# Using both regenerative and mechnical braking systems?



## major (Apr 4, 2008)

jegues said:


> Evening gents,
> 
> I am just trying to understand how motor controllers accommodate both a regenerative and mechanical braking system.
> 
> ...


When you have the motor in torque control, there is no problem. The torque from the motor blends with the torque from the friction brakes. 

If you attempt to run the motor in speed control, it is a nightmare and will be a losing battle.

There are differing strategies used for regen control. All of which I am aware present no problems with the standard braking system and even the ABS. The strategy I prefer is to have the negative torque input signal linked to brake pedal position such that the motor controller gets a proportional command relative to amount of pedal depression. Have this adjusted such that full electric braking is commanded in the pedal travel distance before the service brakes start to engage. Most cars have about an inch or so of brake pedal travel before the service brakes start grabbing. So use that for regen. Then if a quicker stopping time is required, the operator (driver) just presses the brake pedal further and gets into the friction brakes. The torque from the electric and friction brake systems blend. It is seamless and intuitive. No problem.


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## frodus (Apr 12, 2008)

What Motor controller are you using? Does it have a regen input?

Some people have successfully used pressure transducers in the brake line and connected that to the brake input on the controllers.


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## Siwastaja (Aug 1, 2012)

As major stated, It Just Works. You always want to control your EV in some kind of torque control mode, so you ask the controller to "accelerate" or "decelerate", not for any specific speed, so it blends with regular brakes - unless you accidentally ask for acceleration while braking!

Another viable option, which is even easier than what major suggested, is using a single pedal drive approach. Just have enough regen braking for your most daily usage on the gas pedal zero position. Then you can leave the brake pedal untouched and need only one pot and it should be easy to configure if your controller allows that.

But this is very much a matter of taste. 

The two-pedal option is safer in case you borrow the car to someone familiar with traditional controls - they could be surprised of the strong motor braking -, and also that you more often have your foot on the brake pedal which decreases the reaction time to apply full friction brakes in the case of an emergency.

But one-pedal approach is surprisingly fun to drive, and safer in it's own way - when the regen is enough to stop you, you save time for not moving your foot.

Just remember that the 2WD regen braking is not equivalent to friction brakes on bad weather, especially on ice because only two wheels brake. Especially important with an RWD as it spins easily if the real wheels lose traction. Yes, regen can be very strong, much stronger than you can handle in bad conditions.

A 4-motor AWD would solve the problem and you could always use any level of regen braking in all conditions.


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## jegues (Aug 5, 2013)

Hi major! 

Let's take a small step back.



major said:


> When you have the motor in torque control, there is no problem. The torque from the motor blends with the torque from the friction brakes.
> 
> If you attempt to run the motor in speed control, it is a nightmare and will be a losing battle.


Isn't both speed and torque control used simultaneously in most motor controllers? (See figure attached)


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## major (Apr 4, 2008)

jegues said:


> Hi major!
> 
> Let's take a small step back.
> 
> ...


No, not for vehicle propulsion with a human driver.

Think of it like this using a conventional car as an example. Speed control would be like using only the cruise control and never the pedals. You can go at a constant speed, or accelerate at a constant rate, or cost, or decelerate at a given rate. Rates are in miles per hour per second.

Torque control would be like using only the two pedals. The accelerator pedal commands positive (forward) torque. The further you press, the faster you accelerate. The rate depends on the load (mass, friction, grade, aero). For constant speed, you command the exact torque needed to match the load at that speed. If the load increases, like starting uphill, then you must push down with your foot to command more torque to maintain speed.

The other pedal (brake) commands negative torque. When you press it, you slow down, or decelerate. The further you push, the higher the negative torque command the quicker you slow or increase the rate of deceleration.

Attempting to use speed control for a vehicle is a breakneck experience. Ask me how I know 

I guess you can find examples like your diagram in industry where the torque control would actually be torque limits. Toque limits would be set to protect the equipment or process. In the EV controller there would likely be a speed limit, but the primary control is torque.


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## jegues (Aug 5, 2013)

Hi major! 



major said:


> Speed control would be like using only the cruise control and never the pedals.
> 
> Torque control would be like using only the two pedals.
> 
> In the EV controller there would likely be a speed limit, but the primary control is torque.


This makes things much more clear.

Looks like torque control is what I'm after.

With a PI controller such as the one described in the figure attached, my torque reference would be the torque corresponding to any given position of the throttle, while the actual torque is the torque seen at the output of the motor.



major said:


> The strategy I prefer is to have the negative torque input signal linked to brake pedal position such that the motor controller gets a proportional command relative to amount of pedal depression.


In order to achieve the scheme you've mentioned above, I could use positive torque signals to indicate the position of throttle, and a negative torque signals to indicate the position of the brake.

The net addition of the two (with the appropriate scaling of course) could then provide the "new and improved" torque reference signal seen in my diagram. This would allow the torque reference signal to consider both the inputs from the throttle and the brake.

This is the first thing that came to mind, I'm sure there are more eloquent ways of achieving the same result. Is there?


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## major (Apr 4, 2008)

jegues said:


> The net addition of the two (with the appropriate scaling of course) could then provide the "new and improved" torque reference signal seen in my diagram. This would allow the torque reference signal to consider both the inputs from the throttle and the brake.
> 
> 
> 
> ...


It probably isn't a good idea to sum the positive and negative inputs to get the torque reference. For safety you want the negative to take priority so any brake input cancels all positive torque request.

And it is not feasible (or practical) to use actual torque in the control scheme. Such transducers are a big expense and PITA. So the control algorithm is going to rely on another parameter, be it current or slip or something. However I don't see the need to compare torque reference to torque. If the driver feels the need for more torque, he commands a higher torque reference. The actual torque value is not needed.


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## jegues (Aug 5, 2013)

Hi major! 



major said:


> It probably isn't a good idea to sum the positive and negative inputs to get the torque reference. For safety you want the negative to take priority so any brake input cancels all positive torque request.


How are the changes in the torque inputs translated to gradual or rapid changes in actual motor torque?

For example, if I wish brake gradually what prevents the motor from suddenly slamming to a stop? If I want to stop quickly, how does the motor differentiate this and a gradual braking and allow for sudden braking?

Is it accomplished by simply scaling the positive/negative torque signal provided by the position of the throttle/brake?

Also, can't you damage the motor be damaged by commanding it to brake faster than its mechanical parts would actually allow? I'm sure you could easily avoid this problem by limiting the maximum torque that the controller can request.




major said:


> And it is not feasible (or practical) to use actual torque in the control scheme. Such transducers are a big expense and PITA. So the control algorithm is going to rely on another parameter, be it current or slip or something. However I don't see the need to compare torque reference to torque. If the driver feels the need for more torque, he commands a higher torque reference. The actual torque value is not needed.


Sorry I did not make this clear in my original post. As you've recommended, the torque reference will be specified by current of the motor, not by the actual torque of the motor.


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## frodus (Apr 12, 2008)

I asked you:


frodus said:


> What Motor controller are you using?


AC drives, like Curtis, have a neutral regen (occurs when letting off the throttle) and is adjustable, as well as a brake input (variable and occurs when pressing a brake input). Both are tunable. It allows for normal acceleration, then you can let off and go into neutral braking. If you want more braking, you can hook a brake input into the controller. Many people have put a pressure transducer on the brake line that gives a signal into the controller.


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## major (Apr 4, 2008)

jegues said:


> How are the changes in the torque inputs translated to gradual or rapid changes in actual motor torque?


Many inputs to choose from, but take -10V to +10V. 0 = zero torque reference. 5V = 50% torque ref. -10V = 100% negative torque ref. etc. The change in torque is as fast as the motor/control will allow which in most cases to us humans is instant. I've used torque reference filters on the 10 to 100 mS scale.

It's called 4 quadrant torque control. Zero to 100% positive or negative torque at any speed in either direction. Most don't bother with regen in reverse, but the capability is there.



> For example, if I wish brake gradually what prevents the motor from suddenly slamming to a stop? If I want to stop quickly, how does the motor differentiate this and a gradual braking and allow for sudden braking?


See above. 



> Is it accomplished by simply scaling the positive/negative torque signal provided by the position of the throttle/brake?


Yes. There are also other factors to consider which will impose limits.



> Also, can't you damage the motor be damaged by commanding it to brake faster than its mechanical parts would actually allow? I'm sure you could easily avoid this problem by limiting the maximum torque that the controller can request.


Yep. You also use torque reference modifiers to limit the high voltage on regen, or low voltage sag on acceleration near end of charge, etc.


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## subcooledheatpump (Mar 5, 2012)

Speed and torque control can be used at the same time, and it can (and does) work for regenerative braking. 

Advanced motor controllers allow for limits in the rate of torque rise and fall, as well as speed reference inputs. 

In my van, I use an ABB ACS 600, which has DTC. 

I can use either speed or torque control modes. 

with torque control, essentially the van behaves like any other car. you have to use the brakes to stop it.

With speed control, you set the speed that you want to go with the pedal, not the power or torque. 

However there are limits. The rate of torque rise is controlled, so if I command a speed increase I do not get 100% torque instantly, only if the van speed dwells below the commanded speed after a certain time does the torque command reach 100%, at which point I do get full power. 

Same goes for regenerative braking, If I lift off the pedal, I don't get 100% braking torque right away, but eventually it will climb that high if the van isn't slowing quickly enough. The mechanical brakes work together with the regenerative braking, so you cannot tell what is actually happening, all you do is press the brake, and the van stops faster. If you choose to do so, you can let the regenerative braking alone stop the van. 

From what I can tell, usually "made for EV" AC motor controllers don't work this way. They allow for a positive torque input from an accelerator pedal, and a negative torque input from the brake pedal. Makes sense, since it will essentially work like a regular car would with regenerative braking. Although I don't see how one pedal driving is a huge problem, unless you racing.


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## jegues (Aug 5, 2013)

Hi frodus! 



frodus said:


> I asked you:
> 
> 
> AC drives, like Curtis, have a neutral regen (occurs when letting off the throttle) and is adjustable, as well as a brake input (variable and occurs when pressing a brake input). Both are tunable. It allows for normal acceleration, then you can let off and go into neutral braking. If you want more braking, you can hook a brake input into the controller. Many people have put a pressure transducer on the brake line that gives a signal into the controller.


Sorry I didn't answer you before.

I am not using any particular motor controller, I am designing and building my own.


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## jegues (Aug 5, 2013)

Hi major! 


major said:


> Many inputs to choose from, but take -10V to +10V. 0 = zero torque reference. 5V = 50% torque ref. -10V = 100% negative torque ref. etc. The change in torque is as fast as the motor/control will allow which in most cases to us humans is instant. I've used torque reference filters on the 10 to 100 mS scale.


Can you explain what torque reference filters are?

I did some quick googling and it seems as though it is simply a filter that the torque reference signal is passed through in order smooth out the torque reference signal before it is given to the motor controller. Is this correct?



major said:


> Yes. There also also other factors to consider which will impose limits.


What might these factors be?




major said:


> Yep. You also use torque reference modifiers to limit the high voltage on regen, or low voltage sag on acceleration near end of charge, etc.


What is a torque reference modifier? Is it some sort of control that will knowingly modify the reference torque given a particular set of predetermined circumstances? (i.e. high voltage on regen and/or low voltage sag on acceleration etc.)


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## major (Apr 4, 2008)

jegues said:


> Hi major!
> 
> 
> Can you explain what torque reference filters are?
> ...


Yep, that's right. Abrupt changes in torque can cause fast changes in current and these can be a nuisance. Also the filter can take it easy on mechanical driveline components. 




> > Originally Posted by *major*
> > _ There are also other factors to consider which will impose limits._
> 
> 
> What might these factors be?


Anything from some type of traction control to motor temperature. Up to you as the designer.



> What is a torque reference modifier? Is it some sort of control that will knowingly modify the reference torque given a particular set of predetermined circumstances? (i.e. high voltage on regen and/or low voltage sag on acceleration etc.)


Yep. Say the battery voltage is too high during regen downhill first thing out after a charge. As the battery voltage approaches the preset maximum, the torque reference command from the brake pedal or pressure sensor is intercepted and reduced. As the braking torque is cut back, the charge current is lowered and the voltage reduces. The electric torque fades away but the driver simply presses harder on the pedal which applies greater force on the friction brakes.


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## Duncan (Dec 8, 2008)

Just a quick comment
somebody talked about only using re-gen for braking if he had 4WD (no friction brakes)

This is almost certainly a BAD idea as modern brakes can absorb a huge amount of energy very fast
Very few motors would be able to absorb energy at the same rate
So re-gen is good for slowing down but if you need to stop really fast you had best retain the disc brakes

Example
A 1 tonne car breaking at 0.75G at 100mph
7500N x 44m/sec = 333Kw or 444Hp - quite a lot of power for a small car


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## jegues (Aug 5, 2013)

Hello again everyone!

I spoke with another engineer on this subject and one of the solutions he offered was to separate the regenerative braking completely from the mechanical braking.

He suggested that the regenerative braking could be operated by the user turning the throttle in the opposite direction, completely separate from the controls for the mechanical brake. (past the zero throttle point obviously)

We figured this could easily be done via a three terminal potentiometer. When the throttle is at the zero torque position the wiper would sit at the center point of the potentiometer. When positive torque is applied to the throttle, the wiper would move in one direction, while a negative throttle would cause the wiper to move in the other direction. Once the throttle is removed again, the wiper is then pulled back to its center position by some sort of mechanical spring.

Does this sound like something that is feasible? Has it been implemented in the past?

Thanks again for all the help/input!


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## Siwastaja (Aug 1, 2012)

jegues said:


> He suggested that the regenerative braking could be operated by the user turning the throttle in the opposite direction, completely separate from the controls for the mechanical brake. (past the zero throttle point obviously)


This is exactly how it is typically done. It's merely a matter of taste whether you want to apply full regen on the "opposite" direction (as you suggest), or just a little bit and then apply more when brake pedal is pressed.



> We figured this could easily be done via a three terminal potentiometer.


All potentiometers have three terminals . Any position can have any function, it's just a matter of programming.



> When the throttle is at the zero torque position the wiper would sit at the center point of the potentiometer. When positive torque is applied to the throttle, the wiper would move in one direction, while a negative throttle would cause the wiper to move in the other direction.


Yes, this is how it is done. It doesn't need to be exactly the center point - a proper motor controller allows you to calibrate the potentiometer range for acceleration and regen just as you like. You could also choose, for example, 25% point for neutral (no torque) and have more range for acceleration than regen.



> Once the throttle is removed again, the wiper is then pulled back to its center position by some sort of mechanical spring.


... but this is an interesting concept. How would you use the regen side at all? By lifting the pedal with your toes? Sounds difficult. I think it's the safest that the car decelerates when you lift off your foot from the gas pedal, so, the traditional spring that returns the pedal to the upmost position. But then again, a very strong motor braking can be surprising and dangerous when it happens without touching the brake pedal, so it's best to either limit the regen altogether or apply more only when the brake pedal is pressed.


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## RIPPERTON (Jan 26, 2010)

Id never use both together.
In an Electric Vehicle the friction brake is relegated to emergency brake
and the hand brake is the holding brake.
All normal controlled braking and decelerating is done with the regen alone and once the vehicle is stationary the hand brake is applied.
If you have set up your regen properly it will be more than powerful enough 
to handle all controlled braking so there is no need to incorporate the friction brake.


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## PStechPaul (May 1, 2012)

Another way to implement vehicle control could be a joystick, where fore and aft motion would provide acceleration and braking, with a spring return to center. The left/right motion would control steering, which might be accomplished by differential drive to left and right side motors. This would be especially useful in a tractor, and particularly suited to a ZTR design or a track vehicle. It would be interesting to do this on a car with power steering.


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## jegues (Aug 5, 2013)

Hi Siwastaja! 



Siwastaja said:


> ... but this is an interesting concept. How would you use the regen side at all? By lifting the pedal with your toes? Sounds difficult. I think it's the safest that the car decelerates when you lift off your foot from the gas pedal, so, the traditional spring that returns the pedal to the upmost position. But then again, a very strong motor braking can be surprising and dangerous when it happens without touching the brake pedal, so it's best to either limit the regen altogether or apply more only when the brake pedal is pressed.


Allow me to explain. This application is for an electric motorcycle, not a car! 

So to engage the regenerative braking the rider will simply turn the throttle on the handle bars forward.


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