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- Custom Development

i2t

**Important:** this note covers the original I2t protection. If you programmed your Regulate before September 2018 that's what you have. After September 2018 we released an improved version (I²t current limit: Programmable Fuse (v2.0)). The foundation is the same, but handy new features have been added.

FlexSEA-Execute and FlexSEA-Battery/Regulate have an optional I²t protection. This piece of code allows the user to select limits for average and peak currents (of a certain duration). It is essentially a programmable fuse that can be used to protect the circuits, but also the robot that used FlexSEA.

The Matlab/Octave script below allows users to generate the proper constants for the C code. The example uses 5A average, and a peak of 30A no longer than 2.5s.

% I2t current limit: user % JFDuval, Dephy, Inc. 06/14/2017 % Note: use this file to get your #define values. i2t_1.m can be used to test % your values with different profiles. clc; close all; clear all; format short eng % Period, i2t_compute() call: dt = 100e-3; %Every 100ms disp('Program these values in i2t-current-limit.h:') disp('============================================') % The algorithm uses 8-bit values: we need to scale down the maximum current % accordingly. It uses real units (mA). Ex.: ±30000mA sensor => % 30000/256 = 117 => shift 7 (div by 128). 30A will give us 234 I2C_SCALE_DOWN_SHIFT = 7 % Closest bitshift available SCALE = 128; % (Octave only, avoids bitwise operations) % Maximum average current you want to support: maxAvgCurrent = 5000; %mA I2T_LEAK = (maxAvgCurrent / SCALE)^2 % Maximum current you want to support, and duration: currentLimit = 30000; %mA currentLimitTime = 2.5; %s I2T_LIMIT = (currentLimitTime / dt) * ((currentLimit/SCALE)^2 - I2T_LEAK) %At what fraction of the max to you want a warning? (0-1) warn = 0.8; I2T_WARNING = warn * I2T_LIMIT % Plotting: CURR = maxAvgCurrent:1000:30000; tmp = (CURR./SCALE).^2; time = (I2T_LIMIT * dt) ./ ( tmp - I2T_LEAK ); figure() plot(CURR, time) title('Time before reaching the I2t limit') xlabel('Current (mA)') ylabel('Time (s)') disp('Time at 6A:') CURR = 6000; tmp = (CURR./SCALE).^2; time = (I2T_LIMIT * dt) ./ ( tmp - I2T_LEAK )

The script will output constants, and a plot:

Program these values in i2t-current-limit.h: ============================================ I2C_SCALE_DOWN_SHIFT = 7.0000e+000 I2T_LEAK = 1.5259e+003 I2T_LIMIT = 1.3351e+006 I2T_WARNING = 1.0681e+006 Time at 6A: time = 198.8636e+000

The examples below are based on a Pocket board with a ±5A sensor, 1.5A continuous and 5A 500ms limit.

Program these values in i2t-current-limit.h: ============================================ I2C_SCALE_DOWN_SHIFT = 5.0000e+000 I2T_LEAK = 2.1973e+003 I2T_LIMIT = 111.0840e+003 I2T_WARNING = 88.8672e+003 Time at 1.6A: time = 36.6935e+000

- The standby current gets added to the load current. On a 30A board that's negligible, but on a ±5A board with a low limit that can be significant. As you can see, adding 100mA to the 1.5A load will make the protection trip after 37s instead of infinity.
- Why does my board turn off with 1A for 900ms and 5A for 100ms? 1²*0.9 + 5²*0.1 = 0.9 + 2.5 = 3.4, sqrt(3.4) = 1.84A DC. Just the pulse is equivalent to 1.58A DC, enough to kill it.
- How long do I need to wait between 5A 500ms pulses (assuming 0A the rest of the time)? 25*0.5 = 12.5, and our limit is 1.5. 1.5/12.5 = 12% duty cycle, so you need to wait at least 4.17s before the next pulse.

2.5A continuous, 15A for 1s, non-linearity at 22.5A:

I2C_SHIFT = 7.0000e+000 (7) I2T_LEAK = 381.4697e+000 (381) I2T_LIMIT = 133.5144e+003 (133514) I2T_NON_LINEAR_THRESHOLD = 175.7812e+000 (176) Time at given current (mA): CURR = 25.0000e+003 time = 353.5354e-003 timeNL = 87.7193e-003

10A continuous, 15A for 1s, non-linearity at 20A:

I2C_SHIFT = 7.0000e+000 (7) I2T_LEAK = 6.1035e+003 (6104) I2T_LIMIT = 76.2939e+003 (76294) I2T_NON_LINEAR_THRESHOLD = 156.2500e+000 (156) Time at given current (mA): CURR = 25.0000e+003 time = 238.0952e-003 timeNL = 22.6244e-003

i2t.txt · Last modified: 2020/06/16 18:35 by bbaig

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