What happened before:
For almost a year I have a vintage turntable from Perpetuum Ebner at home. It's not mine. It belongs to someone that asked me to fix it. And it turned out that fixing the motor would cost too much. I asked the owner to collect the tt. That hasn't happened yet and the machine is collecting dust at my home.
So I'll take the freedom to attempt a non-intrusive repair with modern components. I'm also thinking about making it an Enchanted Objects Design Challenge. |
In post 1 I present the turntable, talk about my previous repair attempts and brainstorm on some modding ideas.
In post 2 I'm measuring up the different gears, pulleys and wheels, and I calculate the speed of the original motor.
This is the first post on motor control. I'm reviewing the operation of the Infineon DC Motor Control Shield.
I'm going to use this shield to drive a replacement turntable motor.
The shield comes with two half-H-Bridge drivers of type BTN8982. The drivers can be controlled independently.
That gives the option to use it for two unidirectional motors or for one bidirectional one.
Even though I don't need two directions, I think I'm going to wire the bridge for bidirectional use. I may build in a satanic message detector into the turntable.
Driving the motor
The driver is controlled by two pins. The IN and INH pins are the ones we need to drive to get the desired behavior.
Subset of the truth table for driving the bridge. This is the explanation on what happens when you drive the INH and IN pins.
You activate the bridge with INH, You select the speed by providing a PWM signal with correct duty cycle to IN.
Fine-tuning the slew rate
The SR pin is used to tweak the slew rate. I may use this if the controller interferes with the turntable's audio. I want to go for a very slow rise and fall time.
I don't have the BOM for the shield, but measured the resistor between SR and ground in circuit. I get 510 Ohm when I measure in-circuit.
The BTN8982 data sheet defines the slew rater behavior:
Unfortunately for lazy me, the characteristics for the on-board Rsr are not documented in the datasheet. So I'll either have to pull out my calculator or my scope.
...
OK. Decision made. I'll use the scope 
.
All measurements below a re made by driving the input with my function generator's TTL output.
I've measured in half bridge mode on channel 1. The rise time of my signal, with a 15V supply and 150K load (I also used a 22R load for a brief time and it didn't change the rise time) is 380 ns.
The datasheet says 'We recommend a PWM-period at least 10 times the rise-time'.
That means:
Rise-time = fall-time = 380ns.
=> T-PWM = 10 * 380ns = 3.8μs.
=> f-PWM = 263kHz.
However, when I tested this, the regulator started to perform badly at 60KHz and flatlined at 65kHz (see the two captures below):
The below capture gives a correlation between input and output, measured at 56kHz, duty cycle 58.7%:
td f(LS)
tf(LS)
td r(LS)
tr(LS)
Note: the shield schematic
The shield documentation doesn't specify the actual components and schema.
I've been probing the components with my meter and found that the shield is the literal implementation of the full H-bridge reference design form the datasheet:
In the next post I'm going to use the enhanced PWM module of a The specified item was not found. Hercules LaunchPad to test drive the motor.