traction motor torque commands that will be sent to each motor ECM for motor control.

The following operation logic for machine travel control will be used based on the input signals that are received from the throttle pedal position sensor and the shift control lever.

When the shift control lever is in the NEUTRAL or the PARK position, the speed of the Engine will be proportional to the position of the throttle pedal. Engine speed will be a low idle when the pedal is not depressed.
When the shift control lever is in the NEUTRAL or the PARK position, with the throttle pedal depressed and the shift control lever is moved to a travel gear position, the drivetrain ECM will ramp up the torque commands to each motor ECM in order to avoid sudden machine movement into the requested gear.
When the shift control lever is in the DRIVE position, the Drivetrain ECM will send torque commands to each motor ECM that are proportional to the position of the throttle pedal. Engine speed will be controlled independently to meet the system demands efficiently.
When the shift control lever is in the REVERSE or the LOW position, the Drivetrain ECM will send torque commands to each motor ECM that are proportional to the position of the throttle pedal within the lower default or programmed speed limits. Engine speed will be limited to 1300 rpm.
Activation of the retarder lever during machine travel will override the position of the throttle pedal. During retarding, the Drivetrain ECM will ramp down to zero torque commands to each motor control ECM regardless of the position of the throttle pedal.
If the Throttle Pedal Position Sensor signal is interrupted or lost, in addition to activating a diagnostic code for the sensor, the Drivetrain ECM will activate the backup throttle function. The Throttle Lock Switch can be held in the ON position in order to request travel in the selected direction of travel. The Drivetrain ECM will allow limited machine movement in order to move the truck to another location.

Right Steering Cylinder Position Sensor

Right Steering Cylinder Position Sensor connector (arrow)


Right Steering Cylinder Position Sensor connections

The Right Steering Cylinder Position Sensor provides a PWM input signal to the Drivetrain ECM that will indicate the position of the steering cylinder.

The sensor operates at a frequency of approximately 500 hz. The duty cycle of the PWM signal that is supplied to the ECM is measured in small increments. The changes in the duty cycle of the sensor cannot be measured with a digital multimeter.

The Drivetrain ECM will use the position of the steering cylinder to calculate the angle of the front tires. The ECM will use this information to determine the torque commands that are sent to each of the electric drive traction motors.

When the truck makes a turn, the ECM will adjust the torque commands that are sent to each motor ECM. The result will allow for speed adjustment of the inside wheel set as opposed to the outside wheel set. This results in better turning performance and less wear on the rear tires.

The Right Steering Cylinder Position Sensor is not serviceable. If the position sensor fails and must be replaced, the right steering cylinder must be replaced.

The truck can be operated with the Right Steering Cylinder Position Sensor inoperable. However, the Drivetrain ECM will not adjust the speed of the rear wheels in a turn. The result will be increased wear of the rear tires and a reduction in the turning performance of the truck.

MARYGAR

Saturday, December 1, 2012