inverse one motor speed based on input from other motor speed

13 years 10 months ago #18785211 by Deyeme
I'm working on an EV (electric vehicle). This EV has power steering, which is run via a 12v dc motor that pumps fluid thru the rack to offer power assist. When the vehicle is at 0mph, I would like the motor to run at maximum assist. As the vehicle is moving faster, I would like the power assist to decrease (the PS pump motor to slow) to conserve energy. In a way, this is exactly the opposite of tachometer closed-loop control. That is, I want to monitor the speed of a <EM>different</EM> signal (instead of the motor's own speed signal) and make this motor to the exact opposite (speed up if the signal slows, slow down if the signal speeds up)<BR><BR>I realize this is not robotics specific, but I think getting some input from this field might help solve my delimma. The transmission has a speed sensor in it, which runs on +5vdc and generates a square wave representing vehicle speed. Can this input be used, so that as the frequency increases, I can use a controller to slow the motor? I could also attach a new sensor to the axle shaft or wheel hub or whatever. I just need to know if I get one of the available controllers that it will do this job, and what other hardware (sensor) I might need to do it.<BR><BR>Thanks!<BR><BR>

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13 years 10 months ago #18789838 by cosma

This is quite easy to do. The trick is to take your speed sensor signal and make it such that the controller sees 0V when the speed is 0 and 2.5V when speed is max.
This can be achieved with a few resistors. 0V makes the controller run in full speed reverse (a relative term), 2.5V makes it stop.

You would wire the motor between one terminal and ground (instead of the controller's M+ and M- terminals) so that the motor does not start spinning the other way if the voltage at the input rises above 2.5V.

Any of our controllers will work. The choice of one vs the other is a matter of how much current your motor pump will draw. Keeping in mind that if you only have one motor, you would use our controller in single channel (and twice the amps) mode.<BR>

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13 years 10 months ago #18790700 by Deyeme

Thanks for the quick reply!

cosma wrote: This is quite easy to do. The trick is to take your speed sensor signal and make it such that the controller sees 0V when the speed is 0 and 2.5V when speed is max.


The existing VSS is a +5v square wave. When the vehicle is moving, the car's speedometer interprets the <EM>frequency</EM> of the wave and shows vehicle speed in the speedometer. It's not a variable value between 0 and 5 volts, but rather a frequency of 0-5-0-5... square waves. If I need to use a different sensor than the OEM one, that's fine- I just need pointed in the right direction for a sensor or magnetic pickup that would work.


This can be achieved with a few resistors. 0V makes the controller run in full speed reverse (a relative term), 2.5V makes it stop. <BR>

You would wire the motor between one terminal and ground (instead of the controller's M+ and M- terminals) so that the motor does not start spinning the other way if the voltage at the input rises above 2.5V.

So if I want to control a motor in only one direction with your controller, I could hook the motor up to ((M+ and ground)) OR ((M- and ground)) instead of ((M+ and M-)) ?

That makes sense. I actually hadn't thought of the motor reversing, and in this application, I only need the motor to spin one direction.


Any of our controllers will work. The choice of one vs the other is a matter of how much current your motor pump will draw. Keeping in mind that if you only have one motor, you would use our controller in single channel (and twice the amps) mode.

Yes, I only have one motor I need to control.

I'll find out what the fuse is that connects the pump to battery power. I know it won't be above that value, and I'll have a better idea of which controller would suit my needs.


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13 years 10 months ago #18790997 by cosma
If you can find a sensor that will give you a voltage out, you would be in great shape.

The controller firmware could be customized to take your square wave signal directly but the cost for that service may not be justifiable if you are not going to need many controllers.

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13 years 10 months ago #18791464 by Deyeme

cosma wrote: If you can find a sensor that will give you a voltage out, you would be in great shape.<BR><BR>The controller firmware could be customized to take your square wave signal directly but the cost for that service may not be justifiable if you are not going to need many controllers.<BR>


<BR>How does the closed-loop tachometer work?<BR><BR>It might be a worthwhile investment if you can make the controller work with the square wave. If I can get this to work, it has NUMEROUS applications, both in electric vehicles and in racecars that could be converted to speed based electrohydraulic power steering. As an example, if you've ever driven a car with wide drag racing tires in the pits area, you're operating at very low vehicle speed, and you need the power assist. When you're making a pass down the drag strip, though, you want NO power assist. You can't just flip a switch, though, because you want some assist to get you the first 50' or so, as the car might be a little "squirly" <FONT size=1>(and messing with extra switches or buttons is distracting)</FONT>. The logical solution is something that offers the most assist at 0mph and no assist at some adjustable vehicle speed, say around 45mph. The pumps already exist, but the controllers tend to be integrated into the PCM for those specific vehicles. Your controller already has the +5v, which happens to be the same voltage the original speed sensor uses. Your controller already uses PWM. Your controller is already PC programmable.

<BR>I may just have to buy one and find out the hard way!<BR><BR>

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13 years 10 months ago #18792047 by cosma
The need is quite clear.

There is also the need to use sensed speed in the case of drive by wire, so that joystick movements result in smaller wheel steer at higher speed than lower speed.

We typically work with a customer to implement such new features. This ensures that the feature has a need and that it will be tested and verified. It is usual that we charge a token portion of the developent cost (typically between 0.5 to 1K) depending on the complexity and whether this is a desirable feature for the product in general.

In a case like this, the extra challenge is to interface to a "standard" way of picking up speed (if such thing actually exisits).

The tachometer feedback works in a very different way. The user manual has several pages of explanations on the subject.

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13 years 10 months ago #18792748 by Deyeme

cosma wrote: The need is quite clear. <BR>...<BR>In a case like this, the extra challenge is to interface to a "standard" way of picking up speed (if such thing actually exisits).<BR><BR>The tachometer feedback works in a very different way. The user manual has several pages of explanations on the subject.<BR>

I just downloaded the manual, and was reading up on the tach feature, so I think I have a better understanding of how that works.<BR><BR><BR>In a rudimentary sense, this new kind of input would operate like this:<BR>(and I'm only thinking of both things moving in a forward direction, but the principle would be similar)<BR><BR>motor speed 000%. . . . . . . 50% . . . . . . 100%<BR>sensor freq. 100% . . . . . . 50% . . . . . . 000%<BR><BR>or maybe motor and sensor would be both 0% . . . 100%, but with an "INVERT" function checkbox to make the motor do the "opposite" percent of what the sensor is doing. 100% of sensor frequency would be a variable the user would type into a textbox and/or determine by spinning the sensor at a known RPM and recording the resulting frequency value.<BR><BR>Most modern vehicles use either +8v or +5v dc square wave 3-pin speed sensors plugged into the transmission. (sensor power, signal back to PCM/speedo, and sensor ground). The computer just expects the same voltage back that it gives out, so I don't think the sensor itself "requires" one voltage or the other. Sometimes with automatic transmissions there isn't a sensor by itself, but rather a signal given out by the transmission control module. To keep it simple, I'm only thinking about scenarios where I can tap the existing sensor signal or add one that is fairly "industry standard". Fortunately, the manufacturers don't all make their own sensors. Most of the time they're made by Delphi, Bosch, or similar.<BR><BR>I have a VSS from a Dodge Neon I could send you, along with a plug for it, if you'd like to see if it's something you could work with.<BR><BR>In my searches for a power steering pump solution for my EV, I've come across the same question in years' worth of forum posts, and no decent answer. I don't think anyone's thought of looking in this kind of market, although it makes perfect sense. The controllers you sell are like smaller, more programmable versions of the big Curtis controller under the hood of my EV. Then there's the race car market, which, as far as I know, NO ONE makes anything like what I'm proposing. The closest thing you can find is a complete electric power steering from Flaming River. It's more for hotrods, and it's well into the $1000s .<BR>

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13 years 10 months ago #18880191 by Deyeme

Any updates?<BR><BR>Is there any information I could provide that would help? I've looked up aftermarket speedometers on Summit Racing's website, and most of them seem to agree that, after calibration, your existing speed sensor can almost always work with their speedo. That means that dispite minor differences in design (like the shape of the electrical plug) they're all pretty much the same since about 1990, depending on application. If you buy a new one for the Dodge/Plymouth Neon, it even comes with a new style plug (probably so they didn't have to keep manufacturing two versions). It runs about $32, I think.<BR>

I'm eager to move forward on this, as my only other solution is to find and attempt to integrate the computer and sensors from a Toyota MR2.<BR>

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13 years 10 months ago #18898387 by cosma
It will not be possible to wire your pickup to the controller until this capability is added. Unfortunately, this is not on our roadmap at the moment.

You should find a way to convert your sensor's square wave, variable frequency output into a 0V to 2.5V output. This is not too complex an electronic circuit (a 555 wired as a monostable followed by an RC integrator), if you are familiar with electonics.

You will be able to validate the concept and test the market using that extra circuitry.

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13 years 10 months ago #18899055 by Deyeme

cosma wrote: It will not be possible to wire your pickup to the controller until this capability is added. Unfortunately, this is not on our roadmap at the moment.<BR>


I'm sorry to hear that.

<BR>You should find a way to convert your sensor's square wave, variable frequency output into a 0V to 2.5V output. This is not too complex an electronic circuit (a 555 wired as a monostable followed by an RC integrator), if you are familiar with electonics.<BR><BR>...<BR>

So I can go back to your initial response once I have a frequency-to-voltage translator, correct?

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