Archiv Oktober 2019

Integrating wmBus devices into iobroker

After my quite expensive MH-Collector (identical with the easy.MUC from solvimus) died (it survived just a little longer that warranty protects), I decided to collect my wmBus devices‘ data with some home brewn solution. I’m also the owner of a Ubiquity US-24-250W, so the descision to go with PoE supply is quite an easy descision. So what lies closer than using a Raspberry Pi 3B+ with PoE hat and an USB-wmBus stick?

No sooner said than done, I bought the parts, installed raspbian and an iobroker slave. The iobroker master ist now running on a Debian 10 buster VM on my tiny HP Proliant server… How to install the iobroker slave can be found here.

wmBus Hardware

For me, the appropriate hardware was the IMST iM871A-USB (you can buy it directly from IMST or from tekmodul). This wmBus stick provides a serial interface (e.g. /dev/ttyUSB0), is quite cheap and supported by most open source wmBus software. But here comes the tricky part. There are quite some paths you can go, but for me, using the Messhelden heat cost allocators, I found myself in a very frustrating situation. These devices stick to the OMS standard for most of the telegram, but unfortunately do some very shitty stuff at slot 2 and 3, so many decoders fall out of sync just after the first data slot.

wmBus Software

After trying different ioborker adapters (like iobroker.wm-bus) and also deamon solutions (like wmbusmeters), sending the data to some MQTT broker (a server is easy to rise up in iobroker), I ended with the iobroker.wmbus (beware of the dash, it is not the same as above). Somehow the author of this adapter managed to come up with the inconsistencies, I even could not decode manually, looking at each single bit and byte of the wmBus telegram.

After attaching the iM871A-USB stick to the Pi and placing it at some location where it can receive all meters you are interested in, you need to install and configure the iobroker adapter iobroker.wmbus.

Adapter Configuration

The configuration is also quite easy and should look like the following:

It could also be, that your slaves need other modes to be received. One widespread mod for battery driven devices is also mode C. Unfortunately, a single stick can not receive multiple modes. But usually you only run devices with a single mode. Another important setting is the baud rate. For the IMST device, it needs to be 57600. The stick contains some serial converter that attaches the IMST module with a real serial connection.

Add Encrypted Meters

After finishing configuration and starting up the adapter, it is time to have a look into the log. There you will see, if the adapter started up correctly. If it did, you soon should see a line that says „Updated device state:<MANUF>-<ID>“ or an error saying, that it could not decrypt a telegram due to missing decryption key. If this occurs, go to the adapter configuration again. There you should see a new entry with a key „UNKNOWN“. Place the correct key there and push „Save“.

The follwoing telegram of that device should be decrypted correctly and a new state will be created within the object tree of iobroker.

If you see other unencrypted devices that pollute your object tree or your log with encryption failed messages, simply put them below „Blocked Devices“ tab in the adapters configuration. My Pi can see at least 20 unencrypted Techem water meters and heat cost allocators.

Let’s encrypt (also on wmBus)

I don’t know, how they can survive in a time of GDPR (General Data Protection Regulation), but they still have no hurry to encrypt their telegrams with a device-unique key. I think it is a security issue, when burglars can easily find people that do not heat in wintertime or have no water demand currently. But at least, Techem sticks closely to the OMS. If you rent a flat, that still has unencrypted wmBus meters, I would definitely claim to get encrypted meters. Even if encryption of wmBus has some weaknesses, it is by far better than plaintext.

STM32 BLDC Motor Control

Introduction

ST offers quite a broad BLDC controller portfolio, but the most interesting to me seems the STSPIN family of controllers. They include mostly anything except the MOSFETs to drive a BLDC, including a ST32 Microcontroller, a DC/DC-Converter (with external passives), the MOSFET drivers,…

Steps of designing a BLDC control circuit with STM32

First step is to find one or some BLDC motors for your specific need. The available motors range from cheap no-name motors (~4000 rpm) to high performance high turn ratio (>60000 rpm) and from a few watts up to kilo-watts of power. For CNC applications, you can find HF spindles with 2200W and 30000 rpm.

When you have your BLDC, it’s time to find the right controller for your application. When you go with STSPIN, you should think about getting the STEVAL-SPIN320x for prototyping your application, but additionally, you should always get the NUCLEO board with an appropriate BLDC driver hat. Why? Because the STM Motor Profiler Tool only runs with a few specific boards. To find the right board, install the ST Motor Control SDK and open the Motor Profiler Tool. Then you can browse through the supported kits to do Motor Profiling.

For further hands-on example, I will use a „Generic BLDC Motor“ with very poor documentation and quite low performance. It looks like a stepper motor and has the following (known) performance characteristics:

  • 8 pole pairs
  • 4000 rpm
  • 24V

We also measures some characteristics that have not been given by the specs, most important: The winding resistance. Set your power supply to current limiting mode with approx 5% of the nominal current of the motor. Then connect it to two wires of the motor. In our example, the power supply showed 0,36V / 0,3A = 1,2 Ohms, this gives, taking the circuit of three windings in star configuration (we have a simple series configuration of two windings) into account, 0,6 Ohm (1,2 Ohm / 2 = 0.6). Applied with above power, you can easily determine the pole-pair count by turning the shaft a full turn counting the ripples, you should feel the while turning. It will be easier to use a pen to mark the positions. Apply the power only for a minute, otherwise, you could damage the motor…

With above data, you can select the appropriate BLDC controller board. For smaller motors, the X-NUCLEO-IHM16M1 should serve well. For larger types, the X-NUCLEO-IHM08M1 is a good choice. But always be aware, that for every power hat board, there is only a limited set of compatible nucleos.

To be continued…