Wheel Speed Sensors / Vehicle Speed Sensors
 

Hi EveryBody,

I am Looking For a Wheel Speed Sesnors / Vehicle Speed Sensors that may help in calculating Speed and Distance for my application in absence of GPS Data. Any suggestions on what kind of Sensors I should go with . Any body having prior experience in Anit-Lock Brake System who could guide or share technical Knoweledge/Docs in the same. I am New to the field of Automobiles , Sensors ... Some of the questions I have are .


1. If I go for Magnetic Sensors , they do not give the Vehicle Slip So how to accomadate.
2.Radar Sensors Do give the True Ground Speed but they are pretty Costly .
(Any suggestion on RADAR Sensors that are costeffective )
3. If I have to choose among Magnetic Sensors , which would be better .
1. MagnetoResistive
2. Halls / Differential Halls
3. Inductive



Thanks & Regards
Chitu

Re: Wheel Speed Sensors / Vehicle Speed Sensors
 

Hi.
In practice, the amount of slippage between the automotive tire and the road surface is negligible and can be ignored. On a robot (I am guessing that's your application?) slippage can be more significant. But, it very much depends on what exactly your goals are.

If you are trying to actually navigate based on pure dead reckoning, your errors will eventually creep up and get you. It's like driving a car with your eyes closed, steering by stopwatch.

The actual type of sensor you use is less important than how you interact with the surface to be measured. All of those technologies will accurately measure the rotation of a wheel, from which you can measure speed and calculate distance (but not direction). You may find that Hall sensors with conditioned outputs are easier to interface than inductive or magnetic sensors, which may require separate conditioning circuitry.

Inductive sensors need a higher wheel speed to give a useful output than magnetic sensors, but they are very similar otherwise. Hall can detect zero speed, as can optical.

Instead of radar, look at optical. Think of your optical mouse - it's pretty accurate, right?

For the rotational sensors, instead of using a vehicle wheel, use a dedicated measurement wheel which will not slip and can sense direction. Think of, say, a baby carriage front wheel - it rotates to allow steering. There's a type of wheel with an offset axle that rotates around the point of ground contact. Now, instead of a smooth wheel, think something with teeth, like a gear, but bigger teeth, that won't slip at all ever.

As the robot moves, the gear senses motion. If the robot turns, a second sensor senses the turning of the measurement wheel.

I hope this helps. If you can get into more details as to the goal, we all can get more exact in our advice.

Good Luck,
Don

Re: Wheel Speed Sensors / Vehicle Speed Sensors
 

Hi ,
Thanks a Lot, indeed it was a great help , It has imporved my thought Process.

Goals are :
1. Zero Speed Measurement
2. Accuracy 1% of the Velcoity Travelled
3. Fucntions in extreme environmental conditions
4. Cost-Effective [ <500$ ,but subjected to change if necessary]
[note : Vehicle is extensively used on Lawns ]

The Activity we are currently involved is to choose a sensors that would best suite our purspose.


I did check out some optical sensors , they seem to be pretty costly [ Corrsys-Datron]
Any manufacturers where I can get Optic Sensors.

I would also like to know manufactures for Hall Speed Sensors and Radar Speed Sensors too .[ during the Study phase of the sensors I could a list a few but it would be great , If I could add on to my list ]

1. During the process of Studying about the Speed Sensors I could find
Gear Tooth Speed Sensor , Transmission Speed Sensors. can these sensors be used to find the Wheel Speed / Vehicle speed
2. Is there any difference between Gear Tooth Speed Sensors and Wheel Speed Sensors .Will the Gear Tooth Speed sensors be helpful to Find the Speed of the Vehicle.



Thanks & Regards
Chitu

Re: Wheel Speed Sensors / Vehicle Speed Sensors
 

One thing for you to consider;

What you are talking about with hall effect and magnetic sensors is actually making your own encoder. Instead, you may want to consider purchasing a standard encoder. If you get a quadrature type encoder (most of them are) you can get distance/speed as well as direction. And it is environmentally sealed.

Matt b.

Re: Wheel Speed Sensors / Vehicle Speed Sensors
 

Yeah, encoders are very robust and much easier to implement than hall effect/magnetic.

These two encoders are reasonably priced, and both perform well (and if you use a IFI controller for some reason, there is code already available);
Grayhill 63R256 and 61K128
One is 256 count per revolution and the other is 128, the price difference is small so I'd go with the 256 unless that is too many counts per second for your processor to handle.

By the way, what robot controller are you using?

Re: Wheel Speed Sensors / Vehicle Speed Sensors
 

One of the problems with trying to measure distance and direction outdoors (you mentioned a lawn) is that the terrain will affect your measurements if you try to measure using wheelspeed. Slippage on the surface will also affect your measurements, as will the little differences in wheel diameter.

I would suggest not trying to measure by internal sensors, but instead using external references like the following:

In the absence of GPS, I would suggest using a electronic compas like This one and a simple Radar gun (aimed at and using ground speed) like This one.

These will give you direction and distance information that will be far more accurate over larger distances.

Good luck. Hope this helps.

Re: Wheel Speed Sensors / Vehicle Speed Sensors
 

Hi ,


To Select a Speed Sensor (Magnetic) what are the Critical / Important Features that I have to consider Other than Power consumption , Operating Temperature , Environmental Compliance , Price .


Thanks & Regards
chitu

Re: Wheel Speed Sensors / Vehicle Speed Sensors
 

I'm guessing you just want the overall speed of the vehicle, and I'm guessing its outside (since you're worried about slippage) so,

If $99 is an ok price for you, then just get a RS-232 GPS receiver, they spit out speed as part of their very-easy-to-decode databursts. You get 1 second updates, an as an added bonus, you know where your robot is! :D

Here's Microsoft's available at some stores for $99 (on their site $130). Just cut the wire running from the GPS antenna and the USB<->RS232 converter and just solder your serial lines and power supply (+5V) line onto the wires. OR solder on to the board directly, make your own connector, etc. I believe it runs at 4800bps, but you'll have to check that on HyperTerm.

PM me with any questions.

-Q

Re: Wheel Speed Sensors / Vehicle Speed Sensors
 

The application is is used in the absence of GPS Signals [ Absence/ Lose of GPS Signal due to interference of other signals [ Reasons may Vary ] .So the GPS receiver suggested .... will not helpful.

So looking for a sensor with the following criteria

1. That will give exact vehicle speed .
2. Mounting should not be a problem
3. Good Performance in harsh environments and able to give good reading in regions like terrain and others where there could be more slippage than the Normal Roads .
4. Cost Price < 500$


Can Any body give a relative comaprison why one should go for a RADAR Speed Sensor to Otpic Speed Sensor , other than the Cost Factor

Regards
chitu

Re: Wheel Speed Sensors / Vehicle Speed Sensors
 

Chitu,
There is really nothing that will be accurate over any terrain for any great distance. All of the methods will have some built in error that accumulates over time and distance. Even RADAR based sensors will lose track of the reference object from time to time. What kind of distance is your application aimed at? 100 yards, 100 miles? Over short distances, reading wheel rotations is a good choice. It is simple, easy to decode and the errors are relatively small. If you read several wheels and correlate the data you would be able to compensate for turns and any one wheel slipping. Over hilly terrain they won't give you straight line distance traveled, though.

Re: Wheel Speed Sensors / Vehicle Speed Sensors
 

You mentioned keeping your distance accurate when the wheel is slipping. To do this, I'd use an accelerometer, and with some calculations, you can figure out what speed the robot should be at with a given voltage. Then if the robot is going slower than it should be, the wheels are slipping... I don't know how accurate this would be, but I know the kit accelerometers give you milliG's.. They do have to mounted just right though.

Re: Wheel Speed Sensors / Vehicle Speed Sensors
 

I'd actually suggest using a multitude of different sensors.

Use encoders (hall effect, optical, it shouldn't matter too much) to measure the wheel speed. Use a dual-axis accelerometer and gyro, and integrate the results over time to build a simple inertial navigation system. If you are still under budget, look into the RADAR unit.

With many different sensors, you can "vote" among the sensors to find the best data. With multiple sensors, you also have graceful degradation, for if one of the sensors ever failed, the other sensors can still operate the vehicle.