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Overview

The OpenGrab EPM v3 is an electropermanent magnet, combining the advantages of electro and permanent magnets. The device creates a very strong magnetic contact with a ferrous target. It supports UAVCAN, RCPWM and push button operation. OpenGrab EPM v3 has been developed by NicaDrone in cooperation with Zubax Robotics.

  

Theory of operation

Electric shock hazard

The device poses an electric shock hazard. Do not touch exposed parts of the circuit while the magnet is operating.

The NXP LPC11C24 MCU drives a MOSFET connected to a transformer in a flyback configuration to charge the main PET capacitors up to 475 V. A thyristor bridge is used to discharge the capacitor in either direction through the winding inside the AlNiCo material. This results in a short, 20 μs 300 A pulse creating a 100 kAm field in the AlNiCo material. This causes the magnetic domains in the AlNiCo magnets to align in a particular orientation to form a magnetic circuit with a ferrous target. More detailed explanation of the operating principle is available on Wikipedia.

An ON command results in the charging and discharging the capacitors 3 times to achieve full magnetization. An OFF command results in charging and discharging the capacitors several times with changing direction and decreasing amplitude, effectively degaussing the AlNiCo material.

Applications

  • Cargo lifting in UAV and robotic applications.
  • Robot workholding.
  • Education, demonstration of magnetic properties.

Features

  • Steady state power under 50 mW.
  • Short cycle time.
  • Variety of interfaces:
    • RCPWM
    • UAVCAN
    • Push button
  • Open source firmware and hardware.
  • 5 V supply voltage, can be powered via the RCPWM connector or via the UAVCAN port.

Mechanical properties

Safety note

The bottom surface of the magnet should be kept clean, because dirt or metal shavings can be crushed into the surface when the magnet is turning on, causing an insulation breakdown.

The diagrams below document the mechanical arrangement and dimensions (click to enlarge):

Version 3R5C and newer

Version 3R4B and older

Characteristics

SymbolParameterMinimumTypicalMaximumUnit
Tcycle(ON)Time to complete one cycle
0.75
s
Tcycle(OFF)Time to complete one cycle
1.2
s
FmaxMax holding force200300
N
VsupplyOperating voltage4.755.06.5V
IsteadySteady state current draw
10
mA
IpeakPeak current draw during cycle execution

1000mA
mMass of the device
65
g
toperatingOperating temperature-40
+70°C
RHoperatingOperating humidity (non-condensing)0
75%

Human-machine interface

Push button

Pressing this button for at least 200 milliseconds will toggle the EPM.

External button

An external button can be connected to the pin 6 (GND) and 3 (TXD) of the UART header (J9). Pulling pin 3 (TXD) low momentarily toggles the EPM. External button is supported in firmware builds starting from March 2017.

LED indication

Status LED

This LED indicator shows the status of the device derived from the continuous self-diagnostics, according to the UAVCAN node status code:

HealthBlinking ON/OFF duration, milliseconds
OK

50/950

WARNING50/500
ERROR or CRITICAL

50/100

CAN LED

This LED indicates the activity on the CAN bus. Each blink indicates that there was a CAN frame that was successfully transmitted or successfully received during the last few milliseconds. Under a high bus load, this LED indicator is expected to glow constantly.

Note that CAN frames filtered out by the hardware acceptance filters will not cause the LED indicator to blink.

RCPWM interface

Connect an RC receiver or some other hardware capable of producing RCPWM signal (e.g. Pixhawk) to the RCPWM connector.

The device divides the PWM pulse duration into three ranges:

  • Neutral – while the signal is in this range, the device ignores it.
  • OFF – while the signal is in this range, the device will be continuously performing the turn-off sequence.
  • ON – while the signal is in this range, the device will be continuously performing the turn-on sequence.
SymbolParameterMinimumTypicalMaximumUnit
TRCPWM(ON)RCPWM pulse duration to turn ON1.75
2.5ms
TRCPWM(OFF)RCPWM pulse duration to turn OFF0.5
1.25ms
fRCPWMRCPWM input frequency15055Hz
VRCPWM(low)Low-level RCPWM input voltage

0.3×VsupplyV
VRCPWM(high)High-level RCPWM input voltage0.7×Vsupply

V

UAVCAN interface

UAVCAN interconnection with Pixhawk

This section describes the properties specific for this product only. For general info about the UAVCAN interface, please refer to the UAVCAN interface documentation page.

Use the UAVCAN GUI Tool to interact with the magnet from a computer (Windows/Linux/Mac).

Mode and status codes


OpenGrab EPM v3 employs the following UAVCAN-defined operating modes:

UAVCAN operating modeConditions
INITIALIZING

The UAVCAN interface is initializing. The initialization of the interface does not interfere with other functions of the device.

OPERATIONALThe UAVCAN interface and the device itself are fully operational.

The following table describes the meaning of the standard UAVCAN health codes.

UAVCAN health codeConditions
OKEverything is OK; the device is functioning properly.
WARNINGNot used.
ERRORSee below.
CRITICALNot used.

Possible reasons for the health code being ERROR:

  • Invalid input voltage.
  • The high-voltage flyback charger circuit is damaged.

Also, the device reports extended status information via the field uavcan.protocol.NodeStatus.vendor_specific_status_code. The higher byte is used to store the current voltage on the buffer capacitor, the units are 2 V per LSB. The lower byte is used to store implementation-specific status flags.

Services

UAVCAN interconnection with Pixhawk

This device does not invoke any services.

The following service servers are implemented:

Data typeNote
uavcan.protocol.GetNodeInfoName: com.zubax.opengrab_epm_v3

Messages

Input:

Data typeNote
uavcan.equipment.hardpoint.CommandControls the magnet, see below.
uavcan.protocol.dynamic_node_id.AllocationUsed to allocate node ID if dynamic node ID allocation is enabled.

Output:

Data typeNote
uavcan.protocol.NodeStatusDescribed above.
uavcan.equipment.hardpoint.StatusStatus of the magnet, see below.
uavcan.protocol.dynamic_node_id.AllocationUsed to allocate node ID if dynamic node ID allocation is enabled.

uavcan.equipment.hardpoint.Command

This message allows to control the magnet via UAVCAN. The fields are interpreted as follows:

hardpoint_id

If the field does not equal the hardpoint ID of the current device, the message will be ignored.

command

  1. If this field is zero and the magnet is turned on → the magnet will turn off.

  2. If this field is non-zero and the magnet is turned off  the magnet will execute the number of turn on cycles specified in the field, but not less than 3 and not more than 10.

  3. If this field is non-zero, the magnet is turned on, and the field has changed its value  see #2.

  4. In all other cases the command will be ignored.

uavcan.equipment.hardpoint.Status

This message carries the status of the magnet.

hardpoint_id

Hardpoint ID of the current magnet.

payload_weight

Always set to NaN.

payload_weight_variance

Always set to positive infinity.

status

Indicates whether the magnet is turned on or off:

  • 1 - the magnet is turned on.
  • 0 - the magnet is turned off.

CAN bus characteristics

The device will detect the CAN bus bit rate automatically after powering on. The automatic detection is done by means of listening to the bus in the silent mode, alternating between the pre-defined values of supported CAN bit rates (listed in the table) until the first valid CAN frame is received. Unconfigured CAN bus does not interfere with other functions of the device.

SymbolParameterMinimumTypicalMaximumUnit
fCANCAN bit rate (autodetect)
100
125
250
500
1000

Kbps
VCAN(out)dif-domCAN dominant differential output voltage1.503V
VCAN(out)dif-recCAN recessive differential output voltage-50050mV
ICAN(out)domCAN dominant output current4070120mA
ICAN(out)domCAN recessive output current-5
5mA
RCAN(in)diffCAN differential input resistance193052
tCAN(out)to-domCAN dominant time-out time0.3112ms

DIP switch

The device is equipped with a 4-position DIP switch that allows the user to configure the Hardpoint ID and enable or disable UAVCAN dynamic node ID allocation:

#Purpose

1

Hardpoint ID bit 0
2Hardpoint ID bit 1

3

Hardpoint ID bit 2

4

0 - use dynamic node ID allocation;

1 - use fixed node ID

Hardpoint ID

Hardpoint ID is defined in binary by the lowest 3 switches. The table below clarifies the binary encoding:

Hardpoint IDDIP #3DIP #2DIP #1
0000
1001
2010
3011
4100
5101
6110
7111

Node ID selection

If the DIP switch #4 is set to OFF, the device will perform dynamic node ID allocation once the CAN bus bit rate detection is done. This implies that the device will not be available via the UAVCAN interface unless the UAVCAN network contains a functioning dynamic node ID allocation server. Please refer to the UAVCAN specification for more info.

If the DIP switch #4 is set to ON, the device’s node ID will be fixed at (Hardpoint ID + 100). For example, if the Hardpoint ID is set to 5, the fixed node ID will be configured as 105. In this case the device does not require an external dynamic node ID allocation server, and therefore it will be accessible via UAVCAN immediately once the CAN bus bit rate detection is done.

UART interface

The EPM reports error and status messages over this interface. This interface can also be used to update the firmware – please refer to the source repository for instructions (link below).

Parameters of the serial interface are as follows:

ParameterValue
Baud rate115200
Word size8
ParityNone
Stop bits1
New line sequence\r\n (CR-LF)


SymbolParameterMinimumTypicalMaximumUnit
VUART(in-low)Low-level UART input voltage

0.3 VsupplyV
VUART(in-high)High-level UART input voltage0.7 Vsupply

V
VUART(out-low)Low-level UART output voltage

0.4 VsupplyV
VUART(out-high)High-level UART output voltageVsupply - 0.4

V