Electric Vehicles and Electric Motors

A friend of mine finally got delivery of a Tesla Roadster.  This prompted discussion of the drive train and the fact that Tesla has had to go from two speed transmissions which were failing to a transmissionless drive train.  The ultimate mechatronic challenge, the electric car, is also a challenger in terms of the precise  application of electric motor technology.

But it has to be said that the motor and drive solution for the electric car is not where the problem has to be solved.  Any motor can be made to run an electric car.  What is critical is how you apply it.  The starting conditions require high torque at low speed and the running conditions require low torque at high speed.  So, typically, what looks like a small 5 to 15 horsepower running requirement at full speed, becomes a 150 horsepower starting requirement depending on how quickly you would like to start.  If you want to keep up with a Corvette, it uses 450 HP to start.

And this produces a lot of confusion.  Why not use at 2 speed transmission to help the situation.  Fine, but the ones that are available can’t handle the dynamic response of the electric motor.

Can electronics help this situation?  Interestingly, yes.  There is a control algorithm generally called vector control which allows you to manage the rotor torque and stator torque separately.  By varying the phase angle between the two, like advancing and retarding the timing of a mechanical distributor cap on an internal combustion engine, you get different speed torque curves out of the motor.  COOL!  Is there any downside to this?

Yes.  You need more current to produce more torque.  That doesn’t change.  So you have to be able to supply the current, and you have to be able to manage the heat.  The heat is transitory since you only need the high current during starting, but it is best to have sophisticated software running to keep track of the RMS temperature of the motor.  Lower operating temperatures mean longer life and reduced risk of demagnetizing the motor.

So, yes, you can run an electric car with a garden variety AC motor, and with good electronics, you can make it run fairly efficiently.  With higher efficiency motors, the benefit is increased driving range from a given power source.  High efficiency motors are frequently smaller and lighter weigh, but a weight savings in the motor of 50 or even 100 pounds is not that big a factor in the driving range when the curb weight of the vehicle is 3000 pounds.

Basically, its F=ma.  If you can reduce the mass of the vehicle, you reduce the battery payload required to power the car.  Aluminum space frames, like on the Prowler, have been studied by the car industry and can reduce curb weight by 400 pounds and reduce cost by 10% at the same time.  We need to bring all the mechatronic leverage to the situation that we can, if we are going to make electric cars that make sense.  Before its too late for Detroit.

Mechatronic Top Ten – Hard Disk Drives

One of the mechatronic Top Ten applications has to be the hard disk drive.  Strangely, it is not an application that you hear much about.  That’s probably because unless you work on hard disk drive design, you pretty much take for granted that little black box that stores all your information. So the group that is actually pushing the design frontier of hard disk drive technology is a very finite group.  There are only a dozen companies actually making disk drives these days, after consolidation in the market has resulted from acquisitions and mergers over the last decade.

Worldwide consumption of hard disk drives is in the tens of millions per year, and like all things electronic and high volume, the industry produces ever more memory at ever lower prices.  The absolute value of hard disk technology is one of the most incredible bargains in the world.  The current state of the art is about 10 cents per gigabyte which is quite a bargain compared to the 1.5 Megabytes for the old 3.5″ mini floppy disk.  With seek times in the low milliseconds, memory is almost instantly available due to 7200 RPM platter speeds.  The 7200 RPM speed is the equivalent of 75 miles per hour at the edge of the platter.  Higher speeds have been delivered, but the thin aluminum platter is subject to “flutter” which can cause a head crash.

The spindle motor is a 3 phase dc brushless motor that is designed to accelerate the memory platter to the 7200 RPM running speed in just 2 or 3 milliseconds.  This is an incredible feat considering that the power available is limited to a small lithium battery.  Further, the spindle motor must coordinate it’s motion with a linear actuator to place the drive’s read head a few millionths of an inch above the platter surface at the exact target sector on the disk.  So, just getting the platter to spin, which is hard enough given the time constraints, is further complicated by the extreme challenge of coordinating the rotational motion with the linear motion of the read head.

What makes this all even more astounding is that the budget for the motor can only be a few dollars, given a retail selling price of $60 for the whole package including the memory.  I don’t know how these guys come up with the solutions, but they consistently do and they consistently do it at lower prices.  The last thing I remember reading about was the elimination of bearings in favor of fluidized bearings.  At 60 million units, saving money on bearings adds up to a lot of money.

One of the many ironies of the hard disk drive is that it is at the root of many improvements in industrial motion control.  The venerable 33035 controller chip from Motorola was developed specifically to run hard disk drives.  It later appeared in a number of industrial servo amplifier designs delivering precise control of higher current power to a variety of brushless dc servo motors.

You never know where the breakthroughs are going to come from, but we keep them coming.  Keep up the good work!

Force Control Allows Robots to “Feel”

Global robot manufacturers ABB and Kuka are developing technology they claim benefits aerospace production because it can deal with the fiddly jobs. Their robots have the flexibility to help build aircraft at high quality and low cost. Gary Taylor, Kuka’s aerospace sales manager, says: “Automation offers a raft of value-added benefits including better quality, fewer wasted consumables and the removal of downstream corrective rework or rejects.

“Robots can efficiently and reliably automate a vast array of processes at a time when aero component makers are under growing pressure to find alternative solutions to stay one step ahead of their competitors.”

ABB Robotics has developed a technology that allows robots to ‘feel’. The technology, called force control, can be used in different ways for aerospace tasks such as machining and assembly.

Source: Professional Engineering Magazine