Controller Drive model – SINUMERIK 840d SL &SINAMICS S120

In this section the actual configuration of SINUMERIK 840d SL controller and SINAMICS S120 drives is described. As mentioned earlier this composite system enables motion control in the GEISS machine. The figure in the next page illustrates the entire graph of SINUMERIK and SINAMICS modules, how they are connected together to perform control on GANTRY axis. As observed at the heart of the system is the SINUMERIK 840d SL numeric control unit NCU730.3 module. This is the numerical controller that computes all the relevant control signals. The port numbers shown have a prefix of character X. This signifies a DRIVE CLiQ connection. The gantry is composed of Y1 and Y2 axis. They both have one 1FK7 motor each. Each of these motors is driven by a motor module. The motor modules and the NCU controller are both powered by the Active line module. The NCU perform the position control, speed control and current control and finally at the end of the cascaded control operations it generates the direct and quadrature voltages and passes them on to the Motor modules. The motor module is comprised of inverter. They generate the pulse with modulation (PWM) signals and control the motor in essence. Apart from driving the motor, it also performs acquisition activities collecting the encoder signals and the three phase voltage signals feeding it back to the NCU which operates on these signals and the respective set points to compute the error signals. The motor module gives angular position, speed and three phase voltage information to the NCU. But for the NCU to perform its position control activities it needs the linear position signals. This is provided by the Heidenhain LC 183 position encoder. Since this is an external encoder to communicate with the NCU it needs a middle man. Sensor module external (SME) serves this purpose. NCU has a limitation on the number of devices it can control. The NCU is connected to the HMI unit using Ethernet connectivity so the user can input the position and speed set points and perform diagnostic and trace operations.

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AXIS POSITION CONTROL LOOP

The controller for an axis consists of the speed control loop, the current control loop and a position control loop like its presented on Figure 4-8

Figure 4-8 Siemens system control loops (Siemens 2012)

In order to obtain high contour accuracy with an interpolation, the loop gain factor (KV factor) of the

position controller must be large. If the KV factor is too high, on the other hand, this leads to

overshooting, instability and inadmissibly high loading of the machine. The maximum permissible KV factor is dependent on the design and dynamic response of the drive and the mechanical quality of the machine. (Siemens 2012)

Figure 4-9 shows a simplified model of the position control loop and a velocity control loop. The velocity control loop is represented by a first-order transfer function block (Zirn 2005) with the time constant: 𝑇𝑣= { 1 𝑘 𝑓𝑜𝑟 𝑖𝑛𝑑𝑖𝑟𝑒𝑐𝑡 𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑚𝑒𝑛𝑡 𝑘+𝜔₀ 𝑘𝜔0 𝑓𝑜𝑟 𝑑𝑖𝑟𝑒𝑐𝑡 𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑚𝑒𝑛𝑡 Equation 4-1

depending on the position capture location, the additional delay due to the flexibility between motor inertia and load can be assessed by the predominant resonant angular frequency ω0.

Figure 4-9 Simplified model of the position control loop (Zirn 2005)

The transfer function of the simplified position control loop is given by

𝐺𝑥(𝑠) = 𝐾𝑣

𝑇_𝑣𝑠2_{+𝑠+𝐾𝑣} Equation 4-2

86 𝐾𝑣≤_4𝑇1 𝑣= { 𝑘 4 𝑓𝑜𝑟 𝑖𝑛𝑑𝑖𝑟𝑒𝑐𝑡 𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑚𝑒𝑛𝑡 𝑘𝜔₀ 4(𝑘+𝜔₀) 𝑓𝑜𝑟 𝑑𝑖𝑟𝑒𝑐𝑡 𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑚𝑒𝑛𝑡 Equation 4-3

Equation defines the maximum position control gain for one axis. A machine tool requires identical position control gains in all axes to eliminate the static path error. Thus the maximum position control gain has to conform to the “weakest” axis.

The loop gain is converted using the following formula:

𝐾𝑣(𝑒−1) = 𝐾𝑣[𝑚 𝑚𝑖𝑛 ⁄ ]

𝑚𝑚 16.6667 Equation 4-4

VELOCITY CONTROL LOOP

The velocity controller is responsible for damping (proportional gain Kp) and eliminating (integrator

time constant Tn) static loads. The typical commissioning procedure is firstly to tune the proportional

gain and afterwards to decrease the integrator time constant until a well damped control performance is reached. The drive frequency response contains redundant information. Due to the logarithmic scale, uncertainties concerning the current converter scaling factors and the noise in real measurements, the frequency response gain yields only a rough estimation for complete inertia θ and inertia ratioλ. As motor inertia λθ is generally known with sufficient precision from the machine tool design, the inertia ratio can be computed using resonance frequency ω0 and

resonance frequency ωN, which as a rule are clearly identifiable in the frequency response:

λ = (ωN ω₀) 2 = (fN f₀) 2 Equation 4-5

The closed velocity control loop transfer function is:

𝐺𝑣(𝑠) = 𝐾𝑝𝐺(𝑠) 1+𝐾_𝑝𝐺(𝑠)= κ λs2+ω02κ s3κ λs2+ω02s+ω02κ Equation 4-6

With the scaled proportional gain:

κ =κp

θ Equation 4-7

The small material damping in the structure is mostly negligible (damping constant d≅ 0) compared to the damping introduced by the feedback control loop. The root locus of the velocity control loop shows one pole on the negative real axis, which defines the set point response. The conjugated complex pole pair defines the vibration and damping performance. At first, increasing κ leads to an improved damping performance and a faster set point response. Overdrawing κ changes damping for the worse. Thus the commissioning criterion for κ is optimum vibration damping.

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