Delta Tau GEO BRICK LV Manuel D’Utilisation

Ixx31 Derivative Gain Term 

The derivative gain term set by Ixx31 provides a damping effect by providing a contribution to the control 
effort proportional to the actual velocity acting against that velocity.  In this respect it acts much like a 
dashpot or the shock absorber of a vehicle’s suspension.  The higher the derivative gain term, the heavier 
the damping action. 

Some form of derivative action – effectively a velocity loop – is required for a stable position loop.  If a 
well-tuned velocity loop is closed in the amplifier, the Ixx31 derivative gain term in Turbo PMAC can be 
set to 0.  However, if there is no velocity loop closed externally, a positive value of Ixx31 will be required 
for stable operation. 

Note that in the Turbo PMAC, this gain acts on the derivative of the actual position, not on the derivative 
of the position error, as in some other controllers.  This permits the simple use of dual motor-and-load 
feedback with a separate sensor on the motor for derivative action (specified by Ixx04) from the sensor on 
the load for proportional and integral action (specified by Ixx03). 

Ixx33 Integral Gain Term 

The integral gain term set by Ixx33 provides for correction against steady-state errors caused by such 
effects as friction, gravitational loads, cutting loads, and analog offsets.  The integral gain term controls 
how fast the position error integrator term charges up and discharges; the higher the gain, the faster the 
action, 

Ixx34 is a single-bit variable that controls the time in which the integral gain term is active.  At the default 
value of 1, the integral gain is active only when the commanded velocity is zero (at move end).  When 
Ixx34 is 0, the integral gain term is always active.  When tuning the other terms, generally it is best to set 
Ixx34 to 1 to reduce the errors as much as possible without the integrator; if the remaining errors are 
small, it is then usually good practice to set Ixx34 to 0 to let the integrator dynamically compensate for 
the remaining errors. 

A feedback filter is error driven, so an error must exist between the commanded and actual positions 
before it takes any action.  The actions of feedforward, on the other hand, are dependent only on the 
commanded trajectory, and therefore do not require errors to cause action.  The basic idea of feedforward 
is to directly apply your best estimate of the control effort needed to execute the commanded trajectory, 
without waiting for position errors to build up.  The feedback terms then need to respond only to the 
errors in this estimate, which typically are quite small. 

In a well-tuned system, over 95 percent of the control effort can come from the feedforward terms, with 
the feedback terms just providing small corrections for disturbances and imperfections in the estimate.  
Turbo PMAC’s PID filter has velocity and acceleration feedforward terms, covered here, and a non-linear 
friction feedforward term, covered below under the heading Servo Loop Modifiers. 

Ixx32 Velocity Feedforward Term 

The velocity feedforward term Ixx32 adds an amount to the control effort that is directly proportional to 
the commanded velocity, to overcome potential position errors that would be proportional to velocity.  
These errors can come from several sources.  The first source, and the dominant one, is from the velocity 
feedback term that provides the required damping for stability, whether done in the Turbo PMAC (the 
Ixx31 term) or externally.  Other minor sources of velocity related errors include magnetic losses in the 
motor and actual viscous damping losses. 

Properly set velocity feedforward will eliminate following error components that are proportional to 
velocity.  If the Turbo PMAC is closing the velocity loop for the motor, typically the optimal Ixx32 will 
be equal to, or slightly greater than Ixx31.