Abstrict
Methods and systems are provided for controlling the output power
to DC electric heater elements to reduce the input current ripple
and to fine-tune the output power to meet the fine-tuning percentage
requirement. In one embodiment, a method is performed for controlling
output power to heater elements in an electric heater system. The
process includes receiving a power request by the heater system,
providing output power controls for each heater element, and determining
a particular output power control based on output power controls
provided for each heater element and the power requested. Further,
the heater system selectively controls the output power to one or
more of the heater elements to meet the power request.
Claims
What is claimed is:
1. A method for controlling output power to a plurality of heater
elements in an electric heater system, comprising: receiving a power
request; providing output power controls for each heater element;
determining at least one heater element to be pulse width modulation
(PWM) controlled; determining at least one other heater element
to be constant ON/OFF controlled; selectively controlling the output
power to the determined heater elements to meet the power request;
and changing output power control for at least one heater element
from PWM controlled to constant ON/OFF controlled after a predetermined
time period.
2. The method in claim 1, wherein controlling the output power
further includes: setting a current limit based on a number of the
heater elements used to meet the power request; and selectively
turning on or turning off the number of the heater elements in a
predetermined sequence.
3. The method in claim 1, wherein selectively controlling the output
power includes: selectively providing power to the at least one
PWM controlled heater element based on the power request.
4. The method in claim 3, further including: selectively providing
power to the at least one other constant ON/OFF controlled heater
element based on the power request, wherein the power provided to
the at least one PWM controlled heater element and the at least
one other constant ON/OFF controlled heater element collectively
meet the power request.
5. The method of claim 4, wherein selectively providing power to
a PWM controlled heater element includes: determining a duty cycle
for a power signal associated with the at least one PWM controlled
heater element based on the power request and the power provided
to the at least one other constant ON/OFF controlled heater element.
6. An electric heater control system for controlling output power
to a plurality of heater elements in an electric heater system,
comprising: means for receiving a power request; means for determining
at least one heater element to be pulse width modulation (PWM) controlled;
means for determining at least one other heater element to be constant
ON/OFF controlled; means for selectively controlling the output
power to the determined heater elements to meet the power request;
and means for changing output power control for at least one heater
element from PWM controlled to constant ON/OFF controlled after
a predetermined time period.
7. The system in claim 6, wherein the means for controlling the
output power further includes: means for setting a current limit
based on a number of the heater elements used to meet the power
request; and means for selectively turning on or turning off the
number of the heater elements in a predetermined sequence.
8. The system in claim 6, wherein means for selectively controlling
the output power includes: means for selectively providing power
to the at least one PWM controlled heater element based on the power
request.
9. The system in claim 8, further including: means for selectively
providing power to the at least one other constant ON/OFF controlled
heater element based on the power request, wherein the power provided
to the at least one PWM controlled heater element and the at least
one other constant ON/OFF controlled heater element collectively
meet the power request.
10. The system in claim 9, wherein means for selectively providing
power to a PWM controlled heater element includes: means for determining
a duty cycle for a power signal associated with the at least one
PWM controlled heater element based on the power request and the
power provided to the at least one other constant ON/OFF controlled
heater element.
11. A system for controlling output power to a plurality of heater
elements in an electric heater system, comprising: a memory including
program code that performs an operation process when executed, the
operation process including: receiving a power request including
a requested power value; determining a first set of the heater elements
to operate in a constant ON mode based on the power value; determining
a second set of the heater elements to operate in a PWM controlled
mode; and providing power to the first and second sets of heater
elements based on the power value and a predetermined algorithm;
and a microcontroller executing the program code to control power
to the heater elements.
12. The system in claim 11, wherein each of the heater elements
provide identical levels of power.
13. The system in claim 12, wherein determining a first set of
the heater elements further includes: determining a total number
of the first set of heater elements as a quotient of the requested
power value divided by a level of power provided by one of the heater
elements.
14. The system in claim 13, wherein determining a second set of
the heater elements further includes: determining a total number
of the second set of heater elements as one; and determining a duty
cycle of the determined heater element based on a remainder of the
division and a level of power provided by the determined heater
element.
15. The system in claim 11, wherein one or more of the heater elements
provide different levels of power than other heater elements.
Description TECHNICAL FIELD
This disclosure relates generally to electric heater control systems,
and more particularly to systems and methods for providing improved
output power controls to electric heater elements.
BACKGROUND
DC electric heaters usually consist of a plurality of heater elements
connected in parallel, series, or both. When a desirable temperature
range is specified, a control system of the electric heater system
controls the output power to the heater elements by turning on a
determined number of heater elements while turning off the remaining
heater elements to approximately meet the desired temperature. The
resolution of this type of control system, however, is limited by
the number of heater elements. This limitation restricts the DC
electric heaters from meeting certain fine-tuned percentage output
power requirements.
In order to fine-tune the output power to the heater elements,
some conventional systems use Pulse Width Modulation (PWM) to control
the output power to all the heater elements. One such system is
described in U.S. Pat. No. 5,582,756 to Hideki Koyama. The '756
system includes a heater control device that uses a PWM signal for
controlling a switch that turns on and off the entire electric heater.
A PWM circuit works by making a square wave with a variable on-to-off
ratio, also called a duty cycle, such that a variable amount of
power is applied to the load. The duty cycle is a percentage number
calculated by T.sub.on/(T.sub.on+T.sub.- off), where T.sub.on is
the time period when power is applied to the load, T.sub.off is
the time period when power is not applied to the load, and the duty
cycle T is the total of T.sub.on and T.sub.off. If T.sub.on=T.sub.off,
then the duty cycle is 50%, which means 50% of power is applied
to the load. However, to achieve the desirable result, the cycle
period T must be short relative to the load's response time to the
change in ON/OFF state. Therefore, the PWM frequency has to be kept
at a high rate. In such instances, it is not uncommon that the PWM
frequency reaches tens of KHz, sometimes up to one hundred KHz or
even more. As the frequency increases, the fast switching between
ON and OFF states in the load circuitry will generate high input
current ripple. This can affect the lifetime of certain circuitry,
such as a bus capacitor, and may also cause radio frequency interference
(RFI) that affects other electronic components in the DC electric
heater or other nearby electronic equipment.
To address the high input current ripple problem, conventional
DC electric heater systems may use additional input filters. This
solution, however, will inevitably add more complexities to the
circuitry and extra cost to the overall system.
Methods and systems consistent with certain features of the disclosed
specification are directed to solving one or more of the problems
set forth above.
SUMMARY OF THE INVENTION
In one embodiment, a method is performed for controlling output
power to heater elements in an electric heater system. The process
includes receiving a power request by the heater system, providing
output power controls for each heater element, and determining a
particular output power control based on output power controls provided
for each heater element and the power requested. Further, the heater
system selectively controls the output power to one or more of the
heater elements to meet the power request.
In another embodiment, a system is provided for controlling output
power to heater elements in an electric heater system. The system
includes a memory including program code that performs an operation
process including receiving a power request including a requested
power value and, based on the power value, determining a first set
of the heater elements to operate in a constant ON/OFF mode. The
operation process may also include determining a second set of the
heater elements to operate in a PWM controlled mode and providing
power to the first and second sets of heater elements based on the
power value and a predetermined algorithm. Further, the system includes
a microcontroller executing the program code to control power to
the heater elements.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and constitute
a part of this specification, illustrate several aspects of disclosed
embodiments and together with the description, serve to explain
the principle of the invention. In the drawings:
FIG. 1 is a pictorial illustration of a vehicle incorporating an
exemplary DC electric heater system;
FIG. 2 illustrates a block diagram of an exemplary control system
consistent with certain disclosed embodiments;
FIG. 3 illustrates a state machine diagram of an exemplary microcontroller
to perform control functions consistent with certain disclosed embodiments;
and
FIG. 4 illustrates a flowchart of an exemplary operation state
of the microcontroller consistent with certain disclosed embodiments.
DETAILED DESCRIPTION
Reference will now be made in detail to exemplary embodiments that
are illustrated in the accompanying drawings. Wherever possible,
the same reference numbers will be used throughout the drawings
to refer to the same or like parts.
FIG. 1 illustrates an exemplary electric heater system 100 incorporated
into a cabin of a work machine 110. The electric heater system 100
is used to provide variable temperature ranges within the cabin
of work machine 110.
Work machine, as the term is used herein, refers to a fixed or
mobile machine that performs some type of operation associated with
a particular industry, such as mining, construction, farming, etc.,
and operates between or within work environments (e.g., construction
site, mine site, power plants, etc.). Non-limiting examples of mobile
machines include commercial machines, such as trucks, cranes, earth
moving vehicles, mining vehicles, backhoes, material handling equipment,
farming equipment, marine vessels, aircraft, and any type of movable
machine that operates in a work environment. Although FIG. 1 shows
heater system 100 incorporated in a truck type work machine, system
100 may be implemented in any type of work machine, such as those
described above. Further, heater system 100 may also be used in
other environments, such as rooms, booths, or any environment where
a temperature range may be fine-tuned.
FIG. 2 illustrates a block diagram of heater system 100 consistent
with certain disclosed embodiments. As shown in FIG. 2, heater system
100 may include microcontroller unit (MCU) 201, memory module 202,
I/O interface 203, and heater elements 204, 205, 206, and 207. A
host controller 208 communicates with MCU 201 to facilitate the
implementation of control functions for heater system 100.
MCU 201 may be configured as a separate processor module dedicated
to provide output power control functions. Additionally or alternatively,
MCU 201 may be configured as a shared processor module performing
other functions unrelated to output power control functions. MCU
201 may be one or more microcontrollers with on-board memory, dedicated
PWM ports, and I/O ports. Further, MCU 201 may include a microprocessor
supported by various memory modules and peripheral devices. In one
embodiment, MCU 201 communicates with host controller 208 by exchanging
J1939 messages over a CAN bus. Other communication protocols and
bus types, however, may be used.
Memory 202 may be one or more memory devices including, but not
limited to, a ROM, a flash memory, a dynamic RAM, and a static RAM.
Memory 202 may be configured to store information used by MCU 201.
Further, memory 202 may be external or internal to MCU 201. I/O
interface 203 may be one or more input/output interface devices
receiving data from MCU 201 and sending data to MCU 201, such as
interrupt signals. Heater elements 204, 205, 206, and 207 are coupled
in parallel to MCU 201. Each heater element may be coupled in a
way such that it can be either PWM controlled or constant ON/OFF
controlled. Although four heater elements in a parallel configuration
are shown in FIG. 2, any number of the heater elements and configurations
may be implemented.
During operations of heater system 100, MCU 201 may perform status
computation and mode management processes. In one embodiment, such
processes enable MCU 201 to PWM control one or more selected heater
elements 204 207, while controlling any remaining heater elements
through constant ON/OFF control processes (i.e., non-PWM control).
Further, MCU 201 may rotate the duty (i.e., providing heat when
applied power) of heater elements 204 207 to minimize the stress
on the heater elements. Because those heater elements that are constant
ON/OFF controlled do not introduce input current ripples, the amount
of the input current ripple may be reduced to only that introduced
by the selected PWM controlled heater element. This configuration
reduces the input current ripple by a factor equal to the number
of parallel elements, while still allowing heater system 100 to
maintain fine-tuning capabilities. Further, as a part of the status
computation process, MCU 201 monitors and calculates the average
power, the instant value and average value of total current, temperature
readings, average input voltage, and average output voltage on each
heater element. MCU 201 may be configured to provide status information
associated with this determined data (e.g., average power) to host
controller 208.
MCU 201 may also monitor the communication channel between MCU
201 and host controller 208, which is used for receiving commands
from host controller 208 and sending status information back to
host controller 208. MCU 201 may also receive interrupts from I/O
interface 203 based on fault or non-fault conditions detected within
heater systems 100. Such conditions may include over-current conditions,
de-saturation conditions, and/or timeout conditions. After MCU 201
receives communication from host controller 208 or is interrupted
by I/O interface 203, MCU 201 performs some initial processing,
then returns to perform the status computation and mode management
processes based on commands received from host controller 208 or
interrupts received from I/O interface 203 or any other component
of heater system 100.
FIG. 3 shows a state machine diagram of various states implemented
by one or more software programs stored in memory 202 and executed
by MCU 201 while performing the status computation and mode management
processes. The state machine diagram reflects various operational
states of the software programs and the reactions to a particular
event during a particular state. In one embodiment, the state machine
diagram includes eight states: "OFF" state 301, "INITIALIZATION"
state 302, "STANDBY" state 303, "OPERATION"
state 304, "SLEEP" state 305, "FLASH" state
306, "FAULT" state 307, and "SHUTDOWN" state
308. Although FIG. 3 shows eight states, any number of states may
be implemented by the software programs executed by MCU 201.
"OFF" state 301 may be a starting state in which MCU
201 is not initialized, such as when MCU 201 is not operating or
performing any functions. Subsequently, MCU 201 may be turned on
or reset, thus causing the state to transition to "INITIALIZATION"
state 302. On entering "INITIALIZATION" state 302, the
software programs perform various initialization processes and diagnostics
tests, such as register configuration and memory allocation, configuring
system clock oscillator (OSC), initialization of a reset register,
memory management, initialization and test on watchdog circuitry,
CAN, comparators, comparator voltage references, D/A converters,
and PWM capture, etc. If there is any fault detected during the
initialization and diagnostic processes, the state transitions to
"FAULT" state 307. Otherwise, if all the initialization
processes are successful and all the diagnostics tests have passed
without detecting a fault event, the state transitions to "STANDBY"
state 303. In "STANDBY" state 303, the power stage operation
is stopped, thus no output power is applied to the load. Heater
system 100, however, is ready for power stage operations.
While in "STANDBY" state 303, MCU 201 may receive a power
request that may or may not be a request for some amount of power
to be applied to a load. If this occurs, the state transitions to
"OPERATION" state 304. FIG. 4 shows a flow chart of an
operation process performed by MCU 201 while in "OPERATION"
state 304. Initially, MCU 201 may receive a power request from host
controller 208 reflecting an amount of power required for providing
a desired temperature range from heater elements 204, 205, 206,
and 207 (Step 400). MCU 201 may then determine whether the power
request is a non-zero power request (i.e., a request for some power),
as opposed to a zero power request (i.e., a request for no power
reflecting non-use of heater system 100) (Step 409). If the power
request is a non-zero power request (Step 409; Yes), the operation
process continues to Step 401. On the other hand, if the power request
is a zero power request (Step 409; No), MCU 201 transitions from
"OPERATION" state 304 to "STANDBY" state 303.
In Step 401, MCU 201 may determine the total number of available
heater elements 204, 205, 206, and 207 in heater system 100. For
example, if during a diagnostic process, MCU 201 detects a faulty
heater element (e.g., element 204), MCU 201 may determine that the
total number of available heater elements is equal to the total
number of heater elements (e.g., four; elements 204, 205, 206, and
207) minus the number of faulty elements (e.g., one; element 204).
If MCU 201 determines that there are no faulty elements, "OPERATION"
state 304 is placed in a normal mode sub-state (not shown) and the
operation process continues to Step 404 (Step 402; Yes). On the
other hand, if MCU 201 detects a faulty element, MCU 201 may place
"OPERATION" state 304 in a limp mode sub-state (not shown)
(Step 402; No). During limp mode, MCU 201 disables any detected
faulty heater elements (Step 403), and the operation process continues
to Step 404.
In Step 404, MCU 201 determines the number of elements required
to meet the power request received in Step 400. In one embodiment,
MCU 201 may perform a calculation process to determine the number
of elements required to meet the power request. Based on the power
request, MCU 201 determines which heater elements should operate
in a constant ON controlled mode and which (if any) heater elements
should operate in PWM mode. For example, if heater elements 204,
205, 206, and 207 each provide 750 W of power and the power request
is for 1875 W of power, MCU 201 may determine that the number of
required elements to operate in a constant ON controlled mode is
two, and the number of required elements to operate in a PWM mode
is one. This is based on the amount of power provided by heater
elements 204, 205, 206, and 207 in this example. For instance, because
two elements that are operating in a constant ON mode will provide
a total output of 750 W+750 W=1500 W of power, the PWM controlled
output required is 1875 W-1500 W=375 W. Therefore, the required
duty cycle of the PWM mode for the single PWM controlled heater
element is 375 W/750 W=50%. As a result of this calculation, MCU
201 will designate one heater element to be PWM controlled with
a 50% duty cycle, two elements to be constant ON controlled, and
one element to be constant OFF controlled.
In another embodiment, MCU 201 may perform a pre-determined algorithm
when performing the calculation process in the event one or more
heater elements 204, 205, 206, and 207 have different power output
values. The pre-determined algorithm may be based on a numerical
order of the heater elements or a combination of the output value
and physical positions of the heater elements.
In Step 405, MCU 201 sets the current limit for heater system 100
according to the number of elements that are calculated and to be
turned on. Further, in Step 406, particular heater elements are
selected for constant ON, constant OFF, and PWM controlled based
on the calculation process performed in Step 404. MCU 201 then turns
on or off heater elements 204, 205, 206, and 207 according to the
selections in an increasing or decreasing sequence to soften the
instant impact of output power. At this point, the optimized output
power is applied to the heater elements so that a desirable temperature
range is achieved. For example, if the first heater element 204
is PWM controlled, the next two heater elements 205 and 206 are
constant ON controlled, and heater element 207 is constant OFF controlled,
heater element 204 may be turned on first, then heater element 205,
then heater element 206, and finally, if heater element 207 is already
turned on, heater element 207 is then turned off.
In Step 407, the software programs executed by MCU 201 may wait
for a new power request to be received from the host controller
208 or for an expiration of a pre-determined time period for rotating
the duty of the elements. If a new power request is received from
the host controller 208 (Step 408; Yes), the amount of the power
requested is assessed in Step 409. In Step 409, if the amount of
power request is not zero (Step 409; Yes), the process returns to
Step 401 to readjust the output power controls. If, however, the
amount of power request is zero (Step 409; No), the state then transitions
from "OPERATION" state 304 to "STANDBY" state
303. If no new power request is received from the host controller
208 (Step 408; No), MCU 201 determines if a pre-determined time
period has expired. If the time period has not expired (Step 410;
No), the operation program returns to Step 407 to continue waiting
for further events. If, however, the time period has expired (Step
410; Yes), the operation program continues to Step 411.
In Step 411, the duty of the heater elements is rotated so that
the stress on each heater element is evenly distributed to extend
the lifetime of heater elements 204, 205, 206, and 207. The rotation
may be scheduled in different times or sequences. For example, the
rotation may be done by rotating all the heater elements in sequence.
Thus, if heater element 204 is currently PWM controlled, heater
elements 205 and 206 are currently constant ON controlled, and heater
elements 207 is currently constant OFF controlled, the rotation
sequence may result in heater elements 204 and 205 being constant
ON controlled, heater element 206 being constant OFF controlled,
and heater element 207 being PWM controlled. Other rotation sequences
may be implemented and the disclosed embodiments are not limited
to the examples listed above.
Returning back to FIG. 3, while in "OPERATION" state
304, if MCU 201 receives on or more interrupts regarding any fault
conditions, MCU 201 may transition "OPERATION" state to
"FAULT" state 307.
Further, while in "STANDBY" state 303, if MCU 201 receives
a flash program message from host controller 208, the state transitions
to "FLASH" state 306. On entering "FLASH" state
306, MCU 201 downloads a new software program into the memory 202
from host controller 208. Subsequently, MCU 201 replaces a current
version of the software program with the newly downloaded software
program. After the replacement is completed or if MCU 201 receives
a flash-program finished message (optionally followed by a standby
message), the state transitions to "STANDBY" state 303.
Further, while in "FLASH" state 306, if MCU 201 performs
the program-flashing operation unsuccessfully, or detects any other
fault conditions, the state transitions to "FAULT" state
307.
While in the "FAULT" state 307, MCU 201 handles faults
in various manners including, for example, sending status messages
back to host controller 208, presenting a fault related message
on external display devices (not shown), and/or taking actions to
eliminate the fault conditions, such as resetting or disabling the
faulty devices. After all the faults are handled or processed, the
state then transitions to either "STANDBY" state 303 if
continuing operation is desired and possible, or to "SHUTDOWN"
state 308 if the faults cannot be handled properly and shutdown
of heater system 100 is desired.
Also, while in the "STANDBY" state 303, if MCU 201 does
not receive an instruction from host controller 208 for a predetermined
period of time and the bus voltage on the load circuitry is within
a predefined range of a zero voltage value, the state transitions
to "SLEEP" state 305. On entering "SLEEP" state
305, MCU 201 is set to sleep mode in order to conserve power. If
MCU 201 receives a wakeup message from host controller 208, the
state transitions to "STANDBY" state 303 again.
Additionally, while in the "STANDBY" state 303, if MCU
201 receives a shutdown command from host controller 208, or if
MCU 201 determines a shutdown sequence is needed to respond to some
external or internal event, the state transitions to "SHUTDOWN"
state 308. On entering "SHUTDOWN" state 308, the power
stage operation is stopped according to a turn off sequence to soften
the instant impact of output power on the circuitry. That is, the
power to heater elements 204, 205, 206, and 207 is turned off in
an increasing or decreasing sequence. MCU 201 also minimizes execution
of its software programs to prepare MCU 201 for a power shutdown.
The state then transitions to "OFF" state 301.
It should be understood that the sequence of events and steps in
FIGS. 3 and 4 are exemplary and not intended to be limiting. Thus,
other method steps may be used, and even within the steps depicted
in FIG. 4, the particular order of steps may vary. Moreover, certain
steps may be removed, added, or modified to perform functions consistent
with the disclosed embodiments.
INDUSTRIAL APPLICABILITY
Consistent with the disclosed embodiments, methods and systems
may facilitate temperature control in confined-space environments.
In one example, a work machine may have a cabin where an operator
desires to fine-tune the temperature range of the cabin to obtain
a comfortable work environment. Methods and systems consistent with
disclosed embodiments may enable a heater system to provide the
desired temperature range.
In one embodiment, the disclosed embodiments may collectively use
PWM control and constant ON/OFF control processes to fine-tune the
output power to the heater elements while reducing input current
ripple. In this fashion, methods and systems consistent with disclosed
embodiments may extend the lifetime of electrical and electronic
components while reducing radio frequency interference (RFI).
Other embodiments, features, aspects, and principles of the disclosed
exemplary systems may be implemented in various environments and
are not limited to a work site environment. For example, a work
machine with an interface control system may perform the functions
described herein in other environments, such as mobile environments
between job sites, geographic locations, and settings. Further,
the processes disclosed herein are not inherently related to any
particular system and may be implemented by a suitable combination
of electrical-based components. Embodiments other than those expressly
described herein will be apparent to those skilled in the art from
consideration of the specification and practice of the disclosed
systems.
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