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Topic: Vacuum sensor for automotive assisted brakes (Read 2245 times) previous topic - next topic

GordonEndersby



Hi guys,

After a problem over the last few weeks wih my vacuum assisted brakes on my car Id like to monitor them.
Ive had a bit of a google but im not sure what kind of sensor I need.
Ive seen info about monitoring boost and pressure in engine management but I dont get the idea that the sensors being used would be appropriate for measuring the vacuum produced by the vacuum pump on my diesel landrover discovery.

If it helps, the setup is a vacuum pump run mechanicaly off the engine.
The vacuum is used in the brake assist servo connected via a pipe to the pump.
There are no existing sensors I can tap into in the system.
Ide like to add a t-piece to incorporate the sensor and monitor the vacuum for variation over time and failure.

Thanks

Gordon

jackrae

Try this device

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0286692
Not too bad at circa £20

It's a differential device so if you leave the HP port to atmosphere and connect the LP port to your vacuum you'll get a reading proportional to the vacuum.

If you want something with screwed pressure connections it's going to cost you a whole bunch more.

jack

GordonEndersby


Thanks for pointing me there.
I dont need screw on connections as it is all push to fit hosing.
Whats the difference between compensated and non compensated?

There are some non compensated on RS for less cost.
http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=2355863
Where port 1 is left to atmosphere just as you suggest that I leave one connection off to atmosphere on the device you pointed me to. Is it just that these non compensated devices dont have a port to connect a hose to another pressure source and just have a "hole" left to atmosphere?

Thanks

Gordon

jackrae

Gordon,
As far as I can determine from the data sheets, the compensation relates to automatic level output temperature tolerance between 0 and 50 degrees C.  The uncompensated units have no level temperature compensation and you need to use  constant current excitation in an endeavour to provide some degree of compensation.

If you can keep the sensor at a constant temperature (car interior ?) then an uncompensated unit may be OK, but if you are that interested in getting accurate vacuum readings wouldn't the extra cost of the fully compensated unit be worth going for.

Compensation has nothing to do with 2nd port connection.  Gauge sensors (1 port) measure pressure relative to atmospheric pressure and differential sensors (2 port) measure pressure relative as the difference between port 1 and port 2.  Note that either port may be left free to atmosphere, rather than being connected to some preselected reference pressure.

regards

jack

cr0sh

Quote
Ide like to add a t-piece to incorporate the sensor and monitor the vacuum for variation over time and failure.


So you want to "break" the vacuum system, and in effect, introduce four new points of potential failure - for your brakes on the car you drive?

I say four, because now you would have the three joints on the T-connector, plus the sensor itself - as new points of failure; and it doesn't sound like you are even sure it is the vacuum line or pump...

This doesn't sound like the safe thing to do. You don't mention what your problem is exactly (what you are experiencing), but if this was my situation, I would remove the line from the booster, and put a vacuum gauge on it, run the engine to pull the vacuum (via the pump), and see if it compares to what the manual/specs says it should be.

There's probably even a testing procedure outlined in a Haynes/Chilton; likely you run the engine to draw the vacuum, and the shut it down to see if the vacuum holds over time. If it doesn't, then it is the hose or the pump. If it does, then it may be the booster. Depending on the age of the vehicle, you might just want to go ahead and replace all of it.

Its your car, and your risk, of course - but I would definitely think long and hard about messing with the braking system of your vehicle (and potential consequences of doing so)...
I will not respond to Arduino help PM's from random forum users; if you have such a question, start a new topic thread.

Leon Heller

#5
Aug 22, 2010, 02:01 pm Last Edit: Aug 22, 2010, 02:05 pm by leon_heller Reason: 1
It will also invalidate the insurance policy, which will make the OP liable for any damage or injury he causes if the car is involved in an accident, even if it wasn't caused by the modifications.
Leon Heller
G1HSM

GordonEndersby


Thanks that explains the difference between the different types of sensor.

As to the safety.
Im fairly experienced with vehicle mechanics and braking systems.
Ive rebuilt more than I can count on bikes as well as cars.
I have qualifications as well as commercial experience.

The pressures or lack of pressure with the vacuum is fairly low.
On a normally aspirated petrol engine the vacuum is taken from the carb manifold.
So relying on induction process, the sucking in of the fuel air mixture in the carb.
Lots more chance of failure if you take into account the heat and other possible failures in that area.
In this case its a deisel, not so much induction vacuum, so it uses a pump run mechanically by a push rod on a cam I think on the crank shaft. Maybe another lay shaft.
All of the pipe fixtures are press fit and the pipes take a torturous path along the bulkhead with several joins and T's in the system to change the ID of the pipe for the other ancillaries such as the EGR system and one way valves. Plenty of places to tap into the pipe and keep it secure against the bulkhead. Ive already replaced a number of the vacuum hoses that looked like they were on there way out tracing the last problem.

This is an assisted brake system the backup in case of failure is to push on the brake pedal harder.
As I had to do on the motorway the other day when it failed towing a heavy braked trailer full of all our camping gear with 2 adults and 4 teanagers in the car. I wouldnt have wanted to try an emergency stop as I had to practically stand on the brakes as it was. Towing and loading weights were well within the limits for a landrover discovery.
The accepted test for the vacuum pump is to put your finger over the open pipe to see if its sucking.
As to the servo yes you can use a rule of thumb on the change of pressure on the pedal.
I cant find any published figures for the system yet. but im looking.

As anyone whos run a car or bike has found you tend to compensate when things wear or degrade until it stops working. Someone else driving the car may notice its not as good as it could be and you then take notice.
My problem was that the brake performance faded over time with eventual full failure in the vacuum servo and then a week later, after replacement of the servo, the pump finally gave up. In hindsite the old pump didnt suck as well as the new pump but I had nothing to compare it with when I replaced the vaccum servo.
It felt ok to me at the time but there was a compareable difference to the new pump.

The ABS system the car has can can pick up some of the failures or degredation in the braking system but the vacuum assist has no failure warning or monitoring. So I wanted to tap in a sensor and let it monitor over time and let me know of any long term degredation that would warn me of possible failure.

Fairly easy to monitor and report something like this with the Arduino. But Im sure Ill expand it like the other guys did with the car turbo projects.


Gordon

GordonEndersby


The post about insurance came while I was typing.
Thats not such an issue here in the UK.
Many places around the world are tighter on modifications.

But since Im running a vehicle that often have modifications done in normal as well as off road performance instancies. Adding a T piece to a vacuum pipe for the "Assisted" brakes wont be an issue as long as I make sure its seen to have been done properly and is secure.
I also have specialist insurance for an off road vehicle and they expect that most customers will have modified the vehicle in some way. Under body armour, roof racks all the way to uprated suspension and brake systems.
An extra guage will not be a problem.

Gordon

Leon Heller

I'm in the UK, and most insurance policies specifically exclude cover if a car is modified. Your's might not have such an exclusion, of course.
Leon Heller
G1HSM

jackrae

Better not plug your navigator into the cigarette lighter socket then, because if I was an insurance underwriter I'm sure I could argue that was a modification.

Ballcocks to all this can't-do attitude.  Life is meant to be fun, you only get one go at it, "Do and be damned", I say

jack

AWOL

#10
Aug 22, 2010, 09:04 pm Last Edit: Aug 22, 2010, 09:13 pm by AWOL Reason: 1
Quote
Life is meant to be fun

...right up to the point that some jerk with failed brakes and invalidated insurance drives into the back of you at lights.
"Pete, it's a fool looks for logic in the chambers of the human heart." Ulysses Everett McGill.
Do not send technical questions via personal messaging - they will be ignored.

GordonEndersby



All right I promise not to drive behind you.
As long as you dont use your phone while driving or live next to me and plug any of you home made Arduino devices into the mains.

Back to the point.

Now I know a bit more about the sensors and the terminology Ive found a nice cheap alternative.
The ones suggested so far need amplification.
If you want one that is amplified its going to cost £40.00 or more.

After a bit of searching, Ive found the boost pressure sensors fitted to production cars.
They seem to be 5v devices and output between 0 and 5v.
Built for the engine bay and with mounting lugs and a nice plug socket.
All for about £20.00 on ebay.
http://cgi.ebay.co.uk/NEW-MAP-Manifold-Air-pressure-sensor-ALFA-ROMEO-33-/250540286109?pt=UK_CarsParts_Vehicles_CarParts_SM

I think Ill take a punt and see if it works.

Gordon

cr0sh

Quote
..right up to the point that some jerk with failed brakes and invalidated insurance drives into the back of you at lights.


If you're lucky! You might be t-boned at an intersection; though in the UK it might "sideswiped on a roundabout" (wish we had more of those here in the US).

;D

GordonEndersby: It sounds like you know what you are doing, but please be careful. I know that the booster is only one section (and is there to help - not replace - so pump the pedal if you have to, I suppose!). If you can make it as safe as possible, and it won't invalidate your insurance...

Well - good luck with finding the problem!

:)
I will not respond to Arduino help PM's from random forum users; if you have such a question, start a new topic thread.

jackrae

Thought it was a vacuum sensor you were looking for.

By its description I would have thought that a "Boost" pressure sensor is a gauge pressure sensor designed to measure the positive pressure inside an inlet manifold after the turbo booster.  ie used on an injection engine that does not require a negative pressure manifold where the air velocity through a venturii draws petrol from a carburettor.

jack

GordonEndersby



From what Ive read so far the same sensor as was suggested to me earlier has been used for boost pressure projects. The MAP sensor (Manifold Air Pressure / Boost sensor) goes on the inlet which is at lower pressure than the ambient air pressure.
You are never going to get anywhere near a true vacuum. The brakes vacuum assist servo works on the diference between the air pressure and lower pressure on the vacuum pump side.
The pressure on the vacuum pump side is still a positive pressure and should be in the range of the sensor.
The vacuum pump is designed to lower the pressure by the same amount as is in the inlet manifold.
The sensor just needs to be within a working range.

Its on its way so Ill report back when I get it.
If it works it will be a cheap way of getting a robust amplified pressure sensor working at 5v and with an output between 0 and 5v. Ideal for the Arduino.

As to the safety aspect again.
The problem with my brakes has already been fixed with the new pump. I just want to monitor it.
As I said earlier I wasnt going to drive it home without brakes and neither would I drive it if I thought I would endanger myself or others with this very minor and safe modification.
After all its its also got the EGR sensor attached to it on a thinner hose theres no additional risk with adding my extra sensor.

Gordon

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