Controllers: Speed vs (and) Torque Control

pedal map gif from the AEVA forums

Pedal map gif from the AEVA forums

We’re making progress on this whole “How does a controller work?” thing. I still haven’t quite been able to find out what controllers use what kind of control – torque control or speed control – but I decided it would be a better thing to try to learn exactly what the difference was, and why you’d want to use one over another.  What I found was that the hot setup is to use a degree of combining both if you want smooth power delivery.  Especially if you want it to feel like a gas bike.

Here’s the basic breakdown, as I understand it.  Pure speed control will feed 100% of your available torque to the motor to get it up to the “requested” RPM.  (Remember now, volts = RPM, so it’s specifying the voltage and letting the current rip until the RPM gets to the target.)  Pure torque control dials in the current to feed the motor until it gets to the requested speed.  Torque control still has to monitor the speed, and yes, it controls the voltage, but that’s done as a secondary step (to put it in a way that will drive most EEs nuts).  It’s a way of dialing in the rotational force until you reach a desired RPM.

I think it may be better to think about how these two approaches respond to your throttle.  With pure speed control, you twist the throttle and it feeds more voltage at the maximum current it can do.  With pure torque control, you twist the throttle a bit, and it adds more current, turning the motor harder.  The result is, the throttle responds more like a gas bike – you turn it a little, it pulls a little harder.  More, it pulls a lot harder.  When it gets to the speed you want, it tapers off.

Apparently, true speed control isn’t a great thing, whereas true torque control is “tolerable”.  From the AEVA forum post:

True speed control would be very unpleasant. A small change in throttle position would result in a sudden jerk as you accelerated at maximum torque, then another jerk once the set speed is reached and the torque drops down.

Torque control is very smooth, but relies on the driver to regulate the vehicle speed by varying the throttle position.

A DC drive using open loop voltage control is in between the two. It’s basically a softer, less jerky version speed control, but isn’t as smooth as torque control. Some people prefer soft speed control, others prefer torque control.

For AC drives, torque is what’s intrinsically controlled, so you’d have to build a feedback system around that to emulate speed control if that’s what’s desired.

There’s more discussion on this post, also on the AEVA forum:

Requested torque (normalised) is a function of both pedal position and actual speed.  …   Requested speed (rpm, normalised) is based purely on pedal position, and corresponds to that actual speed at which the requested torque would be zero (for the same pedal position).

On a gas engine the throttle controls the fuel/air delivery.  It’s a natural speed-torque feedback loop – almost like a cushion for the power.  You open the throttle, the engine gets more fuel and air, increases toque, and tries to go faster until it tapers off at it’s new top speed for that volume of charge.  The fuel/air volume controls the power output, and power is a function of torque and RPM, right? An electric motor has no such natural feedback loop.

So back to my question.  What’s the difference between running a controller with speed control vs torque control?  It sounds like it all gets down to the feel of the throttle response and the smoothness of the power delivery.  For the most part, an AC motor wants (needs) to control torque, and you can integrate speed control into the programming to give more of a speed control feel.  A DC motor controller is generally using a speed control, however several models I’ve been able to find specs on will allow either torque or speed control (by themselves, or with the addition of a module).

Simple answer?  If you want smoother, more gas-like throttle response, run a controller that allows a balance between torque and speed control, and it sounds like this points squarely at an AC system.

A lot of stuff is starting to make sense, with that in mind.  Running an AC motor isn’t just about easier cooling, higher RPM ranges and less maintenance – it’s also about being able to control the response of the power delivery in a way that, well, feels better to a rider.  And by “better”, mostly I mean “more natural” based on gas bikes – but still, probably better overall.  Smoother, better integration, more control over the torque applied to the wheel…  yeah, it’s all starting to make sense.

 

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One response to “Controllers: Speed vs (and) Torque Control

  1. Pingback: The Controller Thread: Last Installment (Alltrax SPM Controllers) | The Electric Chronicles: Power in Flux·

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