Saturday, August 15, 2015

Power Brakes, kind-of....

New Brake Sensor

The 500 psi brake sensor worked well.  I was able to replace an existing 3/16 union with a T brass connector.  After adding a new value to the digital dash, Can Bus enabled Arduino to show the value the brake sensor A/D in the Kelly KHB 72701 controller, I realized the Mini's brake pressure goes up to about 300 psi.  The 500 psi sensor under heavy braking would generate about 2.1 volts (~110 on a 0-255 scale).  This equates to about 220 psi.  A 300 psi (0V-5V) sensor is available, so after installing this narrower scale sensor, the A/D is now reporting from 25-200 (on the 8 bit scale).  25 corresponds exactly to 0.5V which is the sensor's 0 psi value.  I could only find one reference to the actual PSI ratings on the Classic Mini brake system.  This site reports an upgraded brake system operates up to 330psi.  See here MiniMania

300 PSI Stainless Steel Sensor 5V supply (0.5V- 4.5V output)
Brake Sensor installed in the brake line from the Master Cylinder

With the new sensor functioning it was time to tweak the Regen settings on the Kelly Controller.  After some experimenting, the following values provide reasonable braking with the motor providing most of the braking ahead of the mechanical brakes.  It feels like power brakes, since the amount of force needed on the brake pedal is noticeably less for the same slowing power than what is needed with the mechanical brakes alone.

  • ABS - Disabled - When this was on, the controller would pulse the regen as the car approached a stop.
  • Regeneration - Enabled
  • Brake switch - Enabled - When this is off both the switch and the sensor are ignored.
  • Release Throttle Starts Regen - Disabled - Most agree it is better to coast than to have Regen occurring when your foot is off the accelerator
  • Regen current by Brake Switch On - 5% - This is minimal value and represents how much braking is applied as soon as the brake is pressed.  A relay has been installed from the original Brake Light switch on the pedal to provide the ground signal expected by this input on the controller.
  • Max Regen Current - 70% - Not quite sure but this does scale the Regen.  At 100%, minimal brake pressure was too much braking.  So this seems to affect the transfer function from the sensor, and the best I can tell, it impacts the slope of the linear function.
  • Brake Sensor Type - 0V-5V
  • Brake Sensor Starting Point - 10%  - I think this is close to a value of 25 (10% of 255) 
  • Brake Sensor Ending Point - 70% - This should equate to about 180 (out of 255) or about 210 psi, which is fairly high pressure on the brake pedal.
The Max Current, starting point and ending point are all part of the equation that the value from the sensor is put through to determine the amount of Regen the controller applies.

A few observations
  1. Under heavy braking, the Regen cuts off as the motor slows to about 800 RPM.  It is abrupt but since the brake is already being applied, it is easy to simply increase the mechanical braking to compensate.  
  2. Under normal braking, as the motor slows to about 300 RPM, the Regen shuts off.  This is noticeable but not offensive since the car is almost at a stop anyway, less than 5 MPH.
  3. Sensor controlled regen is the only way to go.  The Kelly controller provides very smooth Regen and it is natural to use the brake pedal to control it.

Timing Breakthrough

Ever since adding the Timing advance circuit between the Motenergy ME0913 motor and the Kelly Controller, a strange behavior developed.  After coasting at speed, when the throttle was re-applied, the motor would attempt to slow the car.  Felt like regenerative braking but the controller did not register any current being generated, there was just a noticeable slowing effect until the throttle was close to matching the car's speed.  When the timing was set to zero-advance, this was not observed, however, when the timing was being advanced, the slowing effect was clearly present.

3 lines of code later, a 1 second delay was added to the timing advance activation triggered by a connection to the micro-switch on the throttle.  The micro-switch signals as soon as the throttle is slightly depressed.   When the throttle is completely off, the timing circuit goes into bypass mode.  As soon as the throttle starts to move, a 1 second timer is started, which at the end of, the timing advancement is engaged.  This 1 second delay gives the Kelly Controller time to sync up with motor while the throttle is being fully applied.  This delay solved the problem and the slow-down effect is no longer felt.  I may try to tweak the 1 second timer to see if a shorter time accomplishes the same results but at 1 second, there is no observable delay in motor power or performance.

A Distance Test

With the Regen and timing working well now, it was time for a range test.   Headed out in 3rd gear (the only gear to use for 0-50 MPH around town driving) and covered 21.5 miles 

Below is a screen shot from the Digital Dashboard GPS  app on my phone.  The area graph at the bottom is speed and the line graph is altitude.  There was a large climb near the end which is when one of the cells started to drop.  The rest were holding steady between 3.0 and 3.1 volts but one cell was down to 2.7.  This cell is also the first to fill-up during charge cycles so it is clearly degraded from the over-discharge event from several months ago.  I suspect that there was still a few more miles in the pack so after I swap out this one weak cell, it will be time for another range test.  

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