WEEDINATOR 2025

None of the previous logs have really gone into any detail about the left hand green MCU PCB, so here we go! The main feature of the board cant actually be seen as it is slotted onto the underside of the board, namely the STM Nucleo  H723ZG MCU. There's absolutely no point in having it on the top surface as it just takes up lots of space which is better occupied by connectors. There can be seen 4 rows of orange Wago connectors on the top surface in the photo below and these correspond to each and every one of the 144 pins on the H723ZG. The board has been carefully designed so that all the pin designations eg PC7 can be easily read on the PCB silk screen to help wiring up the system.

There's another row of these orange connectors on the right hand side and these have permanent connections to 3 banks of blue logic voltage shifter modules which convert the 5V from the 3 electric actuators (steering, throttle A and throttle B) to 3.3V, which is safe for the H723ZG pins. On the other side of the logic level shifter are yellow and white wires soldered in for the quadrature encoders. Other yellow and white wires connect from the MCU direct to PWM and direction terminals on the black Cytron motor control modules on the lower right hand side of the box.

On the very top left on the green MCU PCB is a small green GPS daughter board which is waiting for it's GPS modules to arrive. To the right is a 4G modem waiting for it's sim card to be inserted. Other than this there are two 5V and one 3.3V regulator with large capacitors on the 12V input side. There's also four simple voltage divider circuits, which cant be clearly seen,  for changing the 12V signal from the inductor sensors to about 3V for compatibility with the H723ZG MCU.

If we look really closely, we can see a through hole transistor in the middle of the green PCB which is used to invert the signal from the SBUS receiver so that the MCU serial bus can recognise the data flowing in. The SBUS receiver itself is the black box perched precariously at the top of the box and is due to be moved to somewhere near the top of the machine with cables to extend the SBUS connections. The 4 purple wires at the lower edge of the box are for actuating the 4 engine relays as demonstrated in the previous log (engine stop solenoid, fuel pump, glow plus and starter motor) and are waiting to be assigned pins on the H723ZG.

The video below gives some basic tips about how the MCU PCB and the relay PCBs should be wired up including how the large high capacity cables should be kept away from signal cables. For ease of access to wire up the relays, the MCU PCB needs to be able to tilt on one edge which is enabled by routing the signal wires through one large square aperture in the PCB on the nearest side to the rear of the machine - watch the video! It's essential to label as many of the wires as possible and keep groups of wires eg signal wires to the relays, the same colour. Something the video does not mention is that there are no connectors in the walls of the control panel, just glands. In the past, connectors were used but they take up a lot of space, make the wiring extremely messy and are prone to damage if knocked or pushed. If or when connectors are required, they will be inserted inline instead. Watching the video, one inline connector can already be seen on the engine throttle actuator as the cable supplied by the manufacturer was far too short.

Featuring 4 channels of 16 bit 15A current sensing and 4 channels of 16 bit voltage sensing via voltage divider, this gadget can act as a digital fuse, protecting the motor driver and actuator from damage due to stalling. The steering actuator is particularly prone to this problem, even though it has an induction sensor at the mid position to reset encoder values - it's just too easy during development and commissioning of the machine to make a small mistake and blow up £200 worth of equipment. The resistor bridge can be used for accurately sensing battery voltage and anything else that crops up in the future.

The underside of the board has some heavy duty copper lugs soldered in to connect the current sensing TMCS1123 chips to the large Wago connectors. PCBWay could have supplied extremely heavy 8oz boards, but I chose this option to save cash. BTW, the boards from PCBWay are great quality and the service is superb. They also check with me on the more esoteric features of the board before sending to the press, which guarantees there are no mistakes in interpreting the Gerber files. The chips themselves come in various sensitivity ratings and the ones currently installed are good for +-15A on a 3V3 power supply. The maximum range seems to be +-92 A with the TMCS1123A1A.

This current sensor PCB sits on top of another PCB that has no electrical connections, just accurately placed mounting holes for 3 main modules - the current sensor, an analogue fuse box and a stack of motor controllers. The large white PCB can also be stacked up and there are large holes in the redundant space for cable routing. Mounting of the 3 modules has been checked and everything fits perfectly and the whole thing fits well in the main enclosure box with all the connections in sensible places. Phew !!

Featuring 16 channels with 40 amp automotive relays (relays not plugged in yet) and 2 experimental 4 amp high side switches, this board should be more than capable of illuminating the engine glow plugs (20 amps) and engage the starter motor solenoid (40 amps) for starting the engine. The DIP chips shown in the photo below are ST Stepper Motor Control/Driver L293D chips which can also be used for driving LEDs and relay coils as in this case. There must be some surface mount equivalents somewhere, but not found them yet. The connectors are all by the manufacturer Wago and were selected because of their solid mountings to the PCB and high spring clamping forces. The larger ones could easily break a finger if used wrongly. There's also a SMT 5v regulator for powering the driver chips with 470uF capacitor on the input. The relays could easily have been soldered directly onto the PCB, but using the mounts enable easy replacement if something goes wrong on the machine. The wiring of the relay high current terminals is not connected to the ground plane so a live feed can come out of the analogue fuse box, into the digital fuse box, if required, and then to the glow plugs, for example, which then connect to the machine's common earth.

On the reverse side, as below, there are giant lugs of copper soldered to the relevant relay pins and large grey Wago connectors to carry the 40 amps of current. The relays themselves have fly back protection and are the Durakool 'DR' type. If the 'D' type was used by mistake, there would be plumes of magick smoke emitted from the relays and melting of wire insulation. The PCB, acquired from PCBWay, is 2 oz in weight, which is double the normal thickness and this weight was used to ensure solid connections with the large Wago connectors and no damage to the traces when soldering with a 450 watt soldering iron.

The main systems to enable the machine to drive around on remote control have been installed - steering and the two throttle motors. Yes, this machine has 2 throttles, one to control the engine speed like a normal throttle and another to control the hydrostatic transmission with forwards, reverse and neutral. There are no gears and there is a theoretical infinite range between neutral and 'fast'. In the past we struggled to get fine control and very low speeds and in this version we've installed a 'slow' actuator with a large travel distance (150mm) to get the best out of the in built quadrature encoders. We did tests on the steering with a 300kg spring balance to assess what kind of forces were involved if the wheels were to get stuck in a rut or such like. The balance was attached to the main lever which actuates the Ackerman steering mechanism and we recorded a good 300kg by jamming up the wheels by hand. We maybe should have put an actuator straight onto this lever but instead chose to attach a pulley wheel to the steering column gearbox which would have been a better solution if there was hydraulic assisted steering, of which there is none on the machine. There's a bit of back lash in the gearbox so it will be interesting to see if the machine can still travel in a straight line when using GPS positioning. If it's just too much hassle, we can swap out the pulley wheels and steering column and attach a linear actuator to the main lever instead. 

We were worried about burning out the actuator motor by stalling the steering so installed a fuse box and also a custom made circuit board for sensing currents of about 20A which will provide a 'digital fuse' to help prevent damage.