Tuesday, June 5, 2012

On Power Steering and a "Normal" Feeling Drive

Anyone who's been reading this blog for a while, knows that I have a Toyota MR2 electric hydraulic pump running the power steering system.  Take the ICE engine out of a car, and chances are your power steering pump goes with it. 

There are a few ways of dealing with this problem.  One is to simply fore-go power steering.  But unless you replace the steering rack with one that's geared for non power steering, that makes turning at low speeds rather like arm wrestling with Lou Ferrigno.   The second is to build some apparatus for holding the old power steering pump onto the front of the electric motor, putting a pulley on the tail shaft of the motor and powering it off that.  Of course, if the motor isn't spinning, you have no power steering.  For me and I'm guessing many others, there simply wasn't enough room to incorporate something like that.  You'd be surprised how many times you run out of room and 1/16th of an inch more was all you needed to make something fit.

The third, and perhaps most popular remedy is to use an electric power steering pump.  There are a few OEM cars out their using such devices and the second generation Toyota MR2 was one of them.  They cost somewhere around $300 to $400, they don't take up much room and its relatively inexpensive to have the custom hoses made to add them to the system. 

On a normal car, they run all the time and you really wouldn't care because the alternator will just produce the energy it needs to work.  But in an EV, you want to try and reduce parasitic loads.  That pump can draw up to 85 amps in a hard lock!  So the idea of having it run all the time was troublesome to me.  To resolve that, I devised a system by where the pump stayed off when driving in a straight line, but would come on as soon as you turned the wheel a few degrees.  I also added an off-delay relay so that it would remain on for 10 seconds after I was going straight.  In this way, it wouldn't cycle on an off while I was going through a parking lot or some other place requiring a lot of turns. 

But it wasn't a perfect solution, and I grew to dislike it for two reasons.  One was that it proved to be dangerous.  As is bound to happen when driving, it became necessary for me to quickly dodge something in the street.  The amount of effort it takes to begin turning is massive.  So when the pump kicks in, you just can't react quick enough to release the pressure.  I ended up nearly steering right into the car next to me.  It left me with a case of the sweats as I pondered the fact that I'd nearly caused an accident and destroyed all my hard work.

The second reason was that when I would have other people drive the car (something I do as frequently as possible) they had a difficult time getting used to that sluggish, stiff steering wheel when heading into a corner.  I thought, "how can I convince people that EV's are fun to drive and just like any other car, only better, when they had to wrestle the steering wheel going into any turn?  This was not the message I want to convey. 

I thought I would simply put a latching switch into the system that would allow me to bypass the proximity switch on the steering column which would then turn the system on and leave it on until I pressed the switch again.  Seemed like the perfect plan and I even had a great place to locate the switch in the cabin.   So, before I took the car apart to work on the last motor problem, I ran a few tests to find out just how much power the pump drew when I was traveling in a straight line.  It turns out that it draws less than 2 amps from the high voltage pack.  That translates into something like 18 to 22 amps on the 12 volt system.  Not trivial, but not too bad.  The system is sized to accommodate it.

The point is, I could let the car sit idle running that pump and it would take over 60 hours for it to deplete the battery pack.  That means in a standard commute to work, which takes 20 minutes, the power steering pump uses an extra... let me think... about, Oh I don't care!!  It's seriously not enough for me to worry about AT ALL!  Since the car has been on the road, I've left that switch engaged the whole time.  My previous post detailed how I've experienced a boost in efficiency seemingly from the new brushes, so the change to the system of having this pump run all the time might have had an adverse impact on that, but it's not enough that I care. 

The car drives like a normal car now, in almost all respects.  I don't have to explain to people that drive it why the steering feels weird at first, and they don't have to experience the sharp jerk when the pump kicks on and they aren't expecting it.  This is definitely the way to go. 

By the way, I have turned it off a couple times.  Sitting, waiting for my daughter to finish her choir practice, I left the car on so I could listen to the radio and enjoy the air conditioning, but I turned the pump off.  Honestly, I'm not sure it was worth the effort of lifting my hand to press the switch. 

4 comments:

Boo plus Aroo said...

Hi, just saw your blog. You probably posted this a long time ago, and sounds like you resolved the problem, but might you base your power steering assistance on a certain speedometer trip point instead? Like if you are going 10mph or less, presumably that's when you're in a parking lot, and really the only time you need power steering anyway...
Best

Tim Catellier said...

Boo,

You're absolutely right that this is the proper way to do it. Power steering assistance is modulated vs. the speed of the car. To do that, you need to read the CAN BUS signal put out by the ECU first. Then you need to take that signal and process it so that you can intelligently vary the power to the pump so you get the variable assist steering.

The good news is that the MR2 pump already has a module built in that will vary the power it draws depending on the signal that's sent to it. Here's were I run into problems. I don't know anything about finding or interpreting the signal from the ECU that indicates speed; and I don't know if it's even in the format that would control the pump properly.

The bottom line is that I simply don't know how to do it. If someone smarter than me was willing to stop by and show me how, I'd be eager to learn all about it. But as yet, no one has.

The good news is that since the pump is a DC motor, it tends to draw the current it needs. It's sort of self regulating in that regard. DC motors do a great job of spinning at a specific RPM based on the voltage of the current flowing to them. If you try to slow them down, they simply draw more current to keep the same RPM. Put more load on them, the draw more current until there's no more to draw. Because of that, I really never notice a difference in the way the steering feels whether I'm going slow or fast. It kind of works itself out.

Thanks for the comment.

Tim

Christian Schulz said...

Hi Tim,
I followed your blog. Great inspriation. Converting a Porsche Boxster myself, I have the same problem. I found a guy who build me a custom speed sensitive control unit. The only thin you need is some sort of speed signal which you might find feeding into your radio. The pump stops a a programmable speed lets say 45 mph and starts again at 45 mph. Once you roll to a stoplight it stops the pump after 10 sec delay and starts right when you drive again. The 10 sec. should also cover any parking. The unit is 60 Euros. See my blog incl. a short video. http://eboxster.blogspot.de/2012/10/steuerung-der-servopumpe.html
Best Regards
Christian

Tim Catellier said...

Hello Christian,

I'm eager to see your final design for the power steering pump system I can tell you that for my car, it turned out that simply having the pump on all the time when the car ignition is turned out on to the easiest and safest solution. Having a circuit to adjust it's power based on speed might be nice, but it's unnecessary. The car drives perfect with out it.

The additional power it draws from the battery pack is so small I just do not care. I could turn the car on and it would take 40 hours to use up all the energy in the battery pack. In a 1/2 hour trip, the pump draws only enough power to move the car less than 1 kilometer. It's simply not enough energy for me to worry about.

Tim