Technical Principles

Advantages of Rotary Direct Drives

*MTBF: Mean Time Between Failures

System Advantages

• High dynamics and stiffness
• Extremely smooth motion
• High acceleration
• High velocity
• Compact design
• Easy assembly
• Excellent static and dynamic load rigidity
• No backlash
• Low-wear and low-maintenance system
• Small inertia
• Peak torque T_p: 8.9 – 369 Nm
• Measuring system: Optical measuring principle, several increments depending on model
• Bearing: Compact, high external tilting torque, high stiffness and accuracy, very low axial and radial runout
• Free inner diameter

Standard Design

RDDS Standard Design: component 1 (stage plate) to 7 (base plate) position diagram

The RDDS standard design comprises the following seven components:

Thermal Motor Protection

Monitoring Circuit I and II

Direct drives are often being operated at their thermal performance limits. In addition, unforeseen overloads can occur during operation which result in an additional current load in excess of the permissible nominal current. For this reason, the servo controllers for motors should generally have an overload protection in order to control the motor current. Here, the effective value (root mean square) of the motor current must only be allowed to exceed the permissible nominal current of the motor for a short time. This type of indirect temperature monitoring is very quick and reliable.

IDAM motors are equipped with temperature sensors (PTC and KTY) which should be used for thermal motor protection.

Monitoring Circuit I

The three phase-windings are equipped with three series-connected PTCs to ensure motor protection. A PTC is a positive temperature coefficient thermistor. Its thermal time constant when installed is below 5 s.

Temperature characteristics PTC: resistance R [Ω] vs. temperature ϑ [K]; resistance rises sharply above nominal response temperature T_n

In contrast to a KTY, its resistance increases very sharply when the nominal response temperature T_n is exceeded, increasing to many times the cold value in the process. With three PTC elements connected in series, this behaviour also generates a clear change in the overall resistance even if only one of the elements exceeds the nominal response temperature T_n. The use of three sensors ensures that, even if the motor is at a standstill under an asymmetric phase load, there is a signal for a safe shut-down.

A commercially available motor protection tripping device which is connected downstream will typically trigger between 1.5 and 3.5 kΩ. In this way, overtemperature is detected to within a discrepancy of a few degrees for every winding. The tripping devices also react if the resistance is too low in the PTC circuit, which usually indicates a defect in the monitoring circuit. It also ensures secure electrical separation between the controller and the sensors in the motor. The motor protection tripping devices are not included in the scope of supply.

Monitoring Circuit II

Standard connection of PTC and KTY: three phase-windings (U, V, W) each fitted with PTC; one phase (U) fitted with KTY84-130

On one phase of the motor there is an additional KTY84-130. This sensor is a semiconductor resistor with a positive temperature coefficient.

A temperature-equivalent signal is generated with a delay which depends on the motor type. In order to protect the motor against overtemperature, a shut-off limit is defined in the controller. When the motor is at a standstill, constant currents flow through the windings, with the current depending on the respective pole position.

Temperature characteristics KTY: resistance R [Ω] vs. temperature ϑ [°C] (−40 to 140 °C), measuring current I_max = 2 mA

As a result, the motor does not heat up uniformly, which may cause overheating of non-monitored windings. The PTC and KTY sensors have a basic insulation to the motor. They are not suitable for direct connection to PELV/SELV circuits according to standard DIN EN 50178.

Electrical Connections

The standard cable connections of the RDDS1 matrix are axial. Their position is defined in the drawings. The cable length from the motor output is 1.0 m. The wire ends are open and fitted with end sleeves. The cables which are used are UL approved and suitable for cable drag chains.

Optionally the rotary systems are available with externally mounted plug direct on the casing or with coupler plug on the connecting cables.

Positive Direction of Rotation of the System

The standard direction of rotation of the stage plate is counter-clockwise, but it can vary depending on connecting type.

Pin Layout — Cable Connection (Standard)

Type of cable: 4G1.5 + 2x (2x0.75) KAWEFLEX 5281, Ø 12.6 mm, bending radius dynamically 95 mm, bending radius statically 63 mm

Type of cable: 12x0.08 NJ AWM STYLE 20963, Ø 3.7 mm, bending radius dynamically 40 mm, bending radius statically 8 mm

Pin Layout — Plug Connection (Option)

RDDS1-130xH: 9-pole M17 Mounted Plug

RDDS1-160xH, RDDS1-180xH, RDDS1-230xH and Connecting Variants MA/MU/MD (All Sizes)

RDDS1-160xH, RDDS1-180xH, RDDS1-230xH use an 8-pole M23 mounted plug; connecting variants MA/MU/MD (all sizes) use an 8-pole M23 coupler plug at the cable.

Commutation

Rotary direct drive systems are preferably run in commutated operation. As standard, IDAM torque motors are not equipped with Hall sensors. IDAM recommends measuring system-related commutation, because it is supported by modern servo-inverters and controllers.

Insulation Strength

Insulation strength for link voltages of up to 600 V_DC.

IDAM motors comply with EC directive 73/23/EEC and European standards EN 50178 and EN 60204. Prior to delivery they are tested with differentiated high voltage testing methods and casted under vacuum. Please make sure that the type-related operating voltages of the motors are observed.

Overvoltages at Motor Terminals in Converter Operation

Due to the extremely fast-switching power semiconductors which generate high du/dt loads, significantly higher voltage peaks than the actual converter voltages may occur at the motor terminals, particularly when longer connecting cables (from a length of approx. 5 m) are used between motor and converter. This places a very high load on the motor insulation.

Cooling and Cooling Circuit

Power Loss and Thermal Loss

In addition to the power loss defined by the motor constant k_m, motors are also subject to frequency-dependent losses occurring especially at higher control frequencies (above 50 Hz). These losses jointly cause the motor and other system assemblies to heat up.

The following rule applies at low control frequencies (< 80 Hz) of the motors: Motors with a high motor constant k_m produce lower power losses in relation to comparable motors with a lower motor constant.

The power loss generated during motor operation is transmitted via the motor assembly to attached components. The overall system is carefully designed to control the way in which this heat distribution is influenced and controlled through convection, conduction and radiation.

The continuous torques of liquid-cooled motors are around twice as high as those of uncooled motors. The rotary direct drive systems must be selected and integrated into the machine concept in accordance with the requirements for installation space, accuracy and cooling.

Active cooling should be preferably used on machines with high performance and on equipment with highly dynamic operation and correspondingly high bearing loads.

Cooling Design

The cooling of the systems is designed as jacket cooling and should be connected by the customer to the cooling circuit of a cooling device. The cooling jacket is optionally supplied as a part of the motor or is already an integral part of the machine construction for the customer.

The cooling medium passes from the inlet to the outlet through holes in the cooling fins at different levels. Inlet and outlet connections can be assigned to the two connections as required. The flow area is sealed to the outside with O-rings.

Cooling jacket cross-section: cooling connections, cooling slots and O-ring slots positions

Dependency of Characteristic Data on the Supply Temperature of Cooling Medium

The continuous current I_cw indicated in the data sheet for water cooled operation can be achieved at a rated supply temperature ϑ_nV = 25 °C. Higher supply temperatures ϑ_V result in a reduction of the cooling performance and therefore also the nominal current. The reduced continuous current I_{c red} can be calculated from the following quadratic equation:

Relative continuous current I_{c red} / I_cw [%] vs. supply temperature ϑ_V [°C] (rated supply temperature ϑ_nV = 25 °C)

Positioning Cycle

The following positioning cycle example uses the system RDDS1-160x195-S-B-CA-WM-9000:

Positioning cycle (left): angular acceleration a [rad/s²] and position deviation [arcsec] vs. time t [ms]

Positioning cycle (right): angular acceleration a [rad/s²] and DAC torque [% T_nom/T_p] vs. time t [ms]

T_p: Peak torque; T_nom: Nominal torque

System Configuration

System configuration: RDDS1, IDAM DSMRW, Servo Control E, Servo Drive E and PC (Ethernet/EtherCAT) interconnection, supporting XDDS planar systems and MDDS multi-axis systems

The rotary direct drive system (RDDS1) can be operated with the following control architectures:

• IDAM® DSMRW: servo drive for one axis, or controls of other producers
  • Current feedback
  • Encoder 1 V_ss sin/cos
• IDAM® Servo Control E + IDAM® Servo Drive E (amplifier module for multi-axis systems)
  • Ethernet connection to PC (Embedded PC with Soft-PLC and multi-axis interpolation)
  • Supports IDAM® RDDS1 (rotary), IDAM® XDDS (planar), IDAM® MDDS (multi-axis) systems
• Controls of other producers

Additional Loads

The following images show possible loading cases of the rotary system. External forces respectively additional masses cause certain loadings at the rotary system depending on point of application and position.

Axial Force and Axial Load

Image 1 and Image 2: resulting axial force F_ax = F_a + m × g for centrical arrangement (two assembly orientations)

External forces affecting in the centre, whose line of action is identical with the rotary axis (F_a), as well as centrically arranged additional masses (m), lead to a resulting axial force (F_ax) if the rotary systems are assembled like at image 1 and image 2:

Tilting Torque (Axial Eccentricity)

Image 3: eccentricity r generating tilting torque T_tilt (upright mounting)

Image 4: eccentricity r generating tilting torque T_tilt (inverted mounting)

If the resulting axial force (F_ax) is eccentric to the rotary axis with the distance (r) (images 3 and 4), the rotary system is strained by an additional tilting torque:

Radial Force and Tilting Torque

Image 5: radial force F_r, resulting radial force F_rad, distances a, b, c and tilting torque T_tilt (horizontal mounting)

External forces (F_r), radially affecting the centre, whose line of action is perpendicular to the rotary axis, as well as centrically arranged additional masses (m), lead to a resulting radial strain (F_rad) if the rotary systems are assembled like at image 5:

The point of application of the radial force (F_rad) is generally at a distance (a) from the stage plate and the radial load leads additionally to a strain of tilting torque. The tilting torque is according to image 5:

c is a particular value for any diameter step:

Selection of Direct Drives for Rotary Applications

Cycled Applications

In cycled operation, sequential positioning movements are interspersed with pauses during which no motion takes place. A simple positioning sequence takes the form of a positively accelerated motion followed by a deceleration (negative acceleration of usually the same magnitude, in which case acceleration and deceleration time are equal). The maximum angular speed ω_max is reached at the end of an acceleration phase.

Angular speed ω vs. time t: showing ω_max, −ω_max, motion time t_M and pause time t_P for cycled forward-backward movement

This gives the following α(t)-diagram (α: angular acceleration) as well as the flow of the torque required for the movement:

A cycle is described in the ω(t)-diagram (ω: angular speed, t: time). The diagram shows a forward-backward movement with pauses (t_M: motion time, t_P: pause time).

Angular acceleration α and torque T vs. time t: T₁–T₆ six torque cycles, showing t_M/2 and t_P time segments

According to the torque flow of the desired rhythm cycle, the motor is selected according to three criteria:

• Maximum torque in the cycle M: T_p (peak torque) according to data sheet
• Effective torque in the cycle M: T_c (motor uncooled) or T_cw (water cooling) according to data sheet
• Maximum rotary speed in the cycle M: n_lp according to data sheet

The effective torque equals the root mean square of the torque curve (six torque cycles) in the rhythm cycle:

The safety factor 1.4 in the sample calculation also takes into account the operation of the motor in the non-linear region of the torque-current characteristic, for which the formula for calculating T_eff only applies approximately.

With the torques T₁ = T; T₂ = −T; T₃ = 0; T₄ = −T; T₅ = T; T₆ = 0 and the times t₁ = t_M/2; t₂ = t_M/2; t₃ = t_P; t₄ = t_M/2; t₅ = t_M/2; t₆ = t_P, the effective torque is calculated:

Angular acceleration, maximum angular speed and maximum rotary speed of a positioning movement are calculated with:

If a jerk limit is programmed in the servo-inverter, the positioning times extend accordingly. Constant positioning times require, in this case, greater accelerations.

Selection of Rotary Direct Drive Systems

Example: cycle applications, e.g. for test systems

Conversion of motion angle to radians:

Calculation steps:

Together with the friction torque and the safety factor, this yields the maximum safety factor torque:

In that case the torques for acceleration and braking are equal. The effective torque equals the root mean square of the torque curve (six torque cycles) in the rhythm cycle:

Selection example: angular speed ω vs. time t (ω_max, t_M, t_P)

Selection example: angular acceleration α and torque T vs. time t (T₁–T₃ simplified cycle)