Problem- Servo runaway issue

[EPTech]

Hi,

I own an old Ikegai FT20-Z lathe with fanuc 11T controller. When I go into milling mode (M18) the C-axis servo (DC) runs away. It immediately starts turning at maximum speed. I am pretty shure it is not the motor controller because I have exchanged it with another axis to no result. The command voltage is at 0V and it changes positive and negative as I manually control the axis. However, the TSA voltage is stuck to -15V. I have measured it on the motherboard, and the controller, so no cabling problems. The NC has got a separate IND/RES board with the inductosyn and motor tacho connected to it. Since the inductosyn and tacho are working, I suspect the IND/RES board, the motherboard or some parameters.
The IND/RES board has got 2 unused channels. Is there a way to switch between them with jumpers or parameters?
Does somebody know which IC's (DAC) on the motherboard are responsible for generating the TSA voltage?
I am suspicious of parameters in the 1800 or maybe 9000 range. But I do not have the parameter manual yet. Would somebody point me in the right direction? I need to know what parameter is responsible for encoder selection.

Thanks in advance and kind greetings,
Pascal Eraerts.

[Dan Fritz]

On most Fanucs that use resolvers and tachs, the tach signal is passed directly through to the servo amplifier board from the motor's tach. The ones that have pulse coders use Frequency-to-Velocity converters to generate a tach signal from the pulse coder signals, and that tach signal is passed to the servo board.

In your case, I doubt that there are any ICs on the resolver board to process the tach signal. There wouldn't be a need for any signal processing of the tach that I can see. The TSA and TSB signals should just pass through to the CN1 connector on the servo board.

The most likely scenario is that you have a blown op-amp on the servo board that is putting either +15v or -15v onto the TSA or TSB signal. Here's a test: pull the fuses on the C axis servo so it can't move, then disconnect the TSA signal (pin 7) and TSB (pin 20) from the CN1 connector on the servo board. (You can't just unplug CN1 because the servo would just alarm out and not allow you to perform this test). Enable the servo with the M18 command and measure the voltage of TSA on the servo board. (CH2, I believe). If you see 15v, then the source is on the servo board (probably the first 741 op amp that the tach signal goes to.) If you see 0v on the servo board's CH2, then the 15v is indeed coming from the main board or the resolver board, and I'm all wet.


You didn't mention what type of servo board you have. The 11M used 3-phase DC, PWM DC and AC servos. The boards are all different.

[EPTech]

Thanks for the quick answer Dan,

I'll try it when I get back to my shop. I have more information about the system.
Both the inductosyn and tach signal are hooked up to the IND/RES board (A16B-1210-0460 A). The motor is a DC servo from Fanuc, model 5M. The servo controller is an A06B-6047 H002. You have the grey connector with all the control signals. There is also the single line white connector, but that one only has the two power cables and no tach signal. And then you have all the power connections on the screw terminals.

Kind greetings,
Pascal Eraerts.