Here are some pics :
The heater glued to the original bed
Details on the mounting. I used an extra insulation glued to the bed support to prevent thermal expansion issues with the support through radiation :
New power supply and SSR installed :
Here a graph with the original heater from 24°C to 70°C. One can see that the curve flattens at higher temperatures. 95°C without cork cover on the surface took around 45 minutes. Temperature was also very difficult to keep without covering as much as possible of the bed (Thermal runaway issues you don't want with longer prints).
And here a graph with the new heater in place at full power (750W) from 18°C to 100°C. I will have to throttle the max output power. This goes so fast that it will generate mechanical stress. A couple of minutes longer will not make that difference any more, don't you think? This is like using a jet engine as a hair dryer One can see the bang-bang in action and the overshoot in the beginning. This stabilizes to a ripple from 99°C - 102°C. Should be less with power reduction and I will test PID again too.
I did the same test with 400W (average) output power, changed setting in configuration.h to 136 instead of 255. Heats now in 3 minutes and has a ripple between 97 and 101°C. Less overshoot.
The result of a PID autotune on the BED. Look at the effect on the ripple and what he does with the power :
And this is with a tuned bed. Nice ... Very nice. Look at the ripple. Varies now between 99,8°C and 100,2°C !
Configuration.h part :
// This sets the max power delivered to the bed, and replaces the HEATER_BED_DUTY_CYCLE_DIVIDER option.
// all forms of bed control obey this (PID, bang-bang, bang-bang with hysteresis)
// setting this to anything other than 255 enables a form of PWM to the bed just like HEATER_BED_DUTY_CYCLE_DIVIDER did,
// so you shouldn't use it unless you are OK with PWM on your bed. (see the comment on enabling PIDTEMPBED)
#define MAX_BED_POWER 136// limits duty cycle to bed; 255=full current / 750W = 255 / 375W = 127 / 400W = 136
//#define PID_BED_DEBUG // Sends debug data to the serial port.
#define PID_BED_INTEGRAL_DRIVE_MAX MAX_BED_POWER //limit for the integral term
// #define DEFAULT_bedKp 1351.33
// #define DEFAULT_bedKi 249.94
// #define DEFAULT_bedKd 1826.49
#define DEFAULT_bedKp 65.07
#define DEFAULT_bedKi 12.50
#define DEFAULT_bedKd 84.67
Heating to 120°C, just something I could only dream of before... Same nice control. When you look at the settings in the configuration.h above, you can see that the values are up to 20 times lower. The inertia of the bed is no issue for the power of the heater which was the case before. It is almost like the PID parameters for the hot end. There I have 40W for a small block of metal.