A new limit logic and overall telescope safety plan was discussed on 5/20/2019 for TCSNG. This document is a description of that discussion. Currently this document will focus on Horizon limits but the basic ideas could be ported to Equatorial limits as well.

The new limit logic discussed is a four tiered limit logic. There will be two software limits (S₁ and S₂) and two hardware limits (H₁ and H₂).

S₁

S₁ will act like slew limits in that You can get past S₁ by tracking, with bias rates or with the paddle guide and drift rates. You can move past an S₁ limit but the telescope will stop at the limit. You can the send the move command again and the telescope will continue at a very reduced rate.

S₂

S₂ will be a no go zone by goto functions, bias rates and the paddle. Hitting this software limit will be functionally equivalent to hitting the cancel button on the GUI. The telescope will ramp down and come to a stop. You will be able to back out of this limit. If you try to move past this limit with the paddle you will move a little but tcs will cancel the move and send the telescope back to the limit.

H₁

H₁ will act the same as S₂ in that the telescope will ramp down to a stop when the limit is hit. The only difference will be that this limit is a hardware limit and immune from bad initialization. You will be able to back out of this limit.

Note it is possible that H₁ is above S₂

H₂

This is the last hardware limit and will function the same as the final hardware limit currently at most of our telescopes. As soon as it is hit, it will cut power to the drive system either by powering off the drives or by activating the Safe Torque Off option. You will not be able to back out of this limit with the control system. It will have to be done manually.

We would like to implement a new upper limit on Bias and Paddle (Guide and Drift) rates. There is still discussion about what exactly the upper limit will be but we are currently hovering around 300 arc seconds per second. If a rate is sent to TCS that is higher than the limit one of two things will happen:

1. the rate won't change
2. the rate will be set to the max rate

Note you will not be able to slew with bias rates or the paddle.

At our current TCSNG telescope we have one software horizon limit that does not allow goto but does allow bias, tracking and paddle rates.