Tmote for PWM

What's a Tmote Sky ?????

It looks like its a single board computer for networking sensors. Some basic specs from the website: http://www.moteiv.com/products/tmotesky.php

• 250kbps 2.4GHz IEEE 802.15.4 Chipcon Wireless Transceiver
• Interoperability with other IEEE 802.15.4 devices
• 8MHz Texas Instruments MSP430 microcontroller (10k RAM, 48k Flash)
• Integrated ADC, DAC, Supply Voltage Supervisor, and DMA Controller
• Integrated onboard antenna with 50m range indoors / 125m range outdoors
• Optional Integrated Humidity, Temperature, and Light sensors
• Ultra low current consumption
• Fast wakeup from sleep (<6us)
• Hardware link-layer encryption and authentication
• Programming and data collection via USB
• 16-pin expansion support and optional SMA antenna connector
• TinyOS support : mesh networking and communication implementation
• FCC modular certification : conforms to all US and Canada regulations

There's nothing here that says "PWM" to me, although you might be able to fake it with one of the DAC channels.

Generally speaking, if it's programmable, you can program it for PWM. For instance, you can program PWM into a PIC microcontroller (I mean the ones that don't have the built-in PWM) and I've done motor controllers in FPGAs where I made a PWM circuit from counters. It's just a matter of setting a bit, counting out an on time, then reseting the bit and counting out an off time.

Your discrete I/O should be able to drive an H-Bridge of p and n channel FETs, but if you use a gate driver chip, you can use all n channel FETs, and get some extra builtin capabilities, like over current protection and dead time, which is what I prefer. But you can certainly do without. Check out the gate driver chips at www.irf.com for more details.

Why not use one of the analog outputs and a National Semiconductor LMD18200 H-bridge?

Thanks for the responses. I'm trying to get some working knowledge of this, and every place I look either shows an h-bridge some hobbyist made, or h-bridge schematics.

So, I could output an analog voltage signal from the mote, and the h-bridge would use that signal? Can I use encoder feedback for speed control this way too?

Actually, I lied about the analog input and the National Semi LMD 18200 -- I got it confused with another device. To use the LMD 18200, you would have to supply PWM and direction signals from the Tmote, but it's got all the logic and power devices built into one package so that all you have to do is hook it up to a motor, Tmote, and a couple of capacitors.

Unfortunately, it isn't designed to take an encoder feedback. If speed is not a factor, perhaps the Tmote can read the encoder and close the loop that way.

Let me know if you want the data sheet on LMD18200.

P. S. There's lots of IC's, OEM boards, etc. to do what you want, but I assume you want to do this as inexpensively and easily as possible. Let me know if this ain't so.

Does an H-bridge take a digital or analog signal, or does it vary? I think I can get either from the mote. Regarding the encoder, how else is speed control done without encoder feedback? I've been warned about using the encoder for speed control.

I'm understanding that there are lots of ways to do this which only makes it more complicated! We are looking to do it cheaply, but also within a small space.

Thanks.

A H-Bridge is just an arrangement of MOSFETs or bipolar transistors (or other switches). So the input signal is technically analog (driving the MOSFET gate, or bipolar base, or relay coil), but is often driven by a digital gate, depending on the amount of current required and the switching speed. The signal is digital in the sense that it's either ON or OFF. You want the switches in the H-Bridge to be either 100% ON or 100% OFF, and nothing in between. If your Tmote output is not sufficient, you can use a gate driver chip, a pullup resistor, a high output digital buffer, opto-isolator, opamp or another transistor to get the current/voltage that you need, again depending on how much current you need..

DomerDoc, This particular H-bridge (LMD18200) takes digital signals. Others, such as UC3637 in conjunction with an L298 can take an analog input. If you don't know much about electronics, I wouldn't recommend tackling this one, but it can be done with a little diligence.

Encoders measure rotation; they don't measure speed. Therefore, the number of counts (unit of rotation) must be measured in time to determine speed. Think of it as measuring the number of broken stripes driven past on the highway per minute to gauge the speed of your car. To start with, you won't know how fast you're going until a minute has passed, and in that length of time, your speed could have been checked by a cop with a radar gun and found to be way too fast! In the biz, it's called a "lag" in the control system.

A tachometer (which is just about the same as a DC motor, otherwise known as a DC generator) generates a voltage proportional to speed. Well, a DC motor pretty much runs at a speed proportional to applied voltage. Open-loop speed controllers try to hold applied voltage constant with a modifier for "IR" losses, called "IR Compensation." "IR" in this case means current (I) times resistance (R), and not InfraRed. As load on the motor increases, the armature current increases, and unless it's supercooled and made to be superconducting, the applied voltage, if held constant, will result in the motor slowing down. With IR compensation, the armature current is measured electronically and proportionally increases the applied voltage to try to maintain a constant speed. Since the speed is not measured directly, motor speed will be a function of the current feedback and motor characteristics. Most off-the-shelf motor speed controllers use this method and may be modified to match controller to motor within bounds.

Email me if you get desperate.

I built a system to cotrol model trains from a few odd components lying around a 555 timer and some transistors plus a few caps and the odd resistor, it worked fine.