| Author |
Message |
Ed
Guest
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 Posted:
Sat Nov 19, 2005 1:12 am Post subject:
Multiplexing temperature sensor |
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My engine has a temperature sensor on the right bank for the dash gauge, and
a different one on the left bank for
the electronic fuel injection system. Both sensors are just temperature
sensitive resistors of some kind.
I would like a temperature reading for both banks. Rather than drilling and
tapping a hole for yet another
sensor for the left bank I was thinking about multiplexing the EFI sensor.
Is there an simple circuit that I could
cobble together to do this, perhaps based on an IC? The circuit would
probably also have to modify the
output to match an available temperature guage.
Any ideas?
TIA
Ed
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Ed
Guest
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| Posted:
Sat Nov 19, 2005 1:35 am Post subject:
Re: Multiplexing temperature sensor |
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"John Fields" <jfields@austininstruments.com> wrote in message
news:6pdsn157v1d1u8ksr9j0ih471iba4r74q2@4ax.com...
| Quote: | Find out what the temperature sensor's output looks like with
changes in temp, how it's hooked into the system electrically, and
the characteristics of the gauge on which you want to display
temperature and you'll have a starting point for help.
--
John Fields
|
Here is the EFI sensor resistance curve:
Deg C Ohms
10 3700
20 2500
30 1700
40 1180
50 840
60 600
70 435
80 325
90 250
100 190
Here is the the curve for the sender to the dash temperature guage. If
the output of the
multiplexer circuit produced something close to this I could switch
manually back and forth between
the two, which would be fine:
'C ohms
0 1117
10 871
20 680
30 530
40 412
50 319
60 245
70 187
80 141
88 112
90 105
92 99
100 76
110 54
I used to have access to a Web page with the ECU internals, but it
seems to be down at the moment,
or perhaps has gone away. I'll continue looking for it, but maybe in
the meanwhile the above
will be sufficient to offer suggestions, of course with caveats.
TIA
Ed |
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John Fields
Guest
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| Posted:
Sat Nov 19, 2005 1:35 am Post subject:
Re: Multiplexing temperature sensor |
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On Fri, 18 Nov 2005 19:12:17 GMT, "Ed"
<jag_manR__EM*-0_V_E653@hotmail.com> wrote:
| Quote: |
My engine has a temperature sensor on the right bank for the dash gauge, and
a different one on the left bank for
the electronic fuel injection system. Both sensors are just temperature
sensitive resistors of some kind.
I would like a temperature reading for both banks. Rather than drilling and
tapping a hole for yet another
sensor for the left bank I was thinking about multiplexing the EFI sensor.
Is there an simple circuit that I could
cobble together to do this, perhaps based on an IC? The circuit would
probably also have to modify the
output to match an available temperature guage.
|
---
Find out what the temperature sensor's output looks like with
changes in temp, how it's hooked into the system electrically, and
the characteristics of the gauge on which you want to display
temperature and you'll have a starting point for help.
--
John Fields
Professional Circuit Designer
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Bob Masta
Guest
|
| Posted:
Sat Nov 19, 2005 5:35 pm Post subject:
Re: Multiplexing temperature sensor |
|
|
On Fri, 18 Nov 2005 22:14:32 GMT, "Ed"
<jag_manR__EM*-0_V_E653@hotmail.com> wrote:
| Quote: | Here is the EFI sensor resistance curve:
Deg C Ohms
10 3700
20 2500
30 1700
40 1180
50 840
60 600
70 435
80 325
90 250
100 190
Here is the the curve for the sender to the dash temperature guage. If
the output of the
multiplexer circuit produced something close to this I could switch
manually back and forth between
the two, which would be fine:
'C ohms
0 1117
10 871
20 680
30 530
40 412
50 319
60 245
70 187
80 141
88 112
90 105
92 99
100 76
110 54
I used to have access to a Web page with the ECU internals, but it
seems to be down at the moment,
or perhaps has gone away. I'll continue looking for it, but maybe in
the meanwhile the above
will be sufficient to offer suggestions, of course with caveats.
|
I'd be very careful of messing withthe EFI circuit. You should
read the voltage across the sensor with a high impedance
circuit, so as not to disturb the EFI operation. So, the
actual resistance curve of the sensor is not the issue, it's
the voltage output at whatever current the EFI wants to
drive it with. This may not be a constant current, since
the EFI could be turning it on only during measurement
cycles at whatever update rate it requires... maybe not too
often, given the thermal time constant of the big hunk of
metal it's attached to. So your first order of business
would be to look at the voltage across the sensor while
the engine is running, preferably with a scope. If it
is constant (other than varying with temperature!) you
can proceed to curve-fitting to match the other sensor.
If it pulses at ahigh-enough rate, you may be able to
filter it (after buffering, to not disturb the EFI) to get
a suitable signal.
But curve fitting by analog means can be a bear.
If you want to go whole-hog, you can use table
look-up with a small processor chip. But my guess
is that installing a separate sensor may be a lot
easier in the long run.
Best regards,
Bob Masta
dqatechATdaqartaDOTcom
D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Home of DaqGen, the FREEWARE signal generator |
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Ed
Guest
|
| Posted:
Sun Nov 20, 2005 1:35 am Post subject:
Re: Multiplexing temperature sensor |
|
|
"Bob Masta" <NoSpam@daqarta.com> wrote in message
news:437f2b9e.1729905@news.itd.umich.edu...
| Quote: |
I'd be very careful of messing withthe EFI circuit. You should
read the voltage across the sensor with a high impedance
circuit, so as not to disturb the EFI operation. So, the
actual resistance curve of the sensor is not the issue, it's
the voltage output at whatever current the EFI wants to
drive it with. This may not be a constant current, since
the EFI could be turning it on only during measurement
cycles at whatever update rate it requires... maybe not too
often, given the thermal time constant of the big hunk of
metal it's attached to. So your first order of business
would be to look at the voltage across the sensor while
the engine is running, preferably with a scope. If it
is constant (other than varying with temperature!) you
can proceed to curve-fitting to match the other sensor.
If it pulses at ahigh-enough rate, you may be able to
filter it (after buffering, to not disturb the EFI) to get
a suitable signal.
But curve fitting by analog means can be a bear.
If you want to go whole-hog, you can use table
look-up with a small processor chip. But my guess
is that installing a separate sensor may be a lot
easier in the long run.
|
Thanks, Bob.
This is a very early EFI system, namely the Jaguar adaption of the
Bosch
D-Jetronic system, circa late 1970s. The ECU is an analog
implementation with discrete
components. It is of course driven by a pulse stream from the engine,
picked up from
a trigger board in the distributor. The primary inputs are engine
speed (derived from the
pulse stream), manifold pressure, coolant temperature, and air
temperature. It is described
at http://www.jagweb.com/aj6eng/djetronic.html if you really want to
get into it.
Also, I have now found the site at which the Porsch 914 implementation
of D-Jetronic is
fully reverse engineered. It is at:
http://members.rennlist.com/pbanders/ecu.htm. From that site
you can navigate to http://members.rennlist.com/pbanders/ecu2.jpg
which shows the portion of the
ECU schematic where the cylinder head temperature sensor (since the
914 engine is air cooled). It
looks like the sensor feeds into the base circuit of a transistor. I
have read that this is eseentially the same
as the Jaguar/Lucas implementation, although there are some
differences.
The coolant temperature sensor (CTS) is a thermistor with resistance
curce as given previously. One
side is grounded and the other connects to the ECU. I just put a scope
on the the ungrounded
pin of the CTS in situ with the engine idling, warm. What I see is a
very clean 2.2 volts, varying
only by a negative 0.2 volt blip every 37 ms.
So, are you saying I might be able to just connect a gauge to the CTS
through high impedence,
no multiplexing required?
Thangs again for your interest.
Ed |
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Bob Masta
Guest
|
| Posted:
Sun Nov 20, 2005 5:35 pm Post subject:
Re: Multiplexing temperature sensor |
|
|
On Sat, 19 Nov 2005 19:49:00 GMT, "Ed"
<jag_manR__EM*-0_V_E653@hotmail.com> wrote:
<snip>
| Quote: |
The coolant temperature sensor (CTS) is a thermistor with resistance
curce as given previously. One
side is grounded and the other connects to the ECU. I just put a scope
on the the ungrounded
pin of the CTS in situ with the engine idling, warm. What I see is a
very clean 2.2 volts, varying
only by a negative 0.2 volt blip every 37 ms.
So, are you saying I might be able to just connect a gauge to the CTS
through high impedence,
no multiplexing required?
|
That should work fine, as far as not disturbing the EFI.
You can then use nonlinear circuits to linearize the
voltage response for a digital meter, or you can make
a custom face for an analog meter. (I'm assuming
that you won't have an analog meter that is calibrated
to match the EFI curve.) Or you can digitize and use
lookup tables.
Best regards,
Bob Masta
dqatechATdaqartaDOTcom
D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Home of DaqGen, the FREEWARE signal generator |
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Ed
Guest
|
| Posted:
Sun Nov 20, 2005 5:35 pm Post subject:
Re: Multiplexing temperature sensor |
|
|
"Bob Masta" <NoSpam@daqarta.com> wrote in message
news:4380874d.2748776@news.itd.umich.edu...
| Quote: |
That should work fine, as far as not disturbing the EFI.
You can then use nonlinear circuits to linearize the
voltage response for a digital meter, or you can make
a custom face for an analog meter. (I'm assuming
that you won't have an analog meter that is calibrated
to match the EFI curve.) Or you can digitize and use
lookup tables.
|
Sounds easy! I'm wondering, though, about how to get the
proper relationship between voltage at the sensor and temperature.
Direct calibration is just about impossible for two reasons. First,
it would require fitting another temperature measurement of the
coolant at the same location in the system, which is what I'm trying
to
avoid. Second, covering the range at the high end would require
overheating
the engine. There must be some way to relate the known resistance vs.
temperature
to the voltage at the ungrounded terminal while it functions in the
EFI system.
Would this require details such as resistances etc. in the ECU where
the CTS
connects?
Ed |
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Ed
Guest
|
| Posted:
Mon Nov 21, 2005 1:35 am Post subject:
Re: Multiplexing temperature sensor |
|
|
"Ed" <jag_manR__EM*-0_V_E653@hotmail.com> wrote in message
news:1K1gf.21178$q%.12272@newssvr12.news.prodigy.com...
| Quote: | There must be some way to relate the known resistance vs.
temperature
to the voltage at the ungrounded terminal while it functions in the
EFI system.
Would this require details such as resistances etc. in the ECU where
the CTS
connects?
|
Answering my own question, I would think I could just measure the
resistance
between where the CTS pin and the battery voltage pin at the ECU, then
view that
resistance and the CTS as a "voltage divider."
Ed |
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Jasen Betts
Guest
|
| Posted:
Mon Nov 21, 2005 9:35 am Post subject:
Re: Multiplexing temperature sensor |
|
|
On 2005-11-20, Ed <jag_manR__EM*-0_V_E653@hotmail.com> wrote:
| Quote: |
"Bob Masta" <NoSpam@daqarta.com> wrote in message
news:4380874d.2748776@news.itd.umich.edu...
That should work fine, as far as not disturbing the EFI.
You can then use nonlinear circuits to linearize the
voltage response for a digital meter, or you can make
a custom face for an analog meter. (I'm assuming
that you won't have an analog meter that is calibrated
to match the EFI curve.) Or you can digitize and use
lookup tables.
Sounds easy! I'm wondering, though, about how to get the
proper relationship between voltage at the sensor and temperature.
Direct calibration is just about impossible for two reasons. First,
it would require fitting another temperature measurement of the
coolant at the same location in the system, which is what I'm trying
to
avoid. Second, covering the range at the high end would require
overheating
the engine. There must be some way to relate the known resistance vs.
temperature
to the voltage at the ungrounded terminal while it functions in the
EFI system.
Would this require details such as resistances etc. in the ECU where
the CTS
connects?
|
if you can characterise and simulate the probe pulses sent to the sensor
you can remove the sensor and heat it in a pot of oil with a thermometer while
feeding it simulated pulses...
if the EFI sends different pulses to the sensor depending on the engine
speed or temperature (etc) you have a tougher task.
--
Bye.
Jasen |
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Bob Masta
Guest
|
| Posted:
Mon Nov 21, 2005 5:35 pm Post subject:
Re: Multiplexing temperature sensor |
|
|
On Sun, 20 Nov 2005 20:44:18 GMT, "Ed"
<jag_manR__EM*-0_V_E653@hotmail.com> wrote:
| Quote: |
"Ed" <jag_manR__EM*-0_V_E653@hotmail.com> wrote in message
news:1K1gf.21178$q%.12272@newssvr12.news.prodigy.com...
There must be some way to relate the known resistance vs.
temperature
to the voltage at the ungrounded terminal while it functions in the
EFI system.
Would this require details such as resistances etc. in the ECU where
the CTS
connects?
Answering my own question, I would think I could just measure the
resistance
between where the CTS pin and the battery voltage pin at the ECU, then
view that
resistance and the CTS as a "voltage divider."
|
Measuring the ECU "resistance" probably won't work.
(It's probably a current source, not a simple resistor.)
As I recall, you already have the resistance curve for the CTS.
So shut off the engine and quickly measure the CTS resistance
and that will tell you its temperature at some arbitrary reference
point. The voltage across it just before you shut it off will
allow you to find the ECU driving current from I = V/R.
Now assuming that the ECU uses the same current all
the time, you can apply that to the resistance curve to
convert it to a voltage curve.
Best regards,
voltage
Bob Masta
dqatechATdaqartaDOTcom
D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Home of DaqGen, the FREEWARE signal generator |
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Ed
Guest
|
| Posted:
Mon Nov 21, 2005 5:35 pm Post subject:
Re: Multiplexing temperature sensor |
|
|
"Bob Masta" <NoSpam@daqarta.com> wrote in message
news:4381d776.5087087@news.itd.umich.edu...
| Quote: |
Measuring the ECU "resistance" probably won't work.
(It's probably a current source, not a simple resistor.)
As I recall, you already have the resistance curve for the CTS.
So shut off the engine and quickly measure the CTS resistance
and that will tell you its temperature at some arbitrary reference
point. The voltage across it just before you shut it off will
allow you to find the ECU driving current from I = V/R.
Now assuming that the ECU uses the same current all
the time, you can apply that to the resistance curve to
convert it to a voltage curve.
|
Thanks, Bob. Here is the relevant part of the ECU schematic. Could you
take a look and tell me if it is a constant current source?
http://www.efsowell.us/ed/jag/electrical/ECU_CTS_connection.JPG
Thanks, Ed |
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Bob Masta
Guest
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| Posted:
Tue Nov 22, 2005 5:35 pm Post subject:
Re: Multiplexing temperature sensor |
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|
On Mon, 21 Nov 2005 16:11:19 GMT, "Ed"
<jag_manR__EM*-0_V_E653@hotmail.com> wrote:
| Quote: |
"Bob Masta" <NoSpam@daqarta.com> wrote in message
news:4381d776.5087087@news.itd.umich.edu...
Measuring the ECU "resistance" probably won't work.
(It's probably a current source, not a simple resistor.)
As I recall, you already have the resistance curve for the CTS.
So shut off the engine and quickly measure the CTS resistance
and that will tell you its temperature at some arbitrary reference
point. The voltage across it just before you shut it off will
allow you to find the ECU driving current from I = V/R.
Now assuming that the ECU uses the same current all
the time, you can apply that to the resistance curve to
convert it to a voltage curve.
Thanks, Bob. Here is the relevant part of the ECU schematic. Could you
take a look and tell me if it is a constant current source?
http://www.efsowell.us/ed/jag/electrical/ECU_CTS_connection.JPG
Not constant current, just a plain old series resistor (plus a |
diode). But I'd avoid trying to measure the resistance of the
ECU, just because it is a questionable practice in general,
and also because that diode is in there to mess up your
measurement. But since it isn't a current source, using
a voltage divider approach is probably best. Just measure
the CTS resistance (disconnected from the ECU, of course),
as well as the voltage when it is connected, and you will
have the current at that temperature. The effective ECU resistance
will be 12 minus the CTS voltage divide by the current.
You can probably assume that effective resistance over
the whole range, and just use your CTS curve to derive
the voltage curve.
Best regards,
Bob Masta
dqatechATdaqartaDOTcom
D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Home of DaqGen, the FREEWARE signal generator |
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Ed
Guest
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| Posted:
Wed Nov 23, 2005 1:35 am Post subject:
Re: Multiplexing temperature sensor |
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|
Thanks, Bob.
Ed
"Bob Masta" <NoSpam@daqarta.com> wrote in message
news:438321d3.3522587@news.itd.umich.edu...
| Quote: |
Not constant current, just a plain old series resistor (plus a
diode). But I'd avoid trying to measure the resistance of the
ECU, just because it is a questionable practice in general,
and also because that diode is in there to mess up your
measurement. But since it isn't a current source, using
a voltage divider approach is probably best. Just measure
the CTS resistance (disconnected from the ECU, of course),
as well as the voltage when it is connected, and you will
have the current at that temperature. The effective ECU resistance
will be 12 minus the CTS voltage divide by the current.
You can probably assume that effective resistance over
the whole range, and just use your CTS curve to derive
the voltage curve.
|
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Ed
Guest
|
| Posted:
Wed Nov 23, 2005 1:35 am Post subject:
Re: Multiplexing temperature sensor |
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|
"Bob Masta" <NoSpam@daqarta.com> wrote in message
news:437f2b9e.1729905@news.itd.umich.edu...
| Quote: | But curve fitting by analog means can be a bear.
If you want to go whole-hog, you can use table
look-up with a small processor chip.
|
I remember forom analog computer days a device called a diode function
generator (DFG). As
I recall these were basically a set of diodes and resistors. Is there
an IC that does this sort of thing?
Or, what is the processor chip you are referring to? Microcontroller?
When I do a Google on "diode function generator" it's like a march
back in time... pulls up
all sorts of old papers by analog computer programmers. None of mine
though... sigh.
Ed |
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Bob Masta
Guest
|
| Posted:
Wed Nov 23, 2005 5:35 pm Post subject:
Re: Multiplexing temperature sensor |
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|
On Wed, 23 Nov 2005 00:47:39 GMT, "Ed"
<jag_manR__EM*-0_V_E653@hotmail.com> wrote:
| Quote: |
"Bob Masta" <NoSpam@daqarta.com> wrote in message
news:437f2b9e.1729905@news.itd.umich.edu...
But curve fitting by analog means can be a bear.
If you want to go whole-hog, you can use table
look-up with a small processor chip.
I remember forom analog computer days a device called a diode function
generator (DFG). As
I recall these were basically a set of diodes and resistors. Is there
an IC that does this sort of thing?
Or, what is the processor chip you are referring to? Microcontroller?
When I do a Google on "diode function generator" it's like a march
back in time... pulls up
all sorts of old papers by analog computer programmers. None of mine
though... sigh.
|
I have never really used the traditional diode function
generator... they are a morass of interacting bias resistances,
not suitable for hand-tuning. Nowadays, I imagine that
modeling software or even a special diode function generator
program might make this more straightforward, though.
An "improved" diode function generator uses op-amps to
isolate each breakpoint, so they are somewhat easier
to adjust, but they tend to have sharp corners at each
breakpoint... not what you want here.
If you want to pursue this, I'd first take a long hard look
at the correction you need to apply. You might get
lucky and be able to use some simple circuit to do this.
The processor approach is not specific to any chip.
You read the voltage with an A/D, and use that
value as an address into a lookup table to get the
corresponding temperature. If your chip has a lot
of memory, you can have an entry for every degree.
If not, you can segment the required curve into
segments, and take the high N bits of the A/D
value to select the segment, and apply linear
interpolation to get the degree reading. Since
temperature changes so slowly, even a slow
chip should be plenty fast enough to do a lot
of calculations. Note that I'm no expert at
this, and have no specific chip recommendations here
If you are not already comfortable with embedded development
then you can either use this as an excuse to go off
on a long, involved learning quest, or just fall back to
creating a face for an analog meter.
Best regards,
Bob Masta
dqatechATdaqartaDOTcom
D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Home of DaqGen, the FREEWARE signal generator |
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